Bioaccumulation and toxicity of silver compounds: A review

Authors

  • Hans Toni Ratte

    Corresponding author
    1. Aachen University of Technology, Chair of Biology V (Ecology, Ecotoxicology, Ecochemistry), Worringerweg 1, D-52056 Aachen, Germany
    • Aachen University of Technology, Chair of Biology V (Ecology, Ecotoxicology, Ecochemistry), Worringerweg 1, D-52056 Aachen, Germany
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Abstract

A eview of the literature revealed that bioaccumulation of silver in soil is rather low, even if the soil is amended with silver-containing sewage sludge. Plants grown on tailings of silver mines were found to have silver primarily in the root systems. In marine and freshwater systems, the highest reported bioconcentration factors (BCFs) were observed in algae (>105), probably because of adsorption of the dissolved silver (<0.45 μm fraction) to the cell surface. In herbivorous organisms (e.g., zooplankton and bivalves), the BCF was lower by about two orders of magnitude. Low amounts of silver were assimilated from food with no substantial biomagnification. In carnivores (e.g., fish), the BCF was also lower by one order of magnitude with no indication of biomagnification. Toxicity of silver occurs mainly in the aqueous phase and depends on the concentration of active, free Ag+ ions. Accordingly, many processes and water characteristics reduce silver toxicity by stopping the formation of free Ag+, binding Ag+, or preventing binding of Ag+ to the reactive surfaces of organisms. The solubility of a silver compound, and the presence of complexing agents (e.g., thiosulfate or chloride), dissolved organic carbon, and competing ions are important. In soil, sewage sludge, and sediments, in which silver sulfide predominates, the toxicity of silver, even at high total concentrations, is very low. The highly soluble silver thiosulfate complex has low toxicity, which can be attributed to the silver complexed by thiosulfate. Silver nitrate is one of the most toxic silver compounds. The toxic potential of silver chloride complexes in seawater is and will be an important issue for investigation. Aquatic chronic tests, long-term tests, and tests including sensitive life stages show lower toxicity thresholds (˜1 μg Ag+/L). The organisms viewed as most sensitive to silver are small aquatic invertebrates, particularly embryonic and larval stages.

INTRODUCTION

Silver ion is one of the most toxic forms of a heavy metal, surpassed only by mercury and thus has been assigned to the highest toxicity class, together with cadmium, chromium (VI), copper, and mercury [1, 2]. The sources of silver in the environment were recently reviewed [3]. Annual silver released to the environment from industrial wastes and emissions has been estimated at approx. 2,500 tonnes, of which 150 tonnes gets into the sludge of wastewater treatment plants and 80 tonnes is released into surface waters [4, 5]. Surprisingly, however, the toxicity of silver was investigated after all other heavy metals, beginning about 25 years ago [6]. About 15 years ago, the environmental chemistry of silver was barely understood [7]. Because of huge research efforts by the photographic industry since the early 1990s, initiated primarily by the Silver Coalition and the Silver Council in the United States [8], our knowledge of the environmental fate and toxicity of silver has changed markedly, and several review articles and national and internal reports have been published [9–13].

Obviously, the perception of high silver toxicity has long been due to the fact that most laboratory toxicity experiments tested AgNO3, which readily dissolves, releasing the highly toxic free Ag+ ion. Because of enhanced heavy metal analyses and experimental techniques (e.g., the ultraclean technique [14–18]), a better understanding of total silver concentrations in various environmental compartments and, in particular, of silver speciation has emerged. This research has demonstrated that apparent toxicity is related to individual silver species rather than total silver concentration. In natural waters, where silver contamination can be of concern, evidence of toxicity from the dissolved silver ion is generally less than in laboratory tests because of the rich opportunities for possible covalent, complexing, or colloidal binding silver encounters with a variety of reactants [17, 19].

Overall, silver in the environment can be expected to behave predictably [13]. The majority (>94% [17]) of the silver released into the environment will remain in the soil or wastewater sludge at the emission site. A portion, the toxicity of which should not be underestimated, will be transported for long distances by air. Silver from industrial and public wastewater is bound to the activated sludge of wastewater treatment plants. The remaining portion of the silver enters the aquatic environment and, under freshwater conditions, will be adsorbed to sediments or suspended particles immediately at the discharge site and thus is immobilized. A small amount of silver will be kept in solution by colloidal and complexed material, transported downstream, and enter lakes, estuaries, or the sea. Here the colloidal binding, decreasing with increasing salinity [20], is replaced by complex binding with chloride, which keeps a substantial part of the silver molecule in solution. Silver thus can be distributed by tidal and marine currents, which leads to dilution. In the estuary, the majority of silver is deposited in the sediment, where it is immobilized and to a great extent retained.

The existence of various silver species depends on physicochemical environmental conditions. Results of laboratory experiments indicate that the biological response elicited by a dissolved metal is a function of the concentration of the free metal ion. (Do not confuse this with dissolved silver, which also consists of complexed silver species.) However, as reported for other metals [21], exceptions to this so-called freeion activity model (FIAM) of biovailability have been identified that may also apply to silver. Three prerequisites are necessary for the interaction of trace metals with aquatic organisms [21, 22]: (1) metal speciation in the external surrounding of an organism, (2) metal interactions with biological membranes separating organisms from their environment, and (3) metal distribution within the organisms with a subsequent biological effect. The third prerequisite requires that the metal react with some target site and that the reaction product change the metabolic reactions, which are so disturbing that the adaptation of an organism is impaired.

This review presents a survey of the current knowledge on silver bioaccumulation and toxicity with special emphasis on the most recent results. The toxic thresholds are compared with recently measured silver concentrations in the environment, and areas for further research are identified.

BIOACCUMULATION

Definition

Bioaccumulation is the uptake of substances via body surfaces (bioconcentration) and through food uptake (biomagnification). The bioconcentration factor (BCF) is the ratio between the concentration of a chemical in an organism and that in the surrounding medium (e.g., water) or food. In terrestrial plants, uptake generally is through the roots and leaves. In terrestrial animals, uptake is generally through the body surface (endogeous fauna) or intestine (endo- and epigeous fauna). In the latter, this is solely biomagnification. In aquatic systems, both bioconcentration and biomagnification occur. According to current knowledge, uptake through the body surface predominates. As yet, there is no evidence of substantial biomagnification of silver in aquatic organisms [23–27].

Uptake of a chemical from water proceeds by passive diffusion, active transport, and adsorption. In diffusion, the chemicals enter the body through semipermeable membranes (e.g., gills), mucous membranes, or the digestive tract. In contrast, active transport requires macromolecular carriers in the membrane; these carriers form a reversible complex with the chemical and thus facilitate uptake. Predominantly trace elements, including silver, enter the organism this way. Uptake by adsorption assumes binding of a chemical on surfaces by covalent, electrostatic, or molecular forces. Especially in microorganisms, this way of uptake is important because of their high surface/volume ratio.

Bioaccumulation of some metal compounds via diffusion seems to be possible because of interactions not addressed in the FIAM [22]. For example, metals can form noncharged lipophilic complexes with certain organic ligands so that they pass membranes by passive diffusion. Examples of such ligands are diethyl-dithio-carbamates, 8-hydroxy-quinoline, and the so-called xanthates, which are used as flotation agents in the mining industry. However, it has yet to be proved whether increased metal uptake really occurs in the presence of the prevailing metal ligand, in particular Ag ligand concentrations in the field.

Further deviations from the general applicability of the FIAM are represented by inorganic anions, influencing metal speciation. In the grass shrimp (Palaemonetes pugio), the effects of salinity (chloride) on the uptake of radio-labeled silver (110Ag) was examined [28]. As observed for the simultaneously investigated cadmium uptake, silver uptake decreased with increasing salinity. The silver uptake after 4 d, however, was proportional to the concentration of AgCl0 rather than to the free Ag+ ion. The investigators attributed this to the lipophily of the noncharged AgCl complex, relieving the uptake through the cell membrane. The specific salinity-dependent silver speciation in estuaries results in silver being accumulated more than any other trace element, at least in some marine invertebrates [9].

Furthermore, increased metal bioavailability can be caused by certain hydrophilic complexes with organic ligands of low molecular weight. Normally one would expect that a complexing reaction with organic ligands lowers the bioavailability of metals, which indeed applies in most cases. In particular, it has been demonstrated for copper that low-molecular-weight organic ligands (e.g., glycine, alanine, glutamine acid, citrate, and ethylenediamine) contribute to increased metal uptake (and higher toxicity). A summary of these findings and additional exceptions (some of which are not explainable) from the applicability of the FIAM is given by Campbell [22]. With regard to silver uptake and silver toxicity, these possibilities must be investigated further.

One of the important questions is whether the body concentrations in aquatic organisms are proportional to the metal concentration in the water. For example, metal proportionality is disturbed by regulation of the body metal concentration, which is well known for the essential trace metals (e.g., Cu, Zn, and Fe). Metal regulation is very common among higher organisms [29]. Such regulation has not been demonstrated for nonessential metals like silver. Nonetheless, the proportionality to the surrounding metal concentration can be missing, because detoxification or storage of the metals in an inert form (e.g., in mussel shells or fish bones) may interfere with proportionality. Bivalves, which are regarded as bad regulators, are thus suitable as indicators and have been used correspondingly. Because accumulation of contaminants is also possible via the food chain, it makes sense to discuss bioaccumulation, whenever possible, in the context of the terrestrial, freshwater, or marine food chain.

Bacteria

Silver ions are very toxic to bacteria, and bioaccumulation can be investigated only in silver-tolerant species or at low concentrations, which are found in sewage treatment works with wastewater from silver end users. A bacterial biocoenosis exhibiting an extraordinarily high silver tolerance was isolated by Charley and Bull [30]. The genus Pseudomonas was mainly responsible for the high tolerance and thus became dominant in this biocoenosis, which resulted in a bioaccumulation capacity of >300 g Ag/kg dry weight. The accumulation rate was 21 mg Ag+/g biomass·h−1. These results do not imply an adsorption of silver to the cell surface alone, as discussed in algae (see below). Charley and Bull [30] also discuss the possible use of these bacteria in silver recycling.

TERRESTRIAL ORGANISMS

Higher plants and fungi

In higher plants and fungi, silver accumulation is expected only in areas contaminated with silver, such as tailings from silver mines, areas with cloud seeding (AgI), and soils amended with silver-containing sewage sludge (for results, see Table 1) [31, 32].

Trees have been shown to be suitable monitors for heavy metals [31]. The yearly rings were analyzed and showed contamination in past years. The silver content in these trees was also dependent on exposure to solar rays and seasonal variations. In a study of a variety of agricultural crops compared to controls, elevated silver levels were found only in lettuce [32]. The plants were grown on soils amended with sewage sludge that was experimentally spiked with silver sulfide. Significant toxicity or a reduction of the fertilizing potential of the amended soil was not observed. In Lolium perenne (perennial rye grass) and Trifolium repens (white clover) (grown under laboratory conditions in a culture medium), it was shown that 90% of the silver was immobilized in the root system within 10 d [33]. The silver concentration within the plants decreased gradually with increasing distance from the root system (L. perenne, from 64 to 1.2 mg/kg dry weight; T. repens, from 46 to 1.0 mg/kg dry weight). Grasses and agricultural crops accumulated silver to a far greater extent in the roots [31, 32, 34, 35] than other parts of the plant (for concentrations, see Table 1). Mushrooms were found to have a higher bioaccumulation potential if grown with compost amended with silver and sewage [36].

Table Table 1.. Silver concentrations and bioconcentration factors observed in terrestrial plants and fungi
BiotaQuantityParameteraReference
  1. a BCF = bioconcentration factor; C = silver concentration (mg/kg).

  2. b Experimentally amended soils (sewage sludge plus silver sulfide).

  3. c DW = dry weight.

Higher plants   
Agricultural cropsb<1C, above ground parts[32]
(corn, lettuce, oat, turnip, soybean)2.0–33.8C, roots 
Plants14C, maximum[10]
Grasses (tailings of silver mines)0.77–4.3BCF, roots/soil[34]
 0.017–0.33BCF, grass/roots 
 2,100–124,000BCF, grass/H2O 
Cereal products0.008C, mean 
 0.14C, maximum[105]
Vegetables0.007C, mean 
 0.039C, maximum 
Fruit<0.05C 
Plants (35 species)0.01–16C[72]
Grassesb0.06–5.04BCF, plant/soil 
Trees (AgI-seeding generator site)   
Leaves1C 
Twigs13C, 25 m above ground 
Bark5.3C, 15 m above ground 
Wood3.2C, 13 m above ground 
 2.4C, 22 m above ground 
Tea leaves0.2–2C[106]
Fungi   
General119C, maximum[10]
Mushroomsb120–150 DWcC[36]

Animals

Bioaccumulation of silver by terrestrial animals has been investigated mainly in domestic animals and laboratory models. Using radiolabeled 110Ag, the kinetics of silver uptake and depuration was studied in laboratory rats and mice after intravenous or intramuscular injection of silver (˜0.0011 ng/L each) as well as application by a gastrointestinal tube (10.8 ng/L). Silver was absorbed by only 0.1% within 15 d, after it was first accumulated by a factor of 10 to 100 in the liver, kidney, spleen, skin, bones, and muscles [37, 38]. From these results, it can be concluded that there is no substantial potential for bioaccumulation in mammals. Silver concentrations detected in the tissues of domestic animals, which also apply to humans [13], were relatively low (Table 2). The silver content in the liver of birds feeding on silver-contaminated food was found to be elevated [10]. In terrestrial invertebrates, elevated silver concentrations were sometimes found (e.g., in snails and worms), although there was no bioaccumulation potential in earthworms [39].

FRESHWATER ORGANISMS

Algae

In aquatic environments, the phytoplankton and periphyton form the base of the food chain and are generally responsible for the introduction of contaminants into the chain. Algae possess an extraordinary accumulation potential for dissolved (<0.2-μm particle size) heavy metals, particularly silver (Table 3). Uptake into the algal cells strongly affects the fate of the biogeochemical cycling of metals if the cells are consumed by animals [40].

In experiments with 110Ag and the green alga Scenedesmus obliquus, the accumulation of silver was found to proceed rapidly [41]. Equilibrium was reached after an exposure period of only 24 h, and a high BCF was observed (1.8–25 × 105 dry weight = 3–42 × 104 wet weight). Silver has a biological depuration half-life of 115 h, and accumulation is suggested to be due to adsorption to the cell surface rather than to active uptake into the cells. Neither mechanical disruption of the cells, low pH, nor enzymatic degradation (usually by digestion) was able to remove the silver from the cell walls [40]. Hence, biomagnification of silver in algal herbivores (e.g., in filterfeeding crustaceans) is unlikely [42].

The BCFs appear to be positively correlated with solubility of the silver compound. Differences in accumulation of the same silver compound probably result from different experimental conditions in the laboratory experiments. Very low values under field conditions are probably due to the fact that the accumulated quantity was related to the total silver concentration in the surrounding medium rather than to the dissolved silver concentration; only the bioavailable part of the total silver concentration can really be accumulated.

Table Table 2.. Silver concentration and bioconcentration factors in terrestrial animals
OrganismsQuantityParameteraReference
  1. a BCF = bioconcentration factor; C = silver concentration (mg/kg).

Endogeous fauna   
Oligochaetes   
Worms, general30C, maximum[10]
Lumbricus terrestris (earthworm)1BCF Ag2S[39]
Epigeous fauna   
Mollusks   
Snails320C, maximum[10]
Snails (edible)0.1–10C[107]
Birds   
Liver44C[10]
Mammals   
Beef (meat)0.004–0.024C[107]
Cow (milk)0.037–0.095C[106]
Pig (meat)0.007–0.012C[107]
Sheep (meat)0.006–0.011C 

Herbivorous and detritivorous organisms

In lentic freshwaters, algae are consumed by filter-feeding zooplankton, which consist of protozoans, rotifers, and small crustaceans (pelagic food web). Another food chain, the benthic chain, follows the consumption of the algal layer on the water-body bottom and plant surfaces, sometimes including allochtonous materials (e.g., fallen leaves), and is dominated by snails, some insect larvae (e.g., chironomids), gammarids, bivalves, and worms.

Pelagic food chain

Bioconcentration factors and silver concentrations in tissue for daphnids have been obtained from single-species and indoor microcosm tests. Bioconcentration in daphnids was shown to be markedly lower than in algae (Table 3). In addition, it appears more clearly here that a lower solubility of the silver compound leads to lower bioavailability and bioaccumulation [39]. However, earlier studies revealed significantly higher BCF [24], which probably can be attributed to different experimental conditions or to problems occurring before development of ultraclean techniques. Nonetheless, findings clearly indicate that for the pelagic food chain, substantial bioaccumulation is not probable. In addition, it would be worth characterizing how much of the silver is bound to the outside carapace and thus does not contribute to the toxic potential in daphnids.

Benthic food chain

Benthic invertebrates can take up trace elements in three principal ways: (1) by direct contact of the body surface with contaminated sediment particles, (2) from the interstitial water, and (3) from sediment particles being ingested and digested in the intestine [43]. In gammarids, the BCF reached a maximum within 30 d, and in midge larvae and chironomids, within 60 d, exceeding the BCF obtained in daphnids (Table 3). In benthic macroinvertebrates, the rapid uptake kinetics and the relatively high BCFs from water (sediment absent) suggest rapid silver adsorption to the body surface (including gills and intestine). Because of the very high affinity of silver to sediment, however, the authors excluded a substantial uptake from the pore water. In contrast, silver uptake from ingested and digested sediment particles appears highly probable and is supported by results from studies with other heavy metals [43].

In an indoor microcosm study, freshwater bivalves exposed to silver thiosulfate accumulated less silver than daphnids; this finding was thought to be due to the higher adsorption to the body surfaces in daphnids [24]. In more recent investigations [39], the BCF of bivalves (8.5) corresponded to that in daphnids (9). In contrast to the above-mentioned study [43], backswimmers and chironomids did not accumulate thiosulfate silver.

Zebra mussels are well known for their enormous filtration rates. Using microcosms and radiolabeled isotopes, the trace elements silver, cadmium, selenium, and chromium were examined in combination with various levels of dissolved organic carbon (DOC) [44]. With uptake, different typical food particle fractions were considered (e.g., Diatomeae, Chlorophyceae, natural seston populations, and natural bacteria populations). On the tested metals, the assimilation efficiencies were 4% for silver, up to 56% for cadmium, with biological half-lives of 3 d for silver to 141 d for cadmium. Because a proportionality between tissue concentration and environmental concentrations was found, zebra mussels seem suitable as monitors for environmental concentrations of trace metals. Studies with freshwater oligochaetes (Lumbriculus variegatus) exposed to Ag2S-amended sediment (444 mg Ag/kg) for 28 d, showed virtually no silver bioaccumulation (BCF = 0.18) [45]. The reason was obviously the low bioavailability of the nearly insoluble silver sulfide.

Carnivorous and omnivorous organisms

Pelagic carnivores (e.g., fathead minnows [Pimephales promelas]) showed a relatively low bioaccumulation potential (BCF = 1.8) [39] (Table 3) when exposed to silver thiosulfate complexes compared to that of their prey (e.g., daphnids). Silver obviously does not readily move up the whole food chain and enter the top carnivores.

In the zebra fish (Brachydanio rerio), complex interactions between copper, zinc, silver (AgNO3), selenium, and methyl mercury, affected bioconcentration after aqueous exposure for 12 d [46]. Most striking, a notable interaction occurred between silver and methyl mercury. Silver was present together with methyl mercury in the water, it inhibited mercury bioaccumulation in the fish. On the other hand, the bioconcentration of silver was significantly elevated in the presence of zinc, copper, and selenium.

Table Table 3.. Silver concentration and bioconcentration factors as observed with freshwater organisms (maximum values)
OrganismQuantityaParameterbAg compoundTypec of experimentReference
  1. a DL = detection limit; DW = relation to dry weight; WW = relation to wet weight.

  2. b BCF = bioconcentration factor; C = silver concentration (mg/kg).

  3. c F = field measurement; L = laboratory experiment; LM = indoor microcosm.

Algae     
Selenastrum capricornutum3CAgNO3L[26]
 4.8BCF1 μg/L Ag  
Green algae150BCFNaAgS2O3L[39]
Scenedesmus obliquus1.8–25 × 105 DWBCF110AgCNL[41]
 (= 3–42 × 104 WW    
Periphyton0.1–3.6CAg, riverF[108]
Several species50BCF, cells/sediment   
Scenedesmus obliquus6,818 WWBCF CNaAgS2O3LM[24]
 750 mg/kg WWC   
Macrophytes     
Lemna gibba135.3CAgNO3  
Duckweed1.5BCF6 μg/L Ag  
 25.4BCF118.5 μg/L AgL[26]
Crustaceans     
Daphnia magna61BCFAgNO3L[26]
   0.5 μg/L Ag  
 9BCFNaAgS2O3L[39]
 >2,200BCFNaAgS2OLM[24]
 130C23  
Gammarus pulex1,100BCF, water110AgCNLM[43]
 1.9BCF, sediment   
Insects     
Chironomus luridus1,100BCF, water110AgCNLM[43]
 0.67BCF, sediment   
Chironomus sp.0.5BCFNaAgS2O3FM[39]
Hydophilidae sp.0.08BCFNaAgS2O3FM 
Notonecta sp.0.08BCFNaAgS2O3FM 
Oligochaetes     
Lumbriculus variegatus0.18BCFAg2SL[45]
 80.3C444 mg Ag/kgL 
Mollusks     
Ligumia sp.8.5BCFNaAgS2O3LM[24]
 1,400 WWBCFNaAgS2OLM 
Margaritifera sp.110 WWC   
Amphibians     
Bufo sp.0.5BCFNaAgS2O3FM[39]
Fish     
Lepomis macrochirusBelow DLCNaAgS2O3L[26]
   05 μg Ag/LL 
Oncorhynchus mykiss (168-h static bioassay)335BCFNaAgS2O3L[80]
Campostomum anomalum1.32–2.73CAgF[48]
Lepomis microlophus     
Contaminated site0.458CAgF[47]
Noncontaminated site0.001    
Cyprinus carpio106BCF, water110AgCNLM[43]
 1,100BCF, sediment   
Pimephales promelas1.8BCFNaAgS2O3FM[39]
 303 WWBCFNaAgS2O3LM[24]
 32 WWC   

In three species of sunfish (Lepomis microlophus, Micropterus salmonides, and L. macrochirus) from storm-water ponds, natural lakes, and ponds in the vicinity of Orlando, Florida, USA, the silver concentration was significantly higher in L. microlophus from the storm-water ponds as compared to controls [47] (Table 3). In the remaining two species, the silver concentrations did not differ significantly, and in all three species, there were no significant correlations between silver concentration and fish length or weight. Most fish species are able to accumulate metals from food and directly from water via various membrane surfaces, particularly the gills. The concentration of most metals appeared to be dependent on physical sediment contact, or sediment contact by its prey, rather than trophic position in the food chain. Fish species with the greatest sediment contact (such as L. microlophus) contained the highest metal concentrations [47].

Silver bioavailability was estimated with the stoneroller minnow (Campostomum anomalum) harvested from a creek receiving silver-containing effluents using tissue silver concentrations [48]. After gut removal, the tissue concentrations of fish from the effluent outfall zone were 1.32 to 2.73 μg Ag/g dry weight, compared to 0.4 μg/g in fish from noncontaminated river zones. Although the silver concentrations downstream of the discharge remained relatively constant, the tissue silver concentrations in the minnows decreased exponentially and after ˜5.5 km reached the level of control fish from noncontaminated river zones.

Benthic food chain

The carp (Cyprinus carpio), a representative of this food chain, feeds on not only benthic prey (e.g., chironomid larvae and gammarids) but also plant food. The BCF of the carp increased to a maximum of 106 after 180 d [43]. To evaluate whether biomagnification occurred, silver-contaminated gammarids and chironomids were provided in laboratory experiments. The resulting transfer factors were below 1 (gammarids, 0.023; chironimids, 0.135), indicating that no biomagnification occurred. However, based on theoretical computations that considered all the seasonal uptake pathways of silver in the carp and assuming that these act simultaneously and additively, direct uptake from water and sediment dominates for only a few days before uptake from food predominates. At optimal prey supply, the silver transfer via the food chain amounted to ˜80% after 60 d. This means that with chronic silver contamination in the sediment, rapid transfer of silver to a secondary consumer via the prey could occur during certain seasons. If this is true, the silver would be redistributed in the ecosystem, and the sediment would act as a permanent source for contaminating the aquatic system. The validity of these remarkable model calculations, however, has not been confirmed.

MARINE ORGANISMS

Bioaccumulation and transfer of contaminants through marine food chains represent a topical research area, because marine organisms are a popular constituent of human food. In particular, estuarine sediments that are highly contaminated by metals represent a potential source of silver accumulation in the total marine food chain. Consequently, bioaccumulation of metals by organisms from these zones has been more intensely studied. The general results obtained for heavy metals, and those obtained for silver, were recently reviewed [40, 49] and thus are covered only briefly here.

Algae

As with freshwater algae, marine algae possess an extraordinary ability to accumulate trace metals and remove these from the seawater. The accumulated substances undergo sedimentation as phytodetritus or as constituents of zooplankton feces and are immobilized in the sediment. Although the accumulation potential among the various metals can differ markedly (BCFs = 0–106), the concentration factors (v/v) for silver range from 104 to slightly beyond 105 (Table 4). Particularly, in the nonessential trace metals, this factor is due to passive adsorption to the cell surface. Consequently, dead cells can also absorb metals. On the other hand, higher BCFs are common in algae that possess a higher surface/volume ratio (e.g., the coccale blue greens). This sort of relationship applies all the way up to the larger zooplankton. In algae, adsorption proceeds rapidly and leads to relatively strong binding of silver to the cell surface, as shown for the central diatom Thalassiosira weissflogii [42]. Once incorporated in the cell wall, the silver remains strongly bound, even after ultrasonic disruption of the cells, washing at pH 2.0, or treatment with digestive enzymes. Therefore, it is improbable that the silver can be detached from the cell wall constituents by digestion in herbivorous invertebrates.

Adsorption, however, is possible only with charged or polar silver compounds. Particularly in seawater, nonpolar chloride complexes are formed that, because of their lipophilic and membrane-penetration ability, are incorporated into the cell lumen. Despite this salinity-dependent path of uptake, as a rule, BCFs decrease with increasing salinity [28]. The silver incorporated into the inner cells has a particularly high affinity to sulfhydryl groups and should thus bind predominantly to proteins [42]. However, because of methodical problems, there are no published investigations of the exact location of the silver in the cells.

Also, macroalgae, such as the seaweed Fucus vesiculosus, have a similar accumulation potential and are considered appropriate indicators for dissolved silver concentrations [50].

HERBIVOROUS AND DETRITIVOROUS ORGANISMS

Pelagic food chain

As in freshwater systems, marine macrozooplankton (mainly euphausids and copepods) can take up trace elements either through the body surface or from ingested food. In estuarine and coastal areas, as well as in the open sea, microzooplankton (predominantly protozoans) are seen as important consumers of the smaller algal species and bacteria. In the estuarine ciliate Fabrea salina, concentration factors between 7 × 103 and 40 × 103 have been observed [51]. The silver concentration stemmed partly (50% in one experiment) from ingested algal cells.

For filter-feeding marine zooplankton, the weight-related concentration factors for silver average approx. 5 × 103 [40]. In one experiment, copepods (Arcatia tonsa), in which silver is a trace metals with low accumulation potential, were fed diatoms and assimilated 17% of the silver from food [49]. Higher values were obtained for oyster larvae (Crassostrea virginica, 33%) and larvae of an other bivalve (Mercenaria mercenaria, 16%). In the crustacean zooplankton, the old exoskeleton (carapace) is detached at regular intervals during molting so that the trace elements, adsorbed to the carapace surface, are removed and undergo sedimentation together with parts of the skeleton (see Crangon crangon [52], Table 4). The herbivorous crustacean plankton thus might cause the interruption of trophic silver transfer.

Benthic food chain

In most cases, the benthic organisms are omnivorous and feed on phytoplankton, detritus, and even small prey. Important representatives are various bivalve species, which belong to the sediment infauna and forage by particle filtration, or the ragworms (polychaetes, Nereis diversicolor). Among the epifauna are snails (e.g., periwinkles, Littorina littorea) and crustacean species, particularly shrimp. These deposit feeders forage on the periphyton (e.g., growing on seaweed) or on precipitated surface materials. Shrimps are omnivorous and feed on virtually anything they can find. The shrimp Crangon crangon was studied using the ultraclean technique and had very high BCFs [52]. Nonetheless, a factor of ˜5 between accumulation on the carapace and in body tissue was determined. In the grass shrimp Palaemonetes pugio, rapid incorporation of dissolved silver from brackish water rather than from plankton (Artemia sp.), or detritus (Victorella sp.), both of which were highly contaminated with silver, was observed [42].

Table Table 4.. Silver concentration and bioconcentration factors in marine organisms (maximum values)
OrganismQuantityParameteraAg compoundTypebReference
  1. a BCF = bioconcentration factor; C = silver concentration; DW = relation to dry weight; V = relation to volume.

  2. b F = field experiment; L = laboratory experiment.

Algae     
Blue greens, filamentous66,000BCF, V L[40]
Blue greens, coccale200,000BCF, V L 
Coccolithophores24,000BCF, V L 
Diatoms34,000BCF, V L 
Dinoflagellates39,000BCF, V L 
Green algae24,000BCF, V L 
Fucus vesiculosus0.066C, DWAgF[50]
 (maximum 7.4)    
 40,000BCF F 
Protozoans     
Fabrea salina7,000–40,000BCF, V110AgClL[51]
Crustaceans     
Zooplankton, general5,000BCF, DW L[40]
Crustaceans, generalUp to 2C, DWAgF[4]
Crangon crangon0.32–1.51C, DWAgF[52]
Carapace11,000BCF, DWAgF 
Tissue24,000BCF, DWAgF 
Mollusks     
General1–10C, DWAgF[4]
Crassostrea virginica1.71C, DWAgF[109]
 Ensis sp.0.011–0.029CAgF [52]
Macoma baltica0.46C CAg AgF F[52] [50]
 (maximum 301)    
Macoma baltica20–130CAgL[110]
 8–65BCFAgL 
Mytilus edulis0.4–0.9CAgL[53]
 40–180BCF   
Mytilus californiana0.29C, DWAgF[109]
Mytilus edulis0.18C, DWAgF[109]
Mytilus edulis0.02C, DWAgF[50]
 (maximum 198)    
Mytilus edulis, soft tissue0.15–1.7CAgF[52]
Mytilus sp.0.0060–0.011 AgF[52]
Scrobicularia plana0.098C, DWAgF[50]
 (maximum 259)    
Littorina littorea0.73C, DWAgF[50]
 (maximum 1.01)    
Polychaetes     
Nereis diversicolor0.1C, DWAgF[50]
 (maximum 36.4)    
Fish     
Lepomis microlophus0.458CAgF[47]
Microstomus pacificus     
Meat     
Noncontaminated site0.1C, DWAgF[57]
Contaminated site0.2    
Gonads     
Noncontaminated site0.3C, DWAgF 
Contaminated site<0.1    
Liver     
Noncontaminated site0.5C, DWAgF 
Contaminated site0.4    
Bones, general6CAgF[10]
Fish, general11C, DWAgF[4]

Periwinkles (L. littorea) exhibited a tissue silver concentration elevated by ˜10-fold compared with their substrate, the periphyton on seaweed [50]. The investigators suggested that silver uptake was from both the periphyton and the dissolved phase in the water. The uptake of silver was inhibited in the presence of copper.

In an interestuarine comparison, nereids (ragworms) showed a good correlation between the silver concentration in tissue and that in the surrounding sediment. Because they can move to zones of low salinity, they are considered indicators of silver for estuarine regions. Clam species (Macoma baltica and Scrobicularia plana) had higher silver concentrations in tissue than found in the surrounding sediment. The silver ratio (tissue/sediment) was the highest observed among heavy metals, suggesting that bioavailability was higher than indicated by the silver concentration in the sediment pore water and that additional silver uptake from filtered particles occurred [50].

Experiments with mussels (Mytilus edulis) revealed that the trace metal accumulation was affected by biotic and abiotic environmental factors [53]. At low salinities, the mussels accumulated more silver from the dissolved phase than at higher salinities, where the silver influx decreased, probably as a result of reduced filtration activity. The assimilation efficiency for silver depended on temperature. At lower temperatures, a larger amount of silver was retained in the tissue (probably because of lower protein degradation). The quantity of ingested food had an inverse effect on silver accumulation. In addition, this was affected by the algae species consumed. In the same species, the calculated dissolved uptake rate constant was greatest for Ag, followed by Zn > Am > Cd > Co > Se [54]. Using a bioenergetic-based kinetic model, investigators could predict mussel tissue concentrations in the field (e.g., San Francisco Bay) quite well from laboratory data [49].

At dissolved silver concentrations of 5 to 10 ng/L, the concentration of silver in the soft tissue of Mytilus sp. was 0.4 to 0.8 mg/kg. The silver assimilation efficiency from the food was found to be relatively low (5–20%). The American oyster (Crassostrea virginica) assimilated 44%; clams (Mercenaria mercenaria), 22 to 35%; and Macoma baltica, 47 to 60%. In oysters (C. virginica and C. gigas), silver was taken up almost exclusively from the dissolved phase in the water column and not from silver-contaminated phytoplankton [42, 55]. In contrast to what was observed in mussels, the effect of salinity on silver accumulation in oysters was only slight [56].

Carnivorous organisms

Bioaccumulation of silver by carnivorous organisms from the marine environment, predominantly seawater fish, has been scarcely investigated [40]. The accumulation of heavy metals (including Ag) was studied in tissues of Dover soles (Microstomus pacificus) near an outfall system strongly influenced by contaminated wastewater and at a remote, uncontaminated area [57]. The observed concentrations in the fish (Table 4) do not support a substantial accumulation of silver. Only a small accumulation of chromium was observed.

TOXICITY

Analytically determined concentrations of silver in soil, water, and air, as well as plant and animal tissues, do not reflect its toxicity. However, the accumulation of silver to high concentrations in tissues gives rise to concern about possible sublethal effects, which can be detected only with great effort. Bivalves seem to accumulate heavy metals to great extent without obvious symptoms of toxicity [53]. Conversely, small accumulated amounts of heavy metals can lead to sublethal or chronic effects. The crucial points here are concentrations in target organs and the specific mode of action (e.g., the blocking of ion transport through membranes in gills of fish [58]). Therefore, the toxicity of a metal must be assessed by experimental means: usually acute, life-cycle, or long-term toxicity tests or, in some situations, tests with populations or communities (model ecosystems). As a parameter of toxicity, an effect threshold is established where statistically significant effects for a given endpoint (e.g., mortality or reproduction) occur. In acute tests, it is common to determine 50% effect levels for mortality (i.e., the lethal concentration for 50% of test organisms [LC50]). In some tests, either the concentration for an effect level or an effect threshold is determined (e.g., the median effective concentration [EC50] for reproduction or the median inhibitory concentration). For an effect threshold, the no-observed-effect concentration (NOEC) or maximum acceptable toxic concentration (MATC) is generally reported. The NOEC concept has been criticized [59] because it depends largely on test circumstances (e.g., replication, concentration spacing, and variability) and may overlook effects. Because there is no consensus on alternatives, NOEC values are presented.

Because of its cationic nature and its strong association with various ligands in natural waters, silver's toxicity depends largely on the presence of substances with which it can form covalent, colloidal, or complex bonds. This area was scarcely investigated [60] until recently. In addition, toxicity tests performed before ultraclean techniques and sensitive analyses were developed may have been subject to errors. Although it cannot be proved for individual cases, chances are that in older studies of effect levels or effect thresholds, actual concentrations in the test medium were not determined accurately (e.g., because of contamination of test items) or in many cases not even attempted.

In a few cases, the nominal silver concentration would not have been identical to the exposure concentration because some silver adsorbs to test vessels or food. A fundamental difficulty is that speciation and bioavailability could have been unintentionally influenced by the test medium; this was hardly taken into account in older studies.

Because toxicity depends on the physiological properties and reactions of the organisms, in this section, findings are discussed according to their biological or systematic position. Terrestrial, marine, and freshwater organisms are discussed separately because of the large differences in their environments.

EFFECTS ON PLANTS AND FUNGI

Microflora

Free silver ions (Ag+) are strongly fungicidal, algicidal, and bactericidal at comparatively low doses [61]. The use of silver preparations in medicine and for sterilizing potable or swimming-pool water is based on the particular sensitivity of bacterial metabolism to Ag+-inhibiting thiole enzymes [62]. Bactericidal Ag+ concentrations of 0.01 to 1.0 mg/L are far below dangerous thresholds for humans [63]. In the presence of oxygen, metallic silver is also bactericidal because the resulting silver oxide is sufficiently soluble to release free Ag+. The effect of different heavy metals, among them Ag+ (as Ag2SO4), on heterotrophic activity was measured using radiolabeled glucose [2]. Concentrations of 1 μg/kg Ag+ killed bacteria. The order of toxicity was as follows: Ag+ ≫ Cu2+, Ni2+ > Ba2+, Cr3+, Hg2+ > Zn2+, Pb2+, Na+, Cd2+, As2O. It could be demonstrated that Ag+ inhibits the enzymes for the P, S, and N cycles of nitrifying bacteria: 39% for phosphatase at 10 μmol/g Ag+, 96% for arylsulfatase at 25 μmol/g, and 93% for urease at 5 μmol/g [64]. A number of studies were carried out to check the inhibition of microbial activity in sewage sludge. It must be considered here that toxic effects on microbiological parameters of these biocoenoses are difficult to interpret, because selection of populations and shifts in species composition occur [64]. This adaptability is crucial for the ability to biologically decompose contaminated wastewaters after a period of adaptation. In studies on the effects of photographic wastewaters on sludge respiration, toxicity depended largely on solubility [39].

Table Table 5.. Toxicity parameters obtained for freshwater and marine algae
Species (test criterion)CompoundQuantity (mg/L)Toxicity parameteraReference
  1. a EC50 = median effective concentration; NOEC = no-observed-effect concentration (NOEC + 10% difference from control value either inhibited or enhanced growth or germination).

  2. b Highest concentration tested.

Freshwater algae    
Selenastrum capricornutum (cell no.)AgNO3, AgSO4>0.125EC50[67]
Green algae    
MicrocosmNaAgS2O35NOEC[39]
PondNaAgS2O31bNOEC[39]
Selenastrum capricornutum (cell no., 7 d)NaAgS2O310NOEC[39]
Marine algae    
Gymnodium sp. (cell no., 2 d)Ag+0.002–0.010NOEC[111]

In photographic wastewaters, silver is present as a thiosulfate complex, which in sludge is transformed to insoluble silver sulfide. Neither silver thiosulfate nor silver sulfide show the same inhibitory effects as free silver ions [23, 61, 65]. These results clearly allow the conclusions that the biocoenosis of microorganisms is hardly affected by photographic wastewater and that biological sewage treatment of these wastewaters is possible. The algicidal effects of heavy metals, especially Ag+, are well known and documented [4]. A reduction of growth rates in laboratory cultures of different freshwater algae (Chlamydomonas eugametos, Chlorella vulgaris, Haematococcus capensis, and Scenedesmus accuminata) occurred at silver concentrations of 0.01 mg/L in samples from various lakes [66]. The strain of C. vulgaris stopped growing at 0.05 mg/L Ag+. The investigators stressed the particular advantages of such biotests, which are now the only means to determine bioavailability and toxic potential in environmental samples. Again, the results must be viewed with caution because the study is rather old.

Under certain circumstances, low metal concentrations promote algal growth, whereas high ones inhibit it [67]. In chronic toxicity tests with Selenastrum capricornutum, Ag2SO4 and AgNO3 promoted growth up to 0.094 mg/L (300 nM). The EC50 value for both silver salts was above 0.125 mg/L.

Generally, the presence of complexing agents (e.g., ethylenediaminetetra-acetic acid [EDTA]) or water hardness (e.g., Ca2+) influences the toxicity of metals. Either the bioavailability is lowered by complexing agents and ions, or other ions compete with metals for binding sites on the cell surface. In algal growth inhibition tests with cadmium, cobalt, manganese, and nickel in the presence and absence of calcium, the metals always inhibited growth, but the presence of Ca2+ decreased this effect in all cases. [68]. Table 5 shows recent data from toxicity tests with silver.

Higher plants

The sensitivity of terrestrial plant species exposed to silver varies [4]. Toxic effects of solutions of silver nitrate to seeds of barley, wheat, peas, and crucifers are known from several investigations. Recent results collected following American Society for Testing and Materials standards are listed in Table 6 [39]. In sensitive plant species, growth and germination were impaired at 7.5 mg/L silver nitrate. Highly soluble silver thiophosphate and silver nitrate were markedly more toxic than the nearly insoluble Ag2S and AgCl.

Research with lettuce, oats, turnips, and soybeans grown in sewage sludge spiked with photographic silver waste (5.2 and 120 mg Ag/kg dry weight; control, 0.25 mg/kg) revealed no effect on emergence time and rate [32]. The mean fresh weight of lettuce leaves was significantly higher than in the control, but the dry weight of the plants did not differ. Silver content in the sludge had little effect (1%) on the length of oat sprouts. The fertilizing effect of the sludge was not reduced by silver.

TERRESTRIAL ANIMALS

Invertebrates

For the terrestrial environment, earthworms (L. terrestris) are considered suitable test organisms for soil contamination. Their survival, growth, and bioaccumulation of silver were tested using artificial soil containing increasing concentrations of Ag2S for 28 d [39]. Interestingly, the worms did not accumulate silver after 28 d of exposure but responded with reduced growth (NOEC, 62 mg Ag/kg). The accumulation period of 14 d and the exposure of 28 d to the artificial soil was considered extremely stressful by the investigators. This reduced growth was suggested to be due to contact toxicity, which is independent of bioaccumulation, but has received little attention. In soil (and sediment) exposure via pore water or via direct contact with the particulate fraction by dermal tissues is possible. In another study [69], L. terrestris were exposed to 2,000 mg Ag2S/kg soil for 14 d, and no adverse effects were observed. Hence, additional experiments with this important organism are needed.

Table Table 6.. Toxic effect thresholds for silver in terrestrial plantsa
SpeciesEndpointAg compoundNOECb (mg/L)
  1. a Data from [39].

  2. b NOEC = no-observed-effect concentration.

  3. c Highest concentration tested.

Lactuca sativa (lettuce)Germination (7 d)Ag2S771c
 Growth (7 d)Ag2S771c
 Germination (7 d)AgCl750c
 Growth (7 d)AgCl750c
 Germination (7 d)NaAgS2O100
 Growth (7 d)NaAgS2O310
 Germination (7 d)AgNO30.75
 Growth (7 d)AgNO375
Lolium perenne (ryegrass)Germination (7 d)Ag2S771c
 Germination (7 d)AgCl750c
 Germination (7 d)NaAgS2O3100
 Germination (7 d)AgNO375
Rhaphanus sativus (radish)Germination (7 d)Ag2S771c
 Growth (7 d)Ag2S771c
 Germination (7 d)AgCl7.5
 Germination (7 d)NaAgS2O31,000c
 Growth (7 d)NaAgS2O3100c
 Germination (7 d)AgNO37.5
 Growth (7 d)AgNO37.5
Tagetes patula (marigold)Growth (7 d)Ag2S771c
 Growth (7 d)AgCl75c
 Growth (7 d)NaAgS2O3100c
 Growth (7 d)AgNO375c
Zea mays (maize)Growth (7 d)Ag2S771c
 Growth (7 d)AgCl75c
 Growth (7 d)NaAgS2O310
 Growth (7 d)AgNO37.5

Vertebrates

Silver toxicity in terrestrial animals has been investigated mainly in laboratory bioassays [70–72]. Oral exposure to high doses of silver in drinking water, administered as colloids or silver nitrate, killed rats and guinea pigs (Table 7). Argyrosis, a bluish gray coloration of the skin that the U.S. Environmental Protection Agency (EPA) considers a cosmetic effect, is often observed after chronic intake of small doses of metallic silver. Argyrosis results from silver granules distributed over the whole skin but concentrated in basal membranes and elastic fibers of transpiratory glands. Argyria has been observed mainly in humans but has also occurred in rats exposed to silver nitrate or silver chloride in drinking water or to conditions generally leading to silver deposits in different tissues [72]. In mice exposed to silver nitrate, granules were deposited in the brain and resulted in decreased neural activity as compared to untreated controls [73].

FRESHWATER ANIMALS

Invertebrates

Bioavailability and toxicity of silver for inhabitants of limnic sediments depend strongly on very complex sediment properties [74]. Toxicity is modified by pH, organic carbon, cation exchange capacity, and the amounts of silt and clay. These factors determine the concentration of Ag+ in pore water and overlying water immediately above sediment, which can be regarded as the main exposure route for epifauna and infauna. In experiments with the epifaunal amphipod Hyalella azteca, four sediments differing in these properties were enriched with silver nitrate (2,500 mg/L AgNO3 solution). The 10-d LC50 values were between 1.6 and 397.7 mg/kg Ag dry weight. Hyalella azteca was not adversely affected by any test concentration of AgCl, and the 10-d LC50 value was <2,560 mg/L Ag dry weight. Hyalella azteca was rather insensitive to silver thiosulfate complex, with a 10-d LC50 value of >569 mg/kg Ag dry weight [74], and to silver sulfide, with an LC50 value of >753.3 mg/kg [75]. Toxicity differed because bioavailability was orders of magnitude different as a result of the various properties of the sediments and the silver chemicals used.

Table Table 7.. Summary of toxic effects of silver and silver compounds to mammals
Species (test condition)Compound, exposure routeEffect/threshold dose (mg/kg · d−1 NOAEL)aReference
  1. a DW = dry weight; NOAEL = no-observed-adverse-effect level.

Lethal effects   
Rat (acute exposure, 2 wk, 7 d/wk)AgNO3, oral181.2 DW[70]
Rat (acute exposure, 4 d, 1 time/d)??, oral1,680[71]
Guinea pig (8 wk, 7 d/wk)AgNO3, dermal137.13[112]
Systemic effects   
Rat (37 wk, 7 d/wk; weight loss)AgNO3, AgCl, oral222.2 DW[72]
Guinea pig (8 wk, 7 d/wk; weight loss)AgNO3, dermal137.13[112]
Neuronal effects   
Mouse (37 wk, 7 d/wk; hypoactivity)AgNO3, AgCl, oral18.1 DW[73]

The toxicity of silver to sediment organisms differs strongly with benthic species and test method used. For example, Chironomus sp. larvae were much less sensitive to silver nitrate than Hyalella sp. in 10-d laboratory tests, whereas an NOEC value of 1.0 mg/L as silver thiosulfate (the highest concentration tested) was reported for Chironomus sp. in a pond study [39]. Comparison of species-specific sensitivities is hampered by the few tests performed, by use of different silver compounds, and by the fact that tests with water–sediment systems have only recently been standardized.

Markedly more studies are available for planktonic crustaceans, such as the water flea Daphnia magna [39, 60, 76]. Table 8 clearly demonstrates again that toxicity of silver compounds depends largely on solubility and the formation of free silver ions. Silver nitrate is highly toxic, resulting in LC50s in the low microgram-per-liter range, followed by silver sulfate (also quite soluble), and is less toxic by one order of magnitude. The main waste of photoprocessors, silver thiosulfate, has received little investigative attention but seems to be far less toxic than the complexed silver ion, which has been shown to be the most toxic form of silver.

Water hardness has been reported to modify the toxicity of silver ions [76] (Table 8). In contrast, water hardness has little influence on acute Ag+ toxicity, whereas DOC and chloride ions have the greatest effects [60, 77]. Silver was bound to DOC and kept in solution but could not pass through the 0.45-μm filter (i.e., it was not bioavailable). When 20 μg/L silver nitrate was added, Cl and DOC reduced its solubility similarly. At 40 μg/L silver nitrate, the effect of chloride increased. La Point et al. [78] conducted a partial correlation analysis between survivorship, DOC, chloride, and hardness for toxicity test results with D. magna and Pimephales promelas. They could attribute little or no influence of hardness and chloride on silver toxicity. In contrast to the above findings, DOC did ameliorate the effect of silver, but the response was barely statistically significant. These first results on modification of toxicity by water chemistry and the contradictory findings must be clarified in future investigations.

Vertebrates

According to the literature, freshwater fish and amphibians are the most sensitive vertebrates to dissolved silver, although relatively tolerant species exist. The greatest toxicity reported was observed for silver nitrate, which, according to the MINEQL model in freshwater, is present as 60% AgCl0, 34% Ag+, and 5% AgCl2 [79, 80]. The leopard frog, Rana pipiens, with an LC50 of 10 μg/L silver, was among the more sensitive amphibians. The most sensitive fish were even less tolerant according to recent investigations, with LC50s between 2.5 and 10 μg/L. These acute LC50s for silver nitrate were surpassed by chronic NOEC values and MATC values. For example, with fathead minnows, MATC values for AgNO3 were between 0.4 and 0.7 μg/L [39]. The MATC value (at that time called long-term NOEC) for juvenile rainbow trout (Salmo gairdneri) exposed to silver nitrate was between 0.09 and 0.17 μg/L (18-month exposure) [81]. These values do not reflect potential effects on spawning behavior or reproduction of adult females. Silver nitrate concentrations ≥0.17 μg/L resulted in premature hatching and 15% reduced growth of fry. At one exposure concentration, hatching was completed within 10 d under silver nitrate exposure, whereas controls took 42 d to hatch. The prematurely hatched fry were not completely developed and often dead, and growth of surviving fry was strongly reduced. Insoluble or complexed silver compounds were far less toxic or virtually nontoxic (Table 9 [39]); silver thiosulfate was found to be 15,000 times and AgCln 11,000 times less toxic than silver nitrate [80]. Particulate AgCl is virtually nontoxic. On average, LC50s were 7.6 times higher in 28-d-old fathead minnow exposed to silver nitrate compared to 4-d-old fish [82].

The influence of water chemistry on metal toxicity in fish is currently under systematic investigation. The toxicity of silver compounds was found to be markedly reduced in natural river water as compared with the laboratory test medium [60]. Increasing water hardness decreased the toxicity of silver nitrate (Table 9) [60, 83]. The LC50 at pH 8.6 was three times higher than at pH 7.2. Because of the high affinity of silver for dissolved organic matter, adding humic acid (thus increasing organic carbon) greatly reduced toxicity [60, 82]. These studies sustain the assumption that a large number of differences in test results may be explained by different test conditions. This assumption was supported by the revision of a 1989 EPA ring test by Hogstrand et al. [80]. Median lethal concentration values varied between 11.8 and 280 μg/L, but Hogstrand et al.'s analysis of correlation showed a significant dependence of LC50s on the concentration of Cl (r = 0.981, p < 0.01). Additional experiments demonstrated that the 96-h LC50 value did not change with increasing Cl concentration if the silver concentration was based on the free Ag+ ion rather than total silver [58]. Reduction of silver toxicity by raising chloride concentrations is extremely important in marine environments and is discussed later.

The high sensitivity of rainbow trout (Oncorhynchus mykiss) to silver has led to several studies of the mechanisms of toxicity [83–85]. The toxicity mechanisms found explain the effects of water chemistry on silver toxicity. The primary mechanism is an almost total inhibition of active Na+ and Cl uptake by the gills. It is notable that the effects are not caused by bioaccumulation of silver—in fact, accumulation was four times higher with thiosulfate—but by a “surface effect” on the gills, which contain negatively charged binding sites of phosphate, carboxyl amino, and sulfate groups. Metals and other cations (e.g., Ca2+) are attracted and competitively (e.g., Ag+ and Cu2+) bound. Complex binding with ligands like thiosulfate, DOC, and Cl reduce the concentration of free reactive silver ions. The influence of silver speciation and water hardness on silver toxicity can thus largely be explained by complex formation and competition [86, 87]. Binding of Ag+ to gills of rainbow trout is affected by the cations Ca2+, Na+ and H+, as well as by complexing agents such as DOC, chloride ions, and thiosulfate ions. Cations interacting with the gillbinding sites and complexing agents protect the gills from Ag+ and thus exert a cumulative protection against Ag+ accumulation [88]. The general suggestion from these results and from reanalysis of published data was that silver toxicity can be correlated with the free silver ion (Ag+) and that factors altering Ag+ availability can be expected to modify acute silver toxicity.

Table Table 8.. Effect and threshold concentrations for various silver compounds and test conditions observed for freshwater invertebrates
Species (test conditions)CompoundEffect/threshold concn.Toxicity parameteraReference
  1. a EC50 = median effective concentration; LC50 = median lethal concentration; NOEC = no-observed-effect concentration.

  2. b No toxicity reported at highest concentration tested.

  3. c Estimated from a graph.

Turbellaria (flatworms)    
Dugesia dorotocephala    
Mortality or immobility, 96 hAg2S>1,000bLC50[39]
 AgNO30.03LC50 
 NaAgS2O3>1.3bEC50 
Oligochaetes    
Lumbriculus variegatus    
Mortality, 96 hAg2S>1,000bLC50[39]
Mollusks (snails)    
Planorbella trivolis    
Mortality or immobility, 96 hAg2S>1,000bLC50[39]
 AgNO30.3LC50 
 NaAgS2O3>1.3bLC50 
Aplexa hypnorum    
Mortality or immobility, 96 hAgNO30.4LC50[113]
Mollusks (bivalves)    
Linguinia and Margaritifera spp.    
Microcosm, 10 wkNaAgS2O3 5 NOEC[39]
Crustaceans    
Hyalella azteca    
Ag2S-spiked sediment, 10 dAg2S753.3 mg/kgbLC50[75]
AgNO3-spiked sediment, 10 dAgNO31.6–397.7LC50[74]
Ceriodaphnia dubia    
ReproductionAgNO30.0045NOEC[114]
Daphnia magna    
Unaged water, fed, 48 hAgNO3˜0.008cLC50[60]
Unaged water, not fed, 48 h3 AgNO3˜0.0005cLC50 
Aged water, fed, 48 hAgNO3˜0.011cLC50 
Aged water, not fed, 48 hAgNO3˜0.0015cLC50 
River water, 48 hAgNO3˜0.035cLC50 
Immobility, 48 hAg2S>1,000bEC50[39]
Mortality, 96 hAg2SO40.02LC50 
Mortality, 96 hAgNO30.005LC50 
 NaAgS2O3>1.33bLC50 
Starved, 48 hAgNO30.0009LC50[76]
Fed, 48 hAgNO30.0125LC50 
Reproduction, 21 dAgNO3 EC50 
 + 60 mg/L CaCO30.0029  
 + 75 mg/L CaCO30.0036  
 + 180 mg/L CaCO30.0039  
Caecidotea intermedia    
Mortality, 96 hAg2S>1,000bLC50[39]
Gammarus fasciatus    
Mortality, 96 hAg2S>1,000bLC50[39]
Insects    
Chironomus tentansAgNO30.063 (total)LC50[115]
Third instar larvae, water-only exposure, 10 d 0.057 (dissolved)  
  0.035 (free ion) 
Chironomus tentans    
Emergence, 10 dAgNO3259EC50[116]
Chironomus sp.    
Pond study, 58 dNaAgS2O3>1bNOEC[39]
Notonecta sp.    
Pond study, 58 dAg2S>1.0bLC50 

MARINE ANIMALS

Invertebrates

For marine invertebrates, only a few systematic investigations conforming with standardized methods with silver toxicity have been published. Thus, toxicity results are rarely directly comparable. In comparable biotests, silver (as Ag+), copper, and mercury are the three most toxic metals for marine and estuarine invertebrates [89, 90]. Juvenile bivalves appear to be particularly sensitive to ionic silver. Toxicity ranged from <1 to 14 μg/L Ag+ (Table 10). Several examples of toxicity to marine invertebrates have been reported [91]; for example, 400 μg/L killed 90% of tested Balanus balanoides (barnacles) within 48 h, and 10 to 100 μg/L AgNO3 caused abnormal or delayed development in eggs of sea urchin (Paracentrotus sp.). The effect threshold for development of sea urchin (Arbacia sp.) was 0.5 μg/L.

In larvae of the American oyster C. virginica and the bivalve M. mercenaria, the toxicity rankings were as follows: Hg > Ag > Cu > Ni and Hg > Cu > Ag > Zn > Ni, respectively [92]. Adult bivalves seem to be able to bind silver to sulfide-rich proteins [93, 94] and to enclose it in sulfide-rich granules and basal membranes of cells [95], probably detoxifying it. However, such silver accumulations might be considered indications of physiological damage, which can lead to premature release of germ cells [95], reduction in number of offspring [96], reduced storage of glycogen (which is necessary for egg production) [9], and reduced growth [97].

Vertebrates

Tests with marine vertebrates have been performed exclusively with fish. Considering the much larger number of investigations conducted on freshwater fish, and the moderating action of increasing chloride concentration, silver toxicity would be expected to be lower for marine fish than for freshwater fish. Model calculations with MINEQL+ confirm this and predict the presence of ionic silver (the toxic form) to decrease with increasing chloride concentration [98]. Thus, for marine chloride concentrations, the amount of free Ag ions is negligible. Increasing salinity, as in the course of an estuarine passage, exerts a shift of dominant AgCl(n) from Ag+ and noncharged AgCl(aq) to higher charged silver chloride species with altered toxic properties.

Anadromous rainbow trout (O. mykiss) adapted to brackish water showed markedly lower sensitivity to silver than they did in freshwater [98] (Table 10). Tidepool sculpins (Oligocottus maculosus) were equally sensitive; here, a further increase in salinity decreased toxicity. These results can be explained only by the decrease in the free Ag+ ion concentration due to formation of silver chloride complexes with increasing salinity, which reduced toxicity.

With the exception of larvae and embryos of the flounder Paralichthys dentatus, whose sensitivity equals that of the most sensitive marine invertebrates (Table 10), silver seems to be less toxic to juvenile and adult fish in seawater than in freshwater. The LC50s for marine fish are one to two orders of magnitude above those of freshwater fish. With euryhaline fish like tidepool sculpins, Ag+ toxicity decreases with increasing salinity. The mechanisms of toxicity in rainbow trout (O. mykiss) adapted to seawater are currently under investigation, and the findings obtained to date were recently reviewed [83, 84]. In contrast to findings in freshwater fish, exposing fish to sublethal concentrations up to 250 μg Ag+/L did not disturb Na+ and Cl plasma concentrations, indicating intact transport of both ions through gill membranes. Among some tested parameters in marine fish, only plasma ammonium concentration increased temporarily in the presence of silver. Also, some evidence suggests that elevated ammonium concentrations may be linked to silver toxicity [99]. Even though the mechanisms are not completely clear, current findings indicate that the mechanisms of acute toxicity in seawater are completely different from those in freshwater. Thus, extrapolation of toxicity results from freshwater to seawater is impractical.

CONCLUSIONS

It is well documented that the toxicity of silver in the aqueous environment depends on the concentration of active, free silver ions. Accordingly, many water characteristics reduce silver toxicity, reducing the availability of free silver ions by binding free silver ions. Also, competing cations (e.g., Ca2+) prevent binding of free silver ions to the reactive surfaces of organisms. Solubility of the silver compound and the presence of complexing agents (e.g., thiosulfate or chloride), DOC, and competing ions are all important. In earlier studies (before 1990), these conditions received little attention and were not monitored during tests, so effect thresholds and concentrations may be difficult to interpret.

The high adsorptive potential of silver to some materials and the difficulty of detecting low silver concentrations pose problems in toxicity tests. It is conceivable that use of the nominal concentrations found in earlier studies may lead to NOEC or ECx values that are too high, because silver was adsorbed to test vessels or food and thus not biologically available. Current efforts are made to obtain scientifically sound toxicity data by applying ultraclean techniques not only during the analytical determination but also during the toxicity test [60, 77, 82, 96, 100]. Nonetheless, statements about the relative toxicity of silver compounds can be made on the basis of the earlier data. Silver sulfide is the least toxic of all tested silver compounds because of its low solubility and bioavailability. In soil, sewage sludge, and sediments, in which silver sulfide predominates, the toxicity of silver, even at high total concentrations, is very low. A second route of exposure, via the particulate phase (which can lead to contact toxicity), needs to be considered. Toxicity results in earthworms, which did not accumulate silver, have been mixed. Thus, the significance of the particulate exposure route for silver has not been fully evaluated.

Silver thiosulfate, a highly soluble compound and main component of wastewaters of photoprocessors, has a very low toxicity (e.g., it is 15,000–17,000 times less toxic than silver nitrate). This can be attributed to the silver complexed by thiosulfate, which reduces the bioavailability of free silver ions. Silver nitrate is the most toxic silver compound. The toxic potential of silver chloride complexes in seawater is and will be an important issue for investigation.

Most toxicity data originate from short-term toxicity tests. Chronic tests, long-term tests, and tests including sensitive life stages show lower toxicity thresholds. The organisms viewed as most sensitive to silver are small invertebrates (e.g., daphnids), particularly embryonic and larval stages. Long-term studies, taking into account fieldlike exposure scenarios, are recommended.

Table Table 9.. Effect and threshold concentrations for various silver compounds and test conditions as observed for freshwater vertebrates
Species (test conditions)CompoundEffect/threshold concn.Toxicity parameteraReference
  1. a LC50 = median lethal concentration; MATC = maximum acceptable toxic concentration; NOEC = no-observed-effect concentration.

  2. b Estimated from a graph.

  3. c DOC = dissolved organic carbon.

  4. d Mean of two experiments; hardness = 26–42 mg/L (as CaCO3).

  5. e No toxicity reported at highest concentration tested.

  6. f Hardness = ˜130 mg/L (as CaCO3).

  7. g Median lethal concentration exceeded solubility of compound.

  8. h Mean of three experiments.

Fishes    
Anguilla anguilla    
European eel, 24 h, staticAgNO30.75LC50 (salt water)[117]
  0.1LC50 (freshwater) 
Carassius auratus    
Embryo-larvae, >6 dAgNO30.02LC50[114]
Ictalurus punctatus    
Embryo-larvae, >6 dAgNO30.01LC50 
Micropterus salmonides    
Embryo-larvae, >6 dAgNO30.11LC50 
Oncorhynchus mykiss    
Embryo-larvae, >6 dAgNO30.01LC50 
168 h, staticAg+0.0032LC50[58]
96 h, staticAgNO30.012LC50[80]
168 h, staticAgNO30.0091  
96 h, staticNaAgS2O3161LC50 
168 h, staticNaAgS2O3137  
168 h, staticAgCl>100LC50 
Pimephales promelas (96 h, static)    
48 mg/L CaCO3AgNO3˜5bLC50 
130 mg/L CaCO3AgNO3˜8bLC50[60]
249 mg/L CaCO3AgNO3˜12bLC50 
pH 7.173 AgNO3˜3bLC50 
pH 7.65AgNO3˜5.5bLC50 
pH 8.583 AgNO3˜8.5bLC50 
˜1 mg C/LAgNO3˜5bLC50 
˜3.5 mg C/L3 AgNO3˜15bLC50 
˜11 mg C/LAgNO3˜19bLC50 
River waterAgNO3106LC50 
Pimephales promelas    
24 d old, 96 h, static    
3–60 mg/L chloride, 0 mg/L DOCcAgNO30.0196–0.0243LC50[82]
3–60 mg/L chloride, 5 mg/L DOCAgNO30.0218–0.0262LC50 
3–60 mg/L chloride, 10 mg/L DOCAgNO30.0260–0.338LC50 
 AgNO30.010NOEC 
Pimephales promelas (96 h, static)Ag+  [83]
50 mg/L CaCO3 0.005LC50 
250 mg/L CaCO3 0.013  
3 pH 7.2 0.0025  
pH 8.6 0.008  
4 mg C/L 0.005  
11 mg C/L 0.019  
Pimephales promelas    
96 h, flow-throughAgNO30.0092dLC50[76]
96 h, static, aeratedAgNO30.0065dLC50 
96 h, staticAg2S>1,000eLC50[39]f
96 h, flow-throughAg2S>240eLC50 
30-d-old embryo-larvae, flow-throughAg2S>11eMATC[39, 118]
12 wk, flow-throughAg2S800eNOEC[39]
96 h, staticAgI LC50 
96 h, staticAgSCN0.15LC50 
96 h, staticAgO2CCOCH30.11LC50 
96 h, flow-throughAgNO3/XSNaCl>4.6gLC50 
96 h, staticAg3PO40.28LC50 
96 h, staticAg3AsO40.23LC50 
96 h, staticAgVO3 0.17   
96 h, staticAg2CO30.12LC50 
96 h, staticAgCNO0.23LC50 
96 h, staticAgC6H5CO20.28LC50 
96 h, staticAg2SO40.02LC50 
96 h, staticAgO2CCOH30.02LC50 
96 h, staticAgNO30.02–0.11LC50 
96 h, flow-throughNaAgS2O3>28eLC50 
10-wk, flow-throughNaAgS2O35eNOEC 
30-d-old embryo-larvae, flow-throughNaAgS2O316 < MATC < 35MATC[39, 118]
96 h, staticAgF3CCO20.08LC50[39]
96 h, flow-throughAgNO30.016LC50 
30 d, flow-throughAgNO30.0004 < MATC < 0.0007MATC 
96 h, flow-throughAg2S, dispersion240eLC50[118]
30 d, flow-throughAg2S, dispersion>11eMATC 
96 h, flow-throughAgCl complexes>4.6LC50 
96 h, flow-throughAgNO30.016LC50 
96 h, flow-throughNaAgS2O3280–360LC50 
Pimephales promelas    
96 h, staticAgNO30.014LC50[113]
96 h, flow-throughAgNO30.0067LC50 
Salmo gairdneri (rainbow trout)    
96 h, flow-throughAgNO30.0092dLC50[76]
96 h, static, aeratedAgNO30.079dLC50 
96 h, static, nonaeratedAgNO30.0098dLC50 
Salmo gairdneri (steelhead trout)    
96 h, flow-throughAgNO30.0092LC50[76]
Ictalurus punctatus    
96 h, flow-throughAgNO30.017LC50[113]
Salmo gairdneri    
96 h, flow-through, hardness 25 mg/L CaCO3AgNO30.0065hLC50[81]
96 h, flow-through, hardness 350 mg/L CaCO3AgNO30.013LC50 
Salmo spp. (salmon species)Ag+0.0035LC50[23]
Gasterosteus aculeatus (stickleback)Ag+0.003LC50[23]
Poecilia reticulata (guppy)Ag+0.004LC50[23]
 NaAgS2O3 + AgBr>100:3.75LC50[61]
Amphibians    
A. opacumAgNO30.24LC50[114]
B. fowleriAgNO30.23LC50 
Gastrophryne caroliensisAgNO30.01LC50 
Rana catesbeianaAgNO30.02LC50 
Rana palustrisAgNO30.01LC50 
Rana pipiensAgNO30.01LC50 

In natural waters, ionic silver and some silver complexes are readily adsorbed to particulate matter. As a rule, >25% of the total silver measured in natural waters is dissolved as ion, colloid, and complex. The most recent measurements in rivers, lakes, and estuaries using clean techniques revealed background levels of ˜10 ng Ag/L for pristine, nonpolluted areas, whereas levels of ˜10 to 100 ng/L were found for urban and industrialized areas [101, 102]. If one assumes that only 25% of the measured total silver is biologically effective (which is considered to be a maximum), the toxic concentration of the silver ion appears to be very small (2.5–25 ng/L) and is markedly below the lowest toxic thresholds reported (EC50 for green algae, 125,000 ng/L; LC50 for the waterflea, 900 ng/L, worst-case scenario; MATC for fish, ˜450 ng/L, corresponding to a long-term NOEC).

Cobb et al. [77], however, pointed out that low silver concentrations do not give rise to concern for acute effects but that chronic, sublethal exposure of biota to low ionic silver concentrations could lead to accumulations of silver in various compartments of aquatic ecosystems, which, for long-lived organisms, may lead to toxic body burdens. However, there are too few results from chronic bioassays and long-term studies to judge whether these concerns are justified.

Laboratory experiments with silver-amended sediments from freshwater did not provide evidence of toxicity. The amount of silver released to the pore water was shown to be controlled by the acid-volatile sulfide, and concentrations did not cause any concern [103, 104]. However, model calculations revealed that substantial amounts of silver could be transferred from sediment-dwelling prey exposed to low silver concentrations to their predators (secondary consumers) during certain seasons [43]. Given this, sediments containing silver may act as a permanent source of silver contamination for the food web. This area needs further evaluation.

Table Table 10.. Effect and threshold concentrations for various silver compounds and test conditions observed for marine animals
Species (test conditions)CompoundEffect/threshold concn.Toxicity parameteraReference
  1. a EC50 = median effective concentration; LC50 = median lethal concentration; NOEC = no-observed-effect concentration.

  2. b Adapted to brackish waters.

Echinoderms    
Arbacia lixula (embryo)Ag+0.0005Threshold[119]
Mollusks (snails)    
Ilyanassa obsoleta    
Embryonic developmentAg+<0.001Threshold[120]
Crepidula fornicata    
Larvae release, 24 moAg+0.010Threshold[96]
Mollusks (bivalves)    
Spisula solidissima    
Embryo, 1-h exposureAg+0.014EC50[121]
Gametes, 45 min 0.006  
Crassostrea gigas   [122]
Embryo, larvaeAg+0.014Threshold[123]
Mytilus edulis    
Embryo, 72 hAg+<0.004Threshold[123]
Crassostrea virginica    
LarvaeAgNO30.025LC50[92]
Embryo, 48 hAg+0.006LC50 
Mercenaria mercenaria    
LarvaeAgNO30.032LC50 
Crustaceans    
Arcatia clausii (copepod)    
AdultAg+0.013ThresholdLussier and Cardin, unpublished data
Homarus americanus    
Adult, stress enzymesAg+0.006Threshold[92]
Fishes    
Oncorhynchus mykissb    
96 h, acute, 25‰salinityAgNO30.108NOEC[98]
96 h, acute, 25‰salinityAgNO30.402LC50 
Oligocottus maculosus    
96 h, acute, 25‰salinityAgNO30.331LC50[99]
32‰salinityAgNO30.664LC50 
32‰salinity,    
70.6 mg/L ammoniaAgNO3107.6LC50 
  168 h, acute, 25‰salinity AgNO30.119 LC50 
32‰salinityAgNO3 0.472LC50  
Paralichtys dentatus    
Embryo, larvaeAg+0.005–0.008Threshold[9]
Flounders 0.0047Threshold[107]
Cyprinodon variegatus    
(sheepshead minnow) 1.17LC50[107]

In marine sediments, silver was bioavailable and underwent high bioaccumulation. For coastal and estuarine areas, this was considered very serious [9]. The discharge of wastewaters into these areas increased the silver concentration by a factor of 100 to 200 above the background level (0.1–0.3 ng/L) and led to substantial accumulation in the sediment. The formation of stable chlorocomplexes with chlorine favor the distribution and accumulation of silver. The strong bioaccumulation of silver in marine benthic organisms is the main cause of concern. Although not observed, there may be potential for long-term sublethal effects (e.g., on reproduction) in these organisms. The larval instars of these organisms have been found to be as sensitive to ionic silver as water fleas. At this time, it cannot be determined whether these concerns are justified.

The highly toxic potential of ionic silver would not be expected under natural environmental conditions because silver can be readily transferred into biologically nonreactive compounds. Sulfides, dissolved and particulate organic matter, chloride, and enzymes within the biota have all been shown to reduce the toxicity of ionic silver.

Acknowledgements

This review was supported by the German Photochemical Industry Association e. V. I thank I. Boie, B. Goffart, and R. Hubberts for help in the literature search; and A. Weyers, J. Gorsuch, and S. Klaine for critically reading and editing the manuscript and checking the English.

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