Comparison of microbial community compositions of two subglacial environments reveals a possible role for microbes in chemical weathering processes

Abstract

Viable microbes have been detected beneath several geographically distant glaciers underlain by different lithologies, but comparisons of their microbial communities have not previously been made. This study compared the microbial community compositions of samples from two glaciers overlying differing bedrock. Bulk meltwater chemistry indicates that sulfide oxidation and carbonate dissolution account for 90% of the solute flux from Bench Glacier, Alaska, whereas gypsum/anhydrite and carbonate dissolution accounts for the majority of the flux from John Evans Glacier, Ellesmere Island, Nunavut, Canada. The microbial communities were examined using two techniques: clone libraries and dot blot hybridization of 16S rRNA genes. Two hundred twenty-seven clones containing amplified 16S rRNA genes were prepared from subglacial samples, and the gene sequences were analyzed phylogenetically. Although some phylogenetic groups, including the Betaproteobacteria, were abundant in clone libraries from both glaciers, other well-represented groups were found at only one glacier. Group-specific oligonucleotide probes were developed for two phylogenetic clusters that were of particular interest because of their abundance or inferred biochemical capabilities. These probes were used in quantitative dot blot hybridization assays with a range of samples from the two glaciers. In addition to shared phyla at both glaciers, each glacier also harbored a subglacial microbial population that correlated with the observed aqueous geochemistry. These results are consistent with the hypothesis that microbial activity is an important contributor to the solute flux from glaciers.

FIG. 1.

Locations of sampling sites. (A) Bench Glacier. Ice topography from a Global Positioning System survey (39) is shown, with shading where the topography was not determined. Contour intervals are 10 m on ice and 100 m off ice. (B) John Evans Glacier. The contour interval is 100 m on ice. Letters indicate sampling sites. A detailed description of the sampling sites is in Materials and Methods.

FIG. 2.

Phylogenetic analysis of proteobacterial 16S rRNA gene clones from JEG and BG. This phylogenetic tree was rooted with Bacteroides fragilis (GenBank accession number M61006). A mask of 917 nucleotides, including all nonambiguously aligned positions, spanning nearly the full length of the 16S rRNA gene, was included. The clusters which the Com1212 and Gall217 probes were designed to detect are indicated. Bootstrap values (100 replications) generated by the neighbor-joining method are shown above relevant nodes, and those generated by maximum-parsimony analysis are shown below the nodes. Only bootstrap values above 50 are shown. Sequences from formally named isolates are in italic, sequences from environmental gene clones and unnamed isolates are in plain text, and sequences from the subglacial samples are in boldface. GenBank accession numbers of the sequences from other studies are included.

FIG. 3.

Phylogenetic analysis of Cytophaga/Flavobacterium/Bacteroides 16S rRNA gene clones from JEG and BG. This phylogenetic tree was rooted with Escherichia coli (GenBank accession number J01695). A mask of 1,074 nucleotides, including all nonambiguously aligned positions, spanning nearly the full length of the 16S rRNA gene, was included. Bootstrap values (100 replications) generated by the neighbor-joining method are shown above relevant nodes, and those generated by maximum-parsimony analysis are shown below the nodes. Only bootstrap values above 50 are shown. Sequences from formally named isolates are in italics, sequences from environmental gene clones and unnamed isolates are in plain text, and sequences from the subglacial samples are in boldface. GenBank accession numbers of the sequences from other studies are included.