ECIMMEE Project: Physiological Diversity and Environmental Sensitivities of Benthic Marine Nitrogen Fixation
Much of the marine biosphere is constrained by the amount of fixed nitrogen, making biological nitrogen fixation — the metabolically expensive, prokaryotic process that reduces inert, atmospheric nitrogen gas (N2) into bioavailable ammonia (NH3) — of critical importance to the ecology and function of the ocean. The vast majority of marine N2 fixation studies focus on pelagic N2 fixers (diazotrophs). Several recent studies showing N2 fixation by benthic diazotrophs, often in the presence of abundant fixed nitrogen, imply that benthic N2 fixation could be a greater contributor to oceanic fixed nitrogen inventories than previously thought. However, the physiology and environmental sensitivities of benthic diazotrophs are not well understood. The proposed research seeks to establish the ecophysiologies of important sediment diazotrophs using approaches from molecular ecology, microbial physiology and stable isotope geochemistry. Specific goals include determining how N2 fixation by sulfate reducing and fermenting diazotrophs in pure cultures and sediment mesocosms responds to fixed nitrogen, electron donor and acceptor availability, and sediment heterogeneity. Preliminary results from cultures and coastal sediment mesocosms indicate that N2 fixation by benthic diazotrophs shuts down even at low to moderate fixed N concentrations, adding complexity to an emerging picture of benthic N2 fixation that has thus far focused on characterizing BNF as a process relatively insensitive to fixed nitrogen. To resolve the question of whether, why and where benthic N2 fixation occurs, we are applying experimental and modeling approaches to determine the physiological and biophysical conditions that constrain N2 fixation in benthic diazotrophs. This research, spanning both pure culture and field arenas, will better constrain nitrogen budgets at local and regional scales and improve knowledge of the physiological purpose, metabolic cost, and environmental sensitivity of N2 fixation in marine systems and beyond.
Xinning Zhang is an environmental microbiologist jointly appointed in the Department of Geosciences and the Princeton Environmental Institute. Zhang’s research aims to provide a mechanistic understanding of how microbial metabolism influences biogeochemical cycling and climate in modern and ancient environments. To determine the key parameters explaining microbial activities and their signatures in the rock record, she applies tools from microbial physiology, molecular ecology and stable isotope geochemistry to lab and field research. She earned her Ph.D. in environmental science and engineering from the California Institute of Technology and a B.S. in biological and environmental engineering from Cornell University.