Abstract
Biologically produced methane (CH4) from anaerobic digesters is a renewable alternative to fossil fuels, but digester failure can be a serious problem. Monitoring the microbial community within the digester could provide valuable information about process stability because this technology is dependent upon the metabolic processes of microorganisms. A healthy methanogenic community is critical for digester function and CH4 production. Methanogens can be surveyed and monitored using genes and transcripts of mcrA, which encodes the α subunit of methyl coenzyme M reductase – the enzyme that catalyses the final step in methanogenesis. Using clone libraries and quantitative polymerase chain reaction, we compared the diversity and abundance of mcrA genes and transcripts in four different methanogenic hydrogen/CO2 enrichment cultures to function, as measured by specific methanogenic activity (SMA) assays using H2/CO2. The mcrA gene copy number significantly correlated with CH4 production rates using H2/CO2, while correlations between mcrA transcript number and SMA were not significant. The DNA and cDNA clone libraries from all enrichments were distinctive but community diversity also did not correlate with SMA. Although hydrogenotrophic methanogens dominated these enrichments, the results indicate that this methodology should be applicable to monitoring other methanogenic communities in anaerobic digesters. Ultimately, this could lead to the engineering of digester microbial communities to produce more CH4 for use as renewable fuel.
Highlights
SummaryBiologically produced methane (CH4) from anaerobic digesters is a renewable alternative to fossil fuels, but digester failure can be a serious problem
The enrichment cultures used in these analyses were fed with H2/CO2 as the primary substrates
The exception was that mcrA sequences from Methanosaeta, acetoclastic methanogens adapated to low acetate concentrations (Jetten et al, 1990), represented about 60% of the clones in R4 in one set of analyses, but these contributed to only ∼ 5% of the mcrA transcripts
Summary
Produced methane (CH4) from anaerobic digesters is a renewable alternative to fossil fuels, but digester failure can be a serious problem. Specific methanogenic activity (SMA) assays, methane production rates, biogas composition, chemical oxygen demand (COD) removal, pH, granule morphology, acetate utilization rates, methanethiol concentration and quantification of volatile fatty acids have all been suggested or used to evaluate digester function (Coates et al, 1996; Zitomer et al, 2000; Castellano et al, 2007; Conklin et al, 2008; Molina et al, 2009). While these parameters are closely related to the metabolic functions of the microbial community, they do not directly quantify microorganisms. We report an evaluation of the use of qPCR of mcrA genes and transcripts in comparison with traditional SMA assays on the biomass from four different H2/CO2 enriched bioreactors
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