Molecular evidence for an active endogenous microbiome beneath glacial ice

Abstract

Geologic, chemical and isotopic evidence indicate that Earth has experienced numerous intervals of widespread glaciation throughout its history, with roughly 11% of present day Earth's land surface covered in ice. Despite the pervasive nature of glacial ice both today and in Earth's past and the potential contribution of these systems to global biogeochemical cycles, the composition and phylogenetic structure of an active microbial community in subglacial systems has yet to be described. Here, using RNA-based approaches, we demonstrate the presence of active and endogenous archaeal, bacterial and eukaryal assemblages in cold (0-1 °C) subglacial sediments sampled from Robertson Glacier, Alberta, Canada. Patterns in the phylogenetic structure and composition of subglacial sediment small subunit (SSU) ribosomal RNA (rRNA) assemblages indicate greater diversity and evenness than in glacial surface environments, possibly due to facilitative or competitive interactions among populations in the subglacial environment. The combination of phylogenetically more even and more diverse assemblages in the subglacial environment suggests minimal niche overlap and optimization to capture a wider spectrum of the limited nutrients and chemical energy made available from weathering of bedrock minerals. The prevalence of SSU rRNA affiliated with lithoautotrophic bacteria, autotrophic methane producing archaea and heterotrophic eukarya in the subglacial environment is consistent with this hypothesis and suggests an active contribution to the global carbon cycle. Collectively, our findings demonstrate that subglacial environments harbor endogenous active ecosystems that have the potential to impact global biogeochemical cycles over extended periods of time.

Figure 1

qRT-PCR of SSU rRNA transcripts (rRNA) and genes (rDNA) from subglacial sediment, cryoconite sediment and snow debris. Results are presented as the mean of triplicate qPCR assays; error bars represent the s.d. of replicates. Copy number is normalized to per gram dry weight sediment. Cryo, cryoconite sediment; Sub, subglacial sediment.

Figure 2

Cluster dendograms depicting the Rao phylogenetic relatedness of archaeal (a), bacterial (b) and eukaryal (c) SSU rDNA and SSU cDNA assemblages. The cophenetic correlation coefficient indicating the extent to which the pairwise distances between the original unmodeled dissimilarity data points were preserved in each dendogram was 0.9997, 0.9486 and 0.8791, respectively. Cryo, cryoconite sediment; Sub, subglacial sediment.

Figure 3

Composition of SSU rDNA and SSU cDNA in nucleic acids recovered from subglacial sediment, cryoconite sediment and snow debris. Representative OTUs for each library were binned at the order level for eukarya (a) and bacteria (b) and at the family level for archaea (c). Taxonomic bins that represented<5.0% of the total sequences from each assemblage were pooled and depicted as ‘Other'. Cryo, cryoconite sediment; Sub, subglacial sediment.