Life in a toxic environment: How do extreme redox conditions affect oceanic N2 fixation?
Nitrogen (N) is a limiting element of life in the ocean. Nitrogen fixation, the biological reduction of dinitrogen gas (N2) to ammonium, is quantitatively the most important external source of new nitrogen to the ocean and mostly depends on the availability of a phosphorous (P) and iron (Fe) source. While oxygen (O2) depletion favors N2 fixation, the presence of hydrogen sulfide (H2S) under conditions of extreme anoxia may hinder that process by (i) a direct toxic effect on the diazotrophic community and (ii) immobilization of Fe through precipitation of Fe sulfide minerals. In order to explore the sensitivity of N2 fixation to changes in O2 and H2S, we propose an interdisciplinary field study at the time series station Boknis Eck, located in the Eckernförde Bay (www.bokniseck.de), serving as a natural laboratory. This allows an incomparably detailed monitoring of the diazotrophic response to naturally occurring extreme changes in redox conditions. We further aim to explore the potential of the responsible microbial community to adapt to rapidly changing redox conditions in a chemostat. The results are crucial to understand basic controls of N2 fixation and primary productivity over various geological timescales and to further predict them in a changing ocean.
