Applications of cyanobacteria in biotechnology

Authors


Raeid M. M. Abed, Biology Department, College of Science, Sultan Qaboos University, PO Box 36, AL Khoud 123, Muscat, Sultanate of Oman. E-mail: rabed@mpi-bremen.de

Summary

Cyanobacteria have gained a lot of attention in recent years because of their potential applications in biotechnology. We present an overview of the literature describing the uses of cyanobacteria in industry and services sectors and provide an outlook on the challenges and future prospects of the field of cyanobacterial biotechnology. Cyanobacteria have been identified as a rich source of biologically active compounds with antiviral, antibacterial, antifungal and anticancer activities. Several strains of cyanobacteria were found to accumulate polyhydroxyalkanoates, which can be used as a substitute for nonbiodegradable petrochemical-based plastics. Recent studies showed that oil-polluted sites are rich in cyanobacterial consortia capable of degrading oil components. Cyanobacteria within these consortia facilitated the degradation processes by providing the associated oil-degrading bacteria with the necessary oxygen, organics and fixed nitrogen. Cyanobacterial hydrogen has been considered as a very promising source of alternative energy, and has now been made commercially available. In addition to these applications, cyanobacteria are also used in aquaculture, wastewater treatment, food, fertilizers, production of secondary metabolites including exopolysaccharides, vitamins, toxins, enzymes and pharmaceuticals. Future research should focus on isolating new cyanobacterial strains producing high value products and genetically modifying existing strains to ensure maximum production of the desired products. Metagenomic libraries should be constructed to discover new functional genes that are involved in the biosynthesis of biotechnological relevant compounds. Large-scale industrial production of the cyanobacterial products requires optimization of incubation conditions and fermenter designs in order to increase productivity.

Introduction

Cyanobacteria are prokaryotic oxygenic phototrophs found in almost every conceivable habitat on earth (Ferris et al. 1996; Ward et al. 1997; Nübel et al. 1999, 2000; Abed and Garcia-Pichel 2001; Garcia-Pichel and Pringault 2001). They exist in different morphologies including unicellular and filamentous forms (Castenholz 2001). While unicellular types exist as single cells, suspended or benthic, or aggregates, filamentous types may be thin or thick, single trichome or bundles either with or without a sheath. Cyanobacteria are able to perform different modes of metabolism with the capacity to switch from one mode to another (Stal 1995). All cyanobacteria carry out oxygenic photosynthesis but some cyanobacterial species can switch to the typical bacterial anoxygenic photosynthesis using sulfide as electron donor (Cohen et al. 1986). Under anoxic conditions and during the dark, cyanobacteria carry out fermentation (Stal 1997). Some cyanobacteria form heterocysts and have the ability to fix atmospheric nitrogen (Capone et al. 2005). Phylogenetic analysis of cyanobacteria based on 16S rRNA genes showed that they are a diverse, monophyletic phylum of organisms within the bacterial radiation. Research on cyanobacteria in the last decades focused largely on their ecology, morphology, physiology and 16S rRNA-based phylogeny but relatively little has been done on their potential uses in biotechnology.

The overwhelming available knowledge on the diversity and physiology of cyanobacteria serves as an excellent base for exploring their applications in biotechnology. In the last few years, cyanobacteria have gained much attention as a rich source of bioactive compounds and have been considered as one of the most promising groups of organisms to produce them (Bhadury et al. 2004; Dahms et al. 2006). These cyanobacterial metabolites include antibacterial (Jaki et al. 2000), antifungal (Kajiyama et al. 1998), antiviral (Patterson et al. 1994), anticancer (Gerwick et al. 1994), antiplasmodial (Papendorf et al. 1998), algicide (Papke et al. 1997) and immunosuppressive agents (Koehn et al. 1992). Screening of cyanobacteria for antibiotics has opened a new horizon for discovering new drugs. Some cyanobacteria have also been found to intracellularly accumulate polyhydroxyalkanoates (PHA), which are comparable in properties to polyethylene and polypropylene (Steinbüchel et al. 1997). These biodegradable plastics could replace oil-derived thermoplastics in some fields. Recent research on cyanobacteria has demonstrated that they form ideal consortia with chemotrophic bacteria and can effectively be used to cleanup oil-contaminated sediments and wastewaters (Abed and Köster 2005). Cyanobacteria have many more biotechnological applications that await possible uses in mariculture, food, fuel, fertilizers, colorants and production of various secondary metabolites including toxins, vitamins, enzymes and pharmaceuticals. Considering the diverse potential uses of cyanobacteria in biotechnology, an overview of these uses is presented here.

Cyanobacterial bioactive compounds

Cyanobacteria have been identified as a new and rich source of bioactive compounds (Abarzua et al. 1999; Shimizu 2003; Bhadury et al. 2004; Dahms et al. 2006). Isolated compounds belong to groups of polyketides, amides, alkaloids, fatty acids, indoles and lipopeptides (Abarzua et al. 1999; Burja et al. 2001; Table 1). The literature review showed that to date up to 19 cyanobacterial strains produce more than 20 different bioactive compounds. Most of the bioactive compounds isolated from cyanobacteria tend to be lipopeptides, i.e. they consist of an amino acid fragment linked to a fatty acid portion. The range of biological activity of secondary metabolites isolated from cyanobacteria includes antibacterial, antifungal, antialgal, antiprotozoan, and antiviral activities (Table 1). Only few cyanobacteria produce bioactive compounds that show a broad spectrum of biological activities. For example, the cyanobacterium Phormidium sp. has been reported to inhibit growth of different Gram-positive and Gram-negative bacterial strains, yeasts, and fungi (Bloor and England 1989). Another example is Lyngbya majuscula (Burja et al. 2001) that produces numerous chemicals including nitrogen-containing compounds, polyketides, lipopeptides, cyclic peptides and many others (Shimizu 2003). The biological activity of these compounds is also diverse and includes protein kinase C activators and tumour promoters, inhibitors of microtubulin assembly, antimicrobial and antifungal compounds and sodium-channel blockers.

Table 1.   Bioactive compounds from cyanobacteria
Species of cyanobacteriaBioactive compoundsBiological activityReferences
Family oscillatoriaceae
Lyngbya majusculaMalyngolideAntibacterialBurja et al. (2001)
LyngbyatoxinsPKC activator(Shimizu 2003)
DebromoaplysiatoxinInflammatory 
Curacin AMicrotubulin assembly inhibitors 
KalkitoxinSodium channel blocker 
Cyclic polypeptideAnti-HIV activityRajeev and Xu (2004)
L. lagerheimiiSulpholipidAnti-HIV activityRajeev and Xu (2004)
Oscillatoria raoiAcetylated sulfoglyco-lipidsAntiviralReshef et al. (1997)
Phormidium tenueGalactosyldiacylglycerolsAntialgalMurakami et al. (1991)
Anti-HIVRajeev and Xu (2004)
Phormidium spp.Thermostable enzymesCatalysis of reactionsPiechula et al. (2001)
Spirulina platensisSpirulanAntiviralHayashi et al. (1991)
Gamma linolenic acidPredecessor of arachidonic acidCohen (1999)
Vitamin B and EAntioxidants and co-enzymesPlavsic et al. (2004)
Family hyellaceae
Hyella caespitoseCarazostatinAntifungalBurja et al. (2001)
Family nostocaceae
Nostoc spongiaeformeNostocine AAntialgalHirata et al. (1996)
N. commune NostodioneAntifungalBhadury and Wright (2004)
Nostoc sp.NostocyclamideAntifungalMoore et al. (1988)
N. linckiaCyanobacterin LU-1AntialgalGromov et al. (1991)
N. insulareNorharmaneAntibacterialVolk and Furkert (2006)
N.sphaericumIndolocarbazolesAntiviralCohen (1999)
Anabaena circinalisAnatoxin-aInflammatoryRajeev and Xu (2004)
Family Scytonemaceae
Scytonema hofmanniCyanobactericinAntialgalAbarzua et al. (1999)
S. ocellatumTolytoxinAntifungalPatterson and Carmeli (1992); Patterson and Bolis (1997)
Phytoalexin
S. pseudohofmanniScytophycinsAntifungalBurja et al. (2001)
Family microchaetaceae
Tolypothrix tenuisToyocamycinAntifungalBanker and Carmeli (1998)
T. tjipanasensisTjipanazolesAntifungalBonjouklian et al. (1991)
Family stigonemataceae
Fischerella muscicolaFisherellinAntialgal, antifungalDahms et al. (2006)
Hapalosiphon fontinalisHapalindoleAntifungalBurja et al. (2001)
Family merismopediaceae
Gomphosphaeria aponinaAponinAntialgalBhadury and Wright (2004)
Family chroococcaceae
Microcystis aeruginosaKawaguchipeptin BAntibacterialDahms et al. (2006)
Synechocystis sp.Naienones A-CAntitumouralNagle and Gerwick (1995)
Synechococcus elongatesThermostable enzymeCatalysis of reactionsOhto et al. (1999)
Thermosynechococcus elongatus BP-1Thermostable polyphosphate kinaseProduction of dipeptidesSato et al. (2007)

Secondary metabolites with antibacterial activity are widely produced by cyanobacteria (Dahms et al. 2006; Table 1). These compounds are effective against Gram-positive and/or Gram-negative bacteria. Both toxic and nontoxic strains of cyanobacteria are producers of antibacterial compounds that are distinct from cyanotoxins (Østensvik et al. 1998). Antifungal compounds include fisherellin A, hapalindole, carazostatin, phytoalexin, tolytoxin, scytophycin, toyocamycin, tjipanazole, nostocyclamide and nostodione produced by cyanobacteria belonging to Stigonematales, Nostocales and Oscillatoriales (Table 1). Additionally, cyanobacteria produce a broad spectrum of antialgal compounds, which may be used to control algal blooms. Cyanobacteria probably use these compounds in order to out-compete other micro-organisms. Antialgal compounds produced by cyanobacteria inhibit growth of algae, their photosynthesis, respiration, carbon uptake, enzymatic activity and induce oxidative stress (Dahms et al. 2006). In contrast to the large amount of antibacterial and antialgal compounds isolated from cyanobacteria there are only a few compounds that show antiviral properties (Table 1), although 2–10% of extracts of different cyanobacterial species have been shown to have antiviral activity (Cohen 1999). These include acetylated sulfoglyco-lipids from Oscillatoria raoi (Reshef et al. 1997) and spirulan from Spirulina platensis (Hayashi et al. 1991). The compounds isolated from Lyngbya lagerhaimanii and Phormidium tenue has been shown to have anti-HIV activity (Rajeev and Xu 2004).

Gamma linolenic acid (GLA) found rich in S. platensis and Arthrospira sp. is medically important since it is converted in the human body into arachidionic acid and then into prostaglandin E2. This compound has a lowering action on blood pressure and plays an important role in lipid metabolism. Some of the marine cyanobacteria constitute potential sources for large-scale production of vitamins, such as vitamins B and E (Plavsic et al. 2004). Piechula et al. (2001) demonstrated that some cyanobacteria can produce thermostable enzymes. Out of 21 endonucleases from Phormidium spp. 4 enzymes that catalyse the hydrolysis of DNA and RNA were active in a wide range of temperatures from 15 to 60°C. Prenyltransferases enzymes catalysing the consecutive condensation of homoallylic diphosphate of isopentenyl diphosphates at temperatures above 60°C have been isolated from the thermophilic cyanobacterium Synechococcus elongates (Ohto et al. 1999). Thermostable polyphosphate kinase from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 was successfully employed in an ATP regeneration system that could be used at high temperatures for the effective production of d-amino acid dipeptides (Sato et al. 2007). These enzymes and other heat stable bioactive compounds are of great interest in biotechnology. So far none of the isolated compounds have demonstrated antilarval activity and inhibition of settlement of larvae and algal spores (Dobretsov et al. 2006).

Cyanobacteria have been used to synthesize isotopically labelled compounds such as sugars, lipids and amino acids, which are nowadays commercially available (Patterson 1996). This is achieved by growing the cyanobacetria in photobioreactors and allowing them to photosynthetically transform simple labelled compounds such as 14CO2, 13CO2, 33H2O and 15NO3 into complex organics.

Cyanobacterial bioplastics (polyhydroxyalkanoates, PHAs)

In conditions of excess essential nutrients, many micro-organisms usually assimilate and store nutrients for future consumption. Various storage materials have been identified in micro-organisms, which include glycogen, sulfur, polyamino acids, polyphosphate, and lipid. PHAs are lipoidic material accumulated by a wide variety of micro-organisms in the presence of abundant carbon sources (Anderson and Dawes 1990). The assimilated carbon sources are biochemically processed into hydroxyalkanoate monomer units, polymerized, and then stored in the form of water insoluble inclusions (or granules) in the cell cytoplasm (Fig. 1a). PHA is a crystalline thermoplastic with properties comparable to that of polypropylene (Doi 1990). PHA has been the focus of attention for the past three decades as a potential substitute for nonbiodegradable petrochemical-based plastics. This is because PHA is an ideal biodegradable material that can be completely mineralized into water and carbon dioxide by the action of naturally occurring micro-organisms. In addition, PHA is also a biocompatible material and is being studied for its application in the biomedical and biopharmaceutical fields (Williams et al. 1999; Sudesh 2004). The idea of producing PHA from CO2 is thought to contribute to a carbon neutral process for making plastics. The idea is also commercially attractive because then the production of PHA will be based on a readily available and free carbon and energy source (sunlight).

Figure 1.

 Polyhydroxyalkanoates (PHAs) in micro-organisms. (a) Transmission electron micrograph (TEM) of PHA granules in a cyanobacterial cell. The granules can be readily dissolved in chloroform and precipitated in methanol. The resulting extract resembles traditional thermoplastics such as polypropylene. (b) PHA film cast from solvent. (c) PHA granules accumulated by Spirulina platensis stained with Nile blue A. The PHA granules appear as bright orange particles in the cell cytoplasm when viewed under fluorescence microscope. (d) Transmission electron micrograph showing the discrete PHA granules accumulated in the cytoplasm of S. platensis.

Some cyanobacteria, like S. platensis, can accumulate PHA under phototrophic and/or mixotrophic growth conditions with acetate (Fig. 1c,d). The PHA granules can be stained by Nile blue A (Ostle and Holt 1982) (Fig. 1a) and they appear as discrete electron transparent granules in the cell cytoplasm (Fig. 1b). Cyanobacteria are of particular interest as PHA producers because of their minimal nutrient requirements for growth and capability of accumulating PHA by oxygenic photosynthesis. Like higher plants, cyanobacteria fix CO2 from the atmosphere and turn it into PHA under nitrogen limiting conditions. Most of the known cyanobacteria that are capable of synthesizing PHA usually accumulate PHAs amounting to less than 6 wt% of their cell dry weight (CDW) (Vincenzini et al. 1990; Stal 1992; Arino et al. 1995; Carr 1996). Spirulina platensis and Synechocystis sp. PCC 6803 have been reported to accumulate a maximum of 6 wt% (Campbell et al. 1982) and 7 wt% (Sudesh et al. 2002) of CDW of PHA under mixotrophic conditions. Poly(3-hydroxybutyrate) [P(3HB)] is the most common type of PHA synthesized by most bacteria. P(3HB) is also the most common type of PHA synthesized by cyanobacteria. Vincenzini et al. (1990) found that when Spirulina was cultivated under photoautotrophic conditions, the P(3HB) content was low (0·3 wt% of CDW), while under mixotrophic growth conditions in the presence of acetate, P(3HB) level amounted to about 3 wt% of CDW. It is known that the synthesis of P(3HB) is primarily regulated by 3-ketothiolase, which is inhibited by high concentrations of free coenzyme A (CoA). High concentrations of intracellular acetyl-CoA favours the synthesis and accumulation of P(3HB). Thus, when acetate is present in the growth medium, the intracellular acetyl-CoA concentration increases at the expense of free CoA pool, resulting in enhancement of P(3HB) biosynthesis. The role of P(3HB) in cyanobacteria is to provide cells with a mechanism of removal of excess reducing equivalents resulting from a disruption of the balanced formation of ATP and NADPH from photosynthesis (De Philippis et al. 1992). Table 2 shows a summary of known PHA-producing cyanobacteria, the various carbon sources tested and the maximum amount of PHAs accumulation obtained to date.

Table 2.   PHA-producing cyanobacteria and the types and contents of their PHA compounds
Species of cyanobacteriaTypes of PHACarbon sources testedPHA content (w/w)References
  1. ND, not defined.

Chlorogloea fritschiiP(3HB)CO2NDJensen and Sicko (1971)
C. fritschiiP(3HB)AcetateNDJensen and Sicko (1971)
Spirulina platensisP(3HB)CO26%Campbell et al. (1982)
S. maximaP(3HB)CO20·70%De Philippis et al. (1992)
S. maximaP(3HB)Acetate2%De Philippis et al. (1992)
Gloeothece sp.P(3HB)CO2/acetate6%Stal (1992)
Oscillatoria limosaP(3HV)CO2/acetate6%Stal (1992)
Trichodesmium thiebautiiP(3HB)Natural conditions0·20%Siddiqui et al. (1992)
Gloeothece sp. PCC6909P(3HB)CO2NDArino et al. (1995)
Synechococcus MA19P(3HB)CO27·50%Miyake et al. (1997)
Synechocystis sp. PCC6803P(3HB)Acetate10%Hein et al. (1998)
S. platensisP(3HB)CO2/acetate10%Jau et al. (2005)
Synechocystis sp. PCC6803P(3HB)Fructose/acetate38%Panda and Mallick (2007)
Nostoc muscorumP(3HB)Acetate/glucose45·60%Sharma et al. (2007)

Cyanobacterial consortia for bioremediation purposes

Many studies have reported the ability of cyanobacteria to oxidize oil components and other complex organic compounds such as surfactants and herbicides (Yan et al. 1998; Radwan and Al-Hasan 2000; Raghukumar et al. 2001; Mansy and El-Bestway 2002). Among these cyanobacteria were the nonaxenic cultures of Microcoleus chthonoplastes and Phormidium corium, degrading n-alkanes (Al-Hasan et al. 1998), Oscillatoria sp. and Agmenellum quadruplicatum oxidizing naphthalene to 1-naphthol (Cerniglia et al. 1979, 1980a), Oscillatoria sp. strain JCM that oxidized biphenyl to 4-hydroxybiphenyl (Cerniglia et al. 1980b) and Agmenellum quadruplicatum that metabolized phenanthrene into trans-9,10-dihydroxy-9,10-dihydrophenanthrene and 1-methoxy-phenenthrene (Narro 1985). However, recent studies have demonstrated that it is indeed not the cyanobacteria that are responsible for the degradation of these compounds but the associated aerobic organotrophic bacteria (Radwan et al. 2002; Abed and Köster 2005; Sánchez et al. 2005). In spite of that, the presence of cyanobacteria alongside with the aerobic organotrophs facilitated the degradation process and both groups constituted ideal consortia for degradation of petroleum and other complex organic compounds (Abed and Köster 2005). While aerobic bacteria directly degrade these components, cyanobacteria play an equally important indirect role in biodegradation by (i) providing them with oxygen (byproduct of photosynthesis), which is needed for the breakdown of aromatic rings, (ii) providing them with simple organics (produced by photosynthesis and fermentation) and (iii) providing them with fixed nitrogen (through nitrogen fixing strains), which is often limited in different environments. Abed and Köster (2005) demonstrated the capability of an Oscillatoria-Gammaproteobacteria consortium to degrade phenanthrene, dibenzothiophene, pristine and n-octadecane. The degradation rate of these compounds was enhanced in the presence of the cyanobacterium. Similarly, Microcoleus chthonplastes was found to form consortia with organotrophic bacteria, some of which were able to fix atmospheric nitrogen while others could degrade aliphatic heterocyclic organo-sulfur compounds as well as alkylated monocyclic and polycyclic aromatic hydrocarbons (Sánchez et al. 2005). Bacteria immobilized in the sheaths coating macroalgae were shown to degrade petroleum compounds (Radwan et al. 2002). These indigenous consortia could be ideally used in the bioremediation of polluted sites without the need to add new bacteria or fertilizers to the field. Al-Awadhi et al. (2003) grew these consortia on gravel particles and used them successfully to clean up oil pollution.

Cyanobacteria and their associated bacteria have also been successfully used in wastewater treatment. For example, cultures of Oscillatoria sp. BDU 30501, Aphanocapsa sp. BDU 16 and a halophilic bacterium Halobacterium US 101 were used to treat a factory effluent and resulted in reduction of calcium and chloride to levels that did not inhibit survival and multiplication of fish (Uma and Subramanian 1990). Phormidium valderianum BDU 30501 was used to reduce phenol concentrations (Shashirekha et al. 1997) while Oscillatoria boryana BDU 92181 was used to eliminate melanoidin pigment from distillery effluents (Kalavathi et al. 2001).

Cyanobacterial alternative energy sources

Cyanobacteria have been used to produce hydrogen gas that constitutes an alternative future energy source to the limited fossil fuel resources (Dutta et al. 2005). The advantages of using biological hydrogen as a fuel are its eco-friendly nature, efficiency, renewability and the absence of carbon dioxide emission during its production and utilization (Lindbald 1999). Cyanobacteria produce hydrogen either as a byproduct of nitrogen fixation, when nitrogenase-containing heterocystous cyanobacteria are grown under nitrogen limiting conditions, or by the reversible activity of hydrogenases enzymes. Heterocystous cyanobacteria are thus more efficient in hydrogen production than nonheterocystous types (Pinzon-Gamez et al. 2005). More than 14 cyanobacterial genera including Anabaena, Calothrix, Oscillatoria, Cyanothece, Nostoc, Synechococcus, Microcystis, Gloeobacter, Aphanocapsa, Chroococcidiopsis and Microcoleus are known for their ability to produce hydrogen gas under various culture conditions (Lambert and Smith 1977;Sveshnikov et al. 1997; Masukawa et al. 2001; Table 3). Anabaena spp. are able to produce significant amounts of hydrogen. Nitrogen-starved Anabaena cylindrica cells produce the highest amount of hydrogen (30 ml of H2 per lit culture per hour) (Jefferies et al. 1978). Gloeocapsa alpicola showed increase in hydrogen production under sulfur starvation (Antal and Lindblad 2005) whereas S. platensis could produce hydrogen under complete dark and anoxia (Aoyama et al. 1997).

Table 3.   Cyanobacterial species that are capable of producing hydrogen and the growth conditions at which maximum production occur
Species of cyanobacteriaGrowth conditionsMaximum hydrogen productionReference
Anabaena sp. PCC 7120Air, 20 μE m−2 s−12·6 μmol mg−1 chl a h−1Masukawa et al. (2001)
Anabaena cylindrica IAMM-IAir, 20 μE m−2 s−12·1 μmol mg−1 chl a h−1Masukawa et al. (2001)
Nostoc commune IAMM-I 3Air, 20 μE m−2 s−10·25 μmol mg−1 chl a h−1Masukawa et al. (2001)
Anabaena variabilis AVM13Air and 1% CO2, 100 μE m−2 s−168 μmol mg−1 chl a h−1Happe et al. (2000)
Anabaena variabilis PK84Air and 2% CO2, 113 μE m−2 s−132·3 μmol mg−1 chl a h−1Tsygankov et al. (1998)
Anabaena variabilis ATCC 2941373%Ar, 25%N2, 2% CO2, 90 μE m−2 s−146·16 μmol mg−1 chl a h−1Sveshnikov et al. (1997)
Synechococcus PCC6803Air, photon fluence rate 20 μE m−2 s−10·26 μmol mg−1 chl a h−1Moezelaar et al. (1996)
Synechococcus PCC6301Air, photon fluence rate 20 μE m−2 s−10·09 μmol mg−1 chl a h−1Howarth and Codd (1985)
Microcystis PCC 7820Air, photon fluence rate 20 μE m−2 s−10·16 μmol mg−1 chl a h−1Moezelaar et al. (1996)
Gloeobacter PCC 7421Air, photon fluence rate 20 μE m−2 s−11·38 μmol mg−1 chl a h−1Moezelaar et al. (1996)
Synechocystis PCC 6308Air, photon fluence rate 20 μE m−2 s−10·13 μmol mg−1 chl a h−1Howarth and Codd (1985)
Synechocystis PCC 6714Air, photon fluence rate 20 μE m−2 s−10·07 μmol mg−1 chl a h−1Howarth and Codd (1985)
Aphanocapsa montanaAir, photon fluence rate 20 μE m−2 s−10·40 μmol mg−1 chl a h−1Howarth and Codd (1985)
Gloeocapsa alpicola CALU 743Sulfur free 4% CO2; 25 μmol
photons m−2 s−1
0·58 μmol mg−1 proteinAntal and Lindblad (2005)
Chroococcidiopsis thermalisAr and 1% CO20·7 μmol mg−1 chl a h−1Serebryakova et al. (2000)

Large-scale hydrogen production by Spirulina and Anabaena spp. has been tried using different types of bioreactors that included vertical column reactor, tubular type and flat panel photobioreactor (Dutta et al. 2005). These reactors were designed to make use of solar light for illumination, to maximize the area for incident light (high surface to volume ratio) and to allow sterilization and hydrogen collection with convenience and ease. Nevertheless, the reactors are subjected to continuous modification in order to increase their productivity and to decrease costs of maintenance and production. So far, these modifications succeeded in bringing down the cost of biologically produced hydrogen to $25 per m3 compared to $170 per m3 for the hydrogen produced by splitting of water (Block and Melody 1992). Increased production of hydrogen was also achieved by genetically modifying the nitrogenase enzyme in hydrogen-producing strains. The ongoing research on hydrogen production by cyanobacteria focuses on finding new strains with higher potential to produce hydrogen, optimizing the mass production of hydrogen in bioreactors and modifying the physiology and the genetic system of H2-producing cyanobacteria to ensure maximum production of hydrogen.

Cyanobacteria as biofertilizers

Heterocystous cyanobacteria and several nonheterocystous cyanobacteria are known for their ability to fix atmospheric nitrogen (Capone et al. 2005). The fertility of many tropical rice field soils has been mainly attributed to the activity of nitrogen-fixing cyanobacteria. An estimation showed that more than 18 kg N ha−1 year−1 was added to the soils by cyanobacteria (Watanabe and Cholitkul 1979). Inoculation of cyanobacteria to increase the fertility of soils has been successfully attempted. For example, Azolla was used as an organic fertilizer in rice cultivation in many countries (Kaushik and Venkataraman 1979). Addition of Azolla was found to support the growth of soil micro-organisms including heterotrophic N2 fixers. Recently, nitrogen-fixing cyanobacteria have been reported to dominate desert crusts worldwide (Garcia-Pichel and Pringault 2001). This is believed to contribute significantly to the fertility of desert soils and may eventually facilitate vegetation of deserts.

Cyanobacteria as a healthy food source

Strains of Spirulina, Anabaena and Nostoc are consumed as human food in many countries including Chile, Mexico, Peru and Philippines. Arthrospira platensis (misidentified as S. platensis) is grown in large scale using either outdoor ponds or sophisticated bioreactors but marketed in the form of powder, flakes, tablets and capsules. It is used as a food supplement because of its richness in nutrients and digestibility. It contains more than 60% proteins and is rich in beta-carotene, thiamine and riboflavin and is considered to be one of the richest sources of vitamin B12 (see cyanobacterial active compounds). Nostoc commune is rich in fibres and proteins and can play an important physiological and nutritional role in the human diet. Aphanizomenon sp. is collected from natural blooms in the Lake Klamath (Oregon, USA) to be used as healthy food (Carmichael and Gorham 1980). Marine nitrogen-fixing cyanobacteria have also been tested to feed fishes in aquacultures. The Tilapia fish showed high growth rates when fed with marine cyanobacteria in indoor and outdoor cultures (Mitsui et al. 1983). Phormidium valderianum has been used in India to serve as a complete aquaculture feed source based on its nutritional value and nontoxic nature. In view of the cyanobacterial significance as a food source, very little research has been performed and published about this.

Cyanobacterial emulsifiers

Halophilic cyanobacteria produce large amounts of exopolysaccharides (EPS), which can be relevant to oil recovery by decreasing its surface tension and increasing its solubility and mobility. EPS, when gelated under alkaline conditions, was employed to remove dyes from textile effluent. The halophilic cyanobacterium Aphanocapsa halophytica was used for the production of EPS and its yield was increased to tenfold by immobilizing the cells on light-diffusing optical fibres (Matsunaga et al. 1996). The cyanobacterium Cyanothece sp. ATCC 51142 had a maximum EPS production at 4·5% (w/v) NaCl and pH 7 (Shah et al. 1999). Other EPS-producing cyanobacteria are the halotolerant Anabaena sp. ATCC 33047 (Moreno et al. 1998) and Synechococcus sp. (Matsunaga et al. 1991). It should be pointed out that it is necessary in future research to investigate the potential of additional cyanobacteria in EPS production.

Conclusion and future directions

Our review demonstrates the versatility of cyanobacteria to biotechnological applications. They are potent sources of bioactive compounds, biofertilizers, bioplastics, energy, food and have currently been used in drugs discovery, medical diagnostics, and bioremediation. However, further research needs to focus on the satisfactory axenic culturing of these micro-organisms in order to facilitate their exploitation. Additionally, new methods need to be developed to allow the cultivation of previously ‘uncultivable’ strains. The methods should consider the organism’s requirements in the field and these conditions should be mimicked in the laboratory. The cultivation efforts should be directed towards unique environments, particularly those with extreme conditions of salinity, temperature, pH, UV and light intensity. These environments are likely to contain novel strains with strong potential in biotechnology. To circumvent cultivability problem, metagenomics could serve as an alternative approach (Handelsman 2004). This approach involves construction of metagenomic clone libraries from nucleic acids extracted directly from environmental samples. The clone libraries are then screened for the presence of functional genes that are involved in the biosynthesis of certain biotechnologically significant compounds (Zhang et al. 2005). These genes are transferred to cultivable hosts, which can be directly used for the production of the desired compounds.

Numerous compounds that have antibacterial and antialgal activity have been isolated from different cyanobacteria, however, only limited compounds have been screened for their antifouling, antiviral and antitumour activities (Table 1). Recently, several potent antifouling (Fusetani 2004), antiviral and antitumour compounds (Proksch et al. 2002) have been isolated from macroalgae, sponges and tunicates. These compounds are often present in small quantities and in order to obtain sufficient amounts it is needed to harvest and extract many marine organisms (Dobretsov et al. 2006). In many cases, it is the associated micro-organisms and not the host who are the true sources of bioactive compounds. For example, the major compounds of the sponge Dysidea herbacea are actually produced by its cyanobacterial symbiont Oscillatoria spongeliae (Unson and Faulkner 1993). Unlike eukaryotes, cyanobacteria can produce compounds much more rapidly and in large amounts (Dahms et al. 2006). Furthermore, they can be genetically and chemically easily modified in order to increase compounds yield and bioactivity. Therefore, screening of cyanobacteria for novel bioactive compounds should be an important future direction.

The efficient production of PHA (bioplastics) using cyanobacteria is technologically challenging. Nevertheless, it remains as an attractive approach considering the fact that the carbon source comes directly from atmospheric CO2. In contrast, the more efficient production of PHA by bacteria relies on the use of valuable carbon sources such as sugars from starch and fatty acids from vegetable oils. Because of the growing pressure to reduce CO2 emission, the demands for plant products such as starch and vegetable oils are on the rise for use as starting materials for the production of biofuels and biobased materials. Therefore, the demands for plant products can be expected to increase, which inevitably will require more fertile land to be used for agricultural activities. In such a scenario, using cyanobacteria to produce PHA may become more promising because the large-scale cultivation of cyanobacteria does not require fertile land.

While the biotechnological potential of cyanobacteria is attracting an increasing attention, most of the commercial compounds were isolated from freshwater cyanobacteria. Marine environments with different environmental conditions ranging from shallow euphotic zone to deep-sea hydrothermal vents are likely to be a good source for a variety of cyanobacterial species that may have high biotechnological significance.

Although production of hydrogen by cyanobacteria provides a promising environmentally-friendly source of energy, it still requires optimization in order to lower the production costs and to increase the yield. The design of the bioreactors is essential for large-scale production and must be transparent to allow adequate entry of light. The hydrogen productivity tends to decrease at higher light intensity because photosynthesis diverts the hydrogen production pathway, hence the light regime must be controlled carefully. Liquid circulation time and aeration has an influence on hydrogen productivity since the hydrogen producing enzymes are oxygen sensitive. Therefore, aeration, although needed, must be adjusted to avoid any inhibition of the hydrogen production pathway.

More research on the ecological applications of cyanobacteria needs to be performed. Recently they have been shown to play a significant role in the stabilization of coastal sediments by reducing hydrodynamic erosional forces (Noffke et al. 2003). During low hydrodynamic disturbance, cyanobacteria enhance deposition of sediments by baffling, trapping, and binding while during periods of intensive hydraulic reworking, they shelter their substrata against erosion or they permit flexible deformation of sandy sediments. The exact role of cyanobacteria in fighting erosion has to be investigated, considering different types of sediments and different environmental settings.

Cyanobacteria have been studied for a long time for their interesting morphology, diversity and physiology but pioneering work in the last decades has raised the level of these microbes to be viewed with favour in biotechnology-relevant fields. Therefore, it is essential not only to understand and describe the diversity of cyanobacteria in yet unexplored habitats but also to gainfully exploit them for various industrial applications. This requires combined efforts of taxonomists, molecular biologists, biochemists, engineers and industry-related scientists as well as politicians and policy makers.

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