Nitrogen (N) is a key component of fundamental biomolecules. Hence, the cycling and availability of N is a central factor governing the extent of ecosystems across the Earth. Although hosting the largest habitable environment on the planet, the deep marine biosphere, including those environments below the seafloor in sediments and within the ocean crust, remain poorly understood – in particular with respect to the sources and cycling of nitrogen that sustain these microbial communities.
In the ocean crust – warm fluids move quickly through permeable basalt – allowing the delivery of nutrients and electron acceptors and donors necessary for sustaining life. We are collaborating with several groups that are sampling crustal fluids in a variety of contexts (hydrothermal vents, off-axis boreholes, volcanoes etc.) in order to better understand the nature of nitrogen transformations in these under-explored reaches of our planet.
In the organic-lean sediment porewaters underlying the vast expanses of the oligotrophic ocean, where low levels of microbial activity persist despite limited organic matter delivery from overlying water, the extent and modes of nitrogen transformations have not been widely investigated. We are interested in determining what nitrogen sources are available and what nitrogen transformations and metabolisms are active in these remote and ‘extreme’ environments. To answer these questions, we have been measuring the N and O isotopes of nitrate and nitrite in porewaters of the North Atlantic and developing a predictive reaction-diffusion model for constraining rates and modes of key nitrogen transformations. Our findings indicate that the production of organic matter by in situ autotrophy (e.g., nitrification, nitrogen fixation) supplies a large fraction of the biomass and organic substrate for heterotrophy in these sediments, supplementing the small organic matter pool derived from the overlying euphotic zone. (Center for Dark Energy Biosphere Investigations)