Green Earth Sciences - 367 Panama St, Room 365
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Stanford University
*** Ph.D. Thesis/ Oral Defense ***
Microbial life on and below the seafloor: Trends in anabolic activity in space and deep time
Nicolette Meyer
Wednesday, May 11, 2:00 PM
Green 365/Hybrid
Department of Earth System Science
Advisor: Dr. Anne Dekas
The seafloor covers two thirds of our planet's surface and plays an essential role in maintaining planetary habitability and climate stability. In particular, microorganisms on and below the seafloor are the final gatekeepers of organic carbon burial, affecting oxygen production and carbon sequestration on geologic timescales. Although marine sediment archaea and bacteria are fundamental in Earth's elemental cycles, we know little about their anabolic activity rates, the physicochemical drivers of their metabolisms, the dominant players, and how their activity might change spatially and temporally.
In the research described in this dissertation, I set up around 840 microcosm experiments to measure rates of specific benthic microbial metabolisms (total anabolic activity, heterotrophy, inorganic carbon fixation, and nitrogen fixation) in two global oceans. I used both manipulative and correlative approaches to identify key physicochemical controls on activity, and extrapolated rates of activity through space and deep time. In chapter 1, I improved the quantitation of cell-specific anabolic activity rates by tracking the effect of sample preparation on cellular isotope enrichments. I found that sample preparation decreases isotope enrichments by up to 80% – much more than previously reported. I make recommendations for how to account for this effect experimentally and analytically. In chapter 2, I found that microbial anabolic activity in Pacific and Atlantic deep-sea sediments is primarily controlled by the availability of organic carbon and/or energy, not bioavailable nitrogen, and can be predicted by distance from shore. Using this insight, I demonstrated that shifting continental configurations in the last 400 million years might have significantly impacted benthic microbial activity, making a novel link between tectonics and microbiology. In chapter 3, I further explored carbon and energy limitation in surface sediments from Monterey Bay, California, and discovered that the microbial community is in fact limited by energy, not carbon. And in chapter 4, I found that the hydrothermally-influenced, deep marine biosphere at Guaymas Basin is active, and is exclusively composed of heterotrophs. But interestingly, these deep subsurface heterotrophs obtain ~5% of their carbon from inorganic sources. Together, marine benthic microorganisms live on the micron-scale, but they have outsized, long-term impacts on global biogeochemical cycles due to their sheer abundance and vast distribution.