MICROBES LIVING ON GEOLOGIC TIMESCALES: SINGLE-CELL MICROBIAL METABOLISM AND GROWTH RATES FROM SUBSURFACE ENVIRONMENTS (Invited Presentation)

Thus far, microorganisms appear in nearly every environment we have explored on Earth. They thrive across orders of magnitude differences in temperature, pressure, nutrient concentrations, and energy sources. In addition to their resilience and ubiquity, microbes have had a profound effect on our planet’s evolution and continue to drive the biogeochemical cycles of elements essential to life (e.g. carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur). This is especially true in the vast, rock-powered, subsurface environments where sunlight is not an available energy source. By interrogating subsurface environments, we can improve our understanding of life both under extreme conditions and its evolution and survival on geologic timescales. To characterize the activity of subsurface microorganisms, a series of stable isotope probing (SIP) experiments coupled to single-cell targeted nanometer-scale secondary ion mass spectrometry (NanoSIMS) were conducted from a subsea sedimentary (2 km below seafloor, Shimokita Peninsula) system on IODP Expedition 337. Through a combination of geochemical and genomic techniques, we found active metabolism of a thermally-adapted microbial assemblage representing some of the slowest direct measurements of environmental microbial activity. These results improve our understanding of the microbial role in water-rock reactions on Earth, with astrobiological implications for potential similar systems elsewhere.