Pathway for acetate degradation in the biogas reactors
In the control reactor, acetate degradation was primarily through aceticlastic methanogenesis throughout the operating period. In the experimental reactor, which was subjected to gradually increasing ammonia levels, a shift from aceticlastic acetate degradation to syntrophic acetate oxidation was established between 225 and 442 days of operation, when the ammonia level reached 5.5 and 6.9 g NH4+-N l−1 (Fig. S1). Labelling analysis with [2-14C]-acetate demonstrated occurrence of syntrophic acetate oxidation in reactor SB (Schnürer and Nordberg, 2008; Ek et al., 2010), while in reactor DVX the analysis indicated that aceticlastic methanogenesis was the main pathway for acetate degradation (Fig. S1). The dominance of aceticlastic methanogenesis in reactor DVX was somewhat unexpected, as the ammonia concentration in this reactor exceeded the levels previously shown to cause development of syntrophic acetate oxidation (Schnürer et al., 1999; Schnürer and Nordberg, 2008). Parameters other than ammonia [e.g. substrate change, increased loading rate and decreased hydraulic retention time (HRT)] apparently had an impact on the mechanism developed for methane formation in reactor DVX.
Primer specificity and detection of SAOB in samples with conventional PCR
Specific primer sets for detection of 16S ribosomal RNA (rRNA) genes of C. ultunense, S. schinkii, T. acetatoxydans and T. phaeum (Table 1) generated single PCR products from genomic DNA of the corresponding species. Furthermore, in PCR amplification with primer sets targeting C. ultunense, S. schinkii and T. acetatoxydans, products of the predicted length were generated from template DNA extracted from the experimental reactor on day 442 and day 642 of operation. From all other samples (control and experimental reactor) the quantity of amplified DNA was below the detection limit for visualization using ethidium bromide staining. Furthermore, PCR analysis with the primer set targeting the 16S rRNA gene of the thermophile T. phaeum did not generate any visible product from any reactor sample. Analysis of DNA extracted from samples from reactors SB or DVX only gave a positive result with the primer set targeting the 16S rRNA gene of S. schinkii. The primers all showed high specificity, as PCR products generated from all reactor samples were sequenced and shown to be identical (100% identity over 171, 237 and 127 bp respectively) to the sequences retrieved from pure cultures of the corresponding bacteria. Furthermore, all primer sets showed high specificity to the corresponding SAOB in an evaluation against the GenBank database using BLAST.
Table 1. Primer sets and PCR programs used in the investigation.
|Primera||Target species or group||Sequence (5′3′)b||Position in target speciesc||Tm (°C)||Amplicon size (bp)|
|Cultfe|| Clostridium ultunense ||CCT TCG GGT GGA ATG ATA AA||56–76||57||127|
|Cultre||TCA TGC GAT TGC TAA GTT TCA||162–183|
|THACfd|| Syntrophaceticus schinkii ||ATC AAC CCC ATC TGT GCC||802–820||61||171|
|THACrd||CAG AAT TCG CAG GAT GTC||955–973|
|Tpfd|| Tepidanaerobacter acetatoxydans ||AGG TAG TAG AGA GCG GAA AC||963–983||63||237|
|Tprd||TGT CGC CCA GAC CAT AAA||1182–1200|
|Thfe|| Thermacetogenium phaeum ||GGG TGG TGT GAA GCC ATC||795–813||68||175|
|Thre||AGG TCC GCA GAG ATG TCA AG||970–990|
|Tbff||Total bacteria||GTG ITG CAI GGI IGT CGT CA||1048–1068||61||323|
|Tbrf||ACG TCI TCC ICI CCT TCC TC||1371–1391|
|Mscfg|| Methanosarcinaceae ||GAA ACC GYG ATA AGG GGA||380–397||60||408|
|Mscrg||TAG CGA RCA TCG TTT ACG||811–828|
|MMBfg|| Methanomicrobiales ||ATC GRT ACG GGT TGT GGG||282–299||66||506|
|MMBrg||CAC CTA ACG CRC ATH GTT TAC||812–832|
|Mstfg|| Methanosaetaceae ||TAA TCC TYG ARG GAC CAC CA||702–721||61||164|
|Mstrg||CCT ACG GCA CCR ACM AC||812–832|
| pAfh||Bacteria||AGA GTT TGA TCC TGG CTC AG||8–28||55||1534|
| pHrh||AAG GAG GTG ATC CAG CCG CA||1542–1522|
| Arch46fi|| Archaea ||YTA AGC CAT GCR AGT||46–61||40||971|
| Arch1017rj||GGC CAT GCA CCW CCT CTC||1017–999|
Real-time PCR quantification of SAOB and methanogens
All standard curves for the quantitative PCR analyses, constructed as described in Table S1, had a linear correlation coefficient (r2) ranging between 0.985 and 0.999, and the calculated qPCR efficiency of the reactions varied between 86.2% and 108%.
The qPCR analyses showed a distinct increase in C. ultunense, S. schinkii and T. acetatoxydans in the experimental reactor when the ammonia level increased above 3.3 g NH4+-N l−1 (Fig. 1A). The increase was confirmed by an additional assay with triplicate DNA samples from the experimental reactor on day 225 and day 442, which demonstrated a significant increase (un-paired t-test, P < 0.05) in C. ultunense from 4.1 ± 1.2 × 105 to 2.3 ± 0.9 × 107 gene abundance ml−1, in S. schinkii from 6.3 ± 1.4 × 106 to 6.8 ± 2.1 × 109 gene abundance ml−1, and in T. acetatoxydans from 4.7 ± 2.4 × 105 to 5.7 ± 0.4 × 1010 gene abundance ml−1. In parallel, a decrease in the abundance of acetate utilizing methanogens from the family Methanosarcinaceae occurred from day 225 onwards, when the ammonia concentration exceeded 3.3 g NH4+-N l−1. However, the abundance of the acetate utilizing Methanosaetaceae declined after only 70 days of operation (Fig. 1B). Hydrogenotrophic methanogens of the order Methanomicrobiales initially decreased in abundance between days 70 and 142, but subsequently increased to around their initial abundance by day 642. It is possible that certain members of the Methanomicrobiales declined initially due to ammonia inhibition or pH change and subsequently (> 142 days) ammonia-tolerant members of the Methanomicrobiales were favoured as the ammonia concentration increased. The observed decrease in aceticlastic methanogens and increase in hydrogenotrophic methanogens in response to increasing ammonia levels, most likely caused by a comparatively higher tolerance of Methanomicrobiales to ammonia (Koster and Lettinga, 1984; Sprott and Patel, 1986), have been reported at population level previously (Angenent et al., 2002). However, the present study represents the first detailed analysis of changes in both the population of methanogens and SAOB in response to increasing ammonia concentration.
Figure 1. (A) Abundance of syntrophic acetate-oxidizing bacteria in the control reactor (- - -) and the experimental reactor (—), as determined by qPCR analysis of 16S rRNA genes. C. ultunense●; S. schinkii▴; T. acetatoxydans. (B) Abundance of methanogens in the control reactor (- - -) and the experimental reactor (—) as determined by qPCR analysis of 16S rRNA genes. Methanomicrobiales●; Methanosarcinaceae▴; Methanosaetaceae . Genomic DNA was extracted from three replicate samples (0.3 ml each) from each reactor and sampling point, using the FastDNA Spin kit for soil (Qbiogene, Illkrich, France). The triplicate DNA samples were pooled and the qPCR was performed with a BioRad iCycler (Hercules, CA). Each reaction contained 3 μl DNA template, 1 μl of each primer (10 pmol μl−1), 5 μl of sterile water, 10 μl iQ Supermix PCR reagent (BioRad, Hercules, CA), and SYBR-Green I as the fluorescent DNA intercalating agent (0.2 μl of 100x concentrate, Invitrogen, UK). In qPCR analysis of the methanogenic communities the temperature cycle consisted of: 95°C for 7 min; 55 cycles of 95°C for 40 s; annealing at specific temperatures (Table S1) for 1 min; and 72°C for 40 s. qPCR analysis of SAOB was performed applying the following conditions: 7 min at 95°C; 40 cycles of 95°C for 30 s; annealing at specific temperatures (Table S1) for 1 min; and 72°C for 30 s. At the end of each qPCR assay, a temperature melt curve was performed to verify reaction quality (55–95°C, ΔT = 0.1°C s−1). Logarithmic values of the concentration of the16S rRNA gene were plotted against the threshold cycle (Ct) number and used for estimation of gene abundance in the unknown samples.
Download figure to PowerPoint
In the control reactor, methanogen and SAOB abundance remained stable throughout the 642 days of operation (Fig. 1). Total bacterial abundance in the control reactor and experimental reactor was stable (4.9 ± 1.8 × 1010 and 3.7 ± 2.2 × 1010 gene abundance ml−1 respectively) throughout the operating period. These results supported the presumption that the changes in the microbial communities in the experimental reactor were a consequence of increased ammonia concentration.
16S rRNA genes from T. phaeum were not detected in any of the reactors. This was not surprising, as the temperature range of this thermophilic bacterium is 40–65°C, with an optimum around 58°C. The conditions in the reactors, operating at 37°C, were therefore unfavourable for T. phaeum.
In previous studies, C. ultunense, S. schinkii and T. acetatoxydans proved capable of withstanding rather high levels of ammonium chloride (∼ 8 g NH4+-N l−1) at neutral pH (Schnürer et al., 1996; Westerholm et al., 2010; 2011), an ammonium level that has strong inhibitory effects on aceticlastic methanogens from the families Methanosarcinaceae and Methanosaetaceae (Sprott and Patel, 1986; Hajarnis and Ranade, 1993). The ammonia tolerance of these syntrophic acetate-oxidizers probably gives them a competitive advantage in ammonia-stressed systems. These bacteria, in association with ammonia-tolerant hydrogenotrophic methanogens, may consequently adopt the role of dominant acetate consumers in environments where ammonia restrains aceticlastic methanogenic activity.
In reactor SB, C. ultunense, S. schinkii and T. acetatoxydans were present at significantly (t-test, P < 0.05) greater abundance than in reactor DVX (Fig. 2). The total bacterial gene abundance in reactor SB (1.0 ± 0.4 × 1011 ml−1) was also slightly higher than in reactor DVX (3.2 ± 0.8 × 1010 ml−1). The comparatively low abundance of acetate oxidizers in reactor DVX agreed with the labelling analysis, demonstrating dominance of aceticlastic methanogenesis in this reactor. In contrast, there was no significant difference (t-test, P > 0.05) in mean gene abundance of Methanosarcinaceae or Methanomicrobiales between reactors DVX and SB, and the Methanosaetaceae abundance was even significantly lower in reactor DVX. The high abundance of Methanosaetaceae in reactor SB was unexpected and contradicted results reported by Karakashev and colleagues (2006), showing that acetate oxidation is the dominant pathway only in the absence of Methanosaetaceae. The relatively high abundance of Methanosarcinaceae and Methanosaetaceae in reactor DVX is also noteworthy, indicating occurrence of ammonia-tolerant aceticlastic methanogens in this reactor operating at a high ammonia concentration. However, the accumulation of VFA and the decline in pH demonstrated the instability of aceticlastic methanogenesis in the conditions under which reactor DVX was operated, thereby reflecting the importance of SAOB for the maintenance of process stability in methanogenic systems with high ammonia concentrations.
Figure 2. Abundance of syntrophic acetate oxidizing bacteria and methanogens in reactors SB and DVX. Acetate degradation proceeded through syntrophic acetate oxidation in reactor SB and via aceticlastic methanogenesis in reactor DVX. SB was a large-scale reactor operating with an average HRT of 56 days and was fed with slaughterhouse waste as main substrate (Ek et al., 2010). At the time of sampling the concentrations of volatile fatty acids (VFA) and ammonia-nitrogen in the process were 2.3 g l−1 and 5.3 g NH4+-N l−1, respectively. DVX was a laboratory-scale reactor that was inoculated with sludge from the SB reactor. The process was fed with distiller's waste and operated with an average HRT of ∼40 days at approximately pH 7.8, 1.6 g VFA l−1 and 7.8 g NH4+-N l−1. The OLR of DVX was initially 4 g VS l−1 day−1 and was then gradually increased and had reached 6 g VS l−1 day−1 when sampled. After ∼330 days of operation, high concentrations of VFA (4-5 g l−1) had accumulated in the process and the pH had started to decrease. Triplicate samples from reactors SB and DVX, taken on a single sampling occasion, were analyzed separately and the qPCR analysis was conducted as described in Fig. 1.
Download figure to PowerPoint