Correlating metabolically active microbial communities with geochemistry in an unexplored terrestrial subsurface ecosystem, glacial deposit

Background/Question/Methods

Terrestrial subsurface environments harbor a number of microorganisms with diverse metabolic functions associated with global biogeochemical cycles. Glacial deposits formed by the past glaciations are poorly understood terrestrial subsurface ecosystems, which cover vast areas of the northern part of North American, European, and Asian continents. Due to these large scale glacial events, significant amounts of peats and paleosols have been buried within glacial deposits. Such biodegradable organic matter likely serve as a primary carbon source supporting microbial growth and activities involved in biogeochemical processes. Indeed, previous geochemical studies suggest that methane production has biologically occurred in glacial deposits, suggesting that glacial deposits likely harbor active and unique microbial guilds. However, microbial communities and ecological roles within glacial deposits are still largely unknown.

In this study, we investigated microbial community structure and geochemistry within glacial deposits in Illinois. Specifically, to analyze metabolically active microbial communities with high resolution, we conducted deep sequencing of both 16S rRNA and rRNA genes by Illumina MiSeq system for 13 groundwater samples collected from the Mahomet Aquifer, a large glacial aquifer formed in glacial deposit layers in east-central Illinois. Multivariate geochemical properties of the groundwater samples and their correlation with microbial compositions were also analyzed.

Results/Conclusions

DNA- and RNA-based microbial communities were very similar, indicating that microbes present in the subsurface are active and possibly contribute to functioning of the ecosystem. Abundant functional bacterial groups in the aquifer included anaerobic sulfate-reducers and iron-reducers within Deltaproteobacteria, Clostridia, and Nitrospira, and aerobic/microaerobic iron-oxidizers and methanotrophs and/or methylotrophs classified into Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria, indicating the coexistence of anaerobic and aerobic/microaerobic populations in the subsurface. In the archaeal community, methanogenic archaea predominated, although their relative abundance varied among the samples. Geochemical profiles also differed among the samples; high-methane and low-sulfate concentrations and low-methane and high-sulfate concentrations predominated in the western and eastern parts of the aquifer, respectively, whereas more heterogeneity was observed in the central part of the aquifer. Statistical analyses clearly indicated that changes in microbial community compositions were well correlated with changes in the geochemical profiles, and mainly linked to methane, sulfate, manganese, NH₃-N and dissolved organic carbon. Community resemblance analysis clustered the communities into three groups; high-methane group, high-sulfate group, and others, indicating that geochemical factors dictate the microbial community assembly in the glacial deposits. Our findings enhance better understanding of the ecological roles of microbial guilds involved in biogeochemical cycling in the terrestrial subsurface.