Dark N2 fixation: nifH expression in the redoxcline of the Black Sea

Fixed nitrogen is a limiting nutrient in many marine environments. Only a subset of the microbial community has the ability to fix dinitrogen gas (N2). Here, we document the transcription of nitrogenase reductase subunit nifH in N2-fixing bacteria in the dark suboxic and uppermost sulfidic layers of the northeast Black Sea. In shallower oxic waters, no N2 fixation activity was detected using mRNA, and stable isotopes (δ15N and δ18O) of nitrate supported the lack of N2 fixation in oxic waters. On 2 expeditions in 2007, sampling in the suboxic zone (O2