Microbial community structure and ecosystem function: Linking methane production rate to methanogen community structure in peatland soils

Background/Question/Methods

Methane is a significant greenhouse gas, trapping 25 times as much infrared radiation as carbon dioxide per unit weight. All of the methane produced by the world’s biosphere is produced by methanogens, a polyphyletic group of microorganisms in the domain Archaea, in anaerobic environments. In this study we explored the relationship between methane production and methanogen community structure in peatlands along a hydrogeomorphic gradient in northern Michigan, USA spanning six sites from an ombrotrophic bog to a minerotrophic fen. This gradient has been shown to influence methanogenesis pathways and efficiency of methane production. Methanogenesis rates in peat samples taken throughout 2009 and 2010 were measured in sealed bottles, and the relative contributions of both methanogenesis pathways were determined via addition of 14C tracer to the incubations. When quantifying community structure, we used mcrA, a functional gene that is ubiquitous in methanogens, as a marker for molecular analysis. DNA sequences of mcrA were aligned in MOTHUR and binned into OTUs for putative assignment of taxonomic (and functional) identity.

Results/Conclusions

Community structure differed significantly among sites, and both Bray-Curtis and Jaccard Dissimilarity trees of genus-level OTUs reveal strong clustering of communities by gradient position. A large number of sequences in the most ombrotrophic site in spring 2010 were identified as putative acetoclasts, a surprising finding that matches the unexpectedly high rate of acetoclastic methanogenesis in that site at that time. Total methanogenesis was much higher in minerotrophic sites, and the hydrogenotrophic pathway of methanogenesis increased in dominance in all sites as time progressed. Methanogen richness and diversity also increased dramatically from ombrotrophic to minerotrophic sites, and the methanogen community in each site was dominated by putatively hydrogenotrophic methanogens.Altogether, our results show a link between functional community structure and methanogenesis pathways in peats. We also discuss the realtive utility of Sanger and 454 pyrosequencing techniques for functional gene analysis, and future directions for the use of mRNA sequencing to clarify temporal relationships between community structure and methane production.