Influence of lithogenic energy on subglacial microbial community composition
Dunham, Eric Corwin
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Chemosynthesis, the generation of biomass using chemical energy, supported life on early Earth and continues to sustain contemporary light-independent ecosystems. The mechanisms of nutrient release from the geosphere are critical to understanding the present and historical distribution and diversity of life. Glaciers release such nutrients through comminution of bedrock, continuously resurfacing reactive minerals that can be colonized and exploited by chemosynthetic microorganisms. Bedrock mineralogy influences the nutrients available in these environments, but little is known about which nutrients are most important or how they affect microbial community composition, particularly in catchments overlying igneous bedrock like basalt. Iron and silicate minerals, common in basalt, readily generate both reductants such as H 2 and oxidants such as Fe(III) through interactions with water. Abundant H 2 in meltwaters of the basalt-based Icelandic glacier Kotlujokull (KJ) were found to support sediment microbial communities better adapted to use H 2 in chemosynthetic metabolism than those found beneath the carbonate-based Robertson Glacier (RG), Canada. KJ communities exhibited shorter lag-times and faster rates of net H 2 oxidation and dark carbon dioxide (CO 2) fixation than those from RG. A KJ sediment enrichment culture provided with H 2, CO 2, and Fe(III) produced a chemolithoautotrophic population related to Rhodoferax ferrireducens, which was also detected using molecular techniques in sediments from Kaldalonsjokull (Kal), another basalt-based Icelandic glacier. The abundance and composition of microbial communities that colonized defined minerals incubated for 12 months in Kal meltwater streams were examined by extracting DNA and sequencing PCR-amplifiable 16S rRNA genes. DNA quantities and the composition of 16S rRNA genes recovered from Kal sediments were most similar to those recovered from incubated Fe(III)-bearing minerals hematite and magnetite, with putative Fe(III) reducers dominating all three communities. These findings point to the importance of bedrock mineral composition in influencing the supplies of nutrients like H 2 and Fe(III) that, in turn, influence the diversity, abundance, and activity of microbial communities in subglacial environments. They further indicate the potential for subglacial habitats to serve as refugia for microbial communities in the absence of sunlight, such as during Snowball Earth episodes, or on icy planets without photosynthetic life.