Aerobic microbial activity in deep sea abyssal clay

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Marine sediments are estimated to contain >10^29 microbial cells, which extend as far as 2,500 meters below the seafloor (mbsf). Microbial cells in these very stable and ancient settings catabolize up to one million times more slowly than model organisms in nutrient-rich cultures and are estimated to grow on millennial-timescales rather than hours to days. Because of the extremely low rates of activity, it is challenging to study the specific activities of subseafloor microbes. However, the transcriptional activity of subseafloor microbial communities can now be studied via metatranscriptomics, i.e. applying high-throughput DNA sequencing to actively transcribed message RNA (mRNA) extracted from sediment samples. Abyssal clays exhibit oxygen penetration to basement, which is due to low sedimentation rates in the ultra-oligotrophic ocean. Oxygen consumption is driven by slowly respiring microbial communities, whose cell numbers and rates of respiration are kept extremely low by low amounts of organic matter deposited from the overlying ultra oligotrophic ocean. The cellular mechanisms underlying the long-term subsistence of these aerobic microbial communities remain unknown. In 2014, an expedition to the North Atlantic successfully cored oxygenated deep-sea abyssal clay. Preliminary metatranscriptomic analyses of these unique and rare samples demonstrate that the metatranscriptomic approach can be successfully applied to the aerobic microbial communities in these abyssal clays. We thus propose to apply this method with a high level of replication, in 300 samples from four locations. This effort will enable us to test hypotheses regarding subsefloor cellular activities with unprecedented statistical support. We will determine aerobic metabolism supporting long-term subsistence in abyssal clay, identify subsistence strategies in aerobic and anaerobic subseafloor communities, and characterize secretomes and their potential for organic matter degradation. This will address the questions: How does life survive in the subsurface over geological timescales under aerobic conditions? What are the ubiquitous and unique mechanisms promoting long-term cellular survival under aerobic and anaerobic conditions? What are the effects of sediment depth, and organic matter availability, on microbial production of extracellular hydrolases under aerobic and anaerobic conditions? This will provide a better understanding of how microbial activities are distributed in the subseafloor and the deep biospheres role in biogeochemical cycles, as well as improve our understanding of how life survives over geological timescales under extreme energy limitation.