The deep sea, roughly defined as >200 m water depth, is the largest and least explored habitat on the surface of our planet. It covers nearly two thirds of the Earth’s surface, contains ~75% of ocean waters, and is home to the majority of all marine microorganisms. Deep-sea microbes have the potential to play important roles in biogeochemical cycling and climate, including by producing and consuming greenhouse gases (e.g., CO2, CH4, and N2O), attenuating the biological pump via remineralization of photosynthetically-derived organic matter, and producing, storing, and transporting nutrients that fuel surface waters in upwelling regions. However, the microbial ecology and activity of deep-sea water and sediment is sorely understudied relative to that of the surface ocean. In this talk I will explore the diversity and activity of microbes in this realm, with a focus on two critical metabolisms: nitrogen fixation (conversion of N2 to NH3) and chemoautotrophy (conversion of inorganic carbon to biomass).  Organisms mediating these processes unlock a reservoir of nitrogen and carbon, respectively, that is inaccessible to most microorganisms, and therefore play an outsized role in microbial community structure and function. Combining field sampling, metagenomics, metatranscriptomics, and stable isotope probing experiments, we explore the who, where, how, how much, and so what of these metabolisms in the dark ocean. 

Lunch served.