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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 < 10 μM, H2S below detection) and in deeper sulfidic waters yielded mRNA transcripts of nifH, even though NH4 was 1−5 μM. Multiple phylogenetic groups expressed nifH. Three uncultured groups of Cluster III type transcripts were detected, as well as 2 groups of Cluster I type sequences related to known sulfur oxidizers in the ε-proteobacteria and Halorhodospira. The depth range where N2 fixation was found was also the depth range of chemoautotrophic production, as determined by a maximum in suspended organic nitrogen concentrations and from 16S rRNA at these depths, which was dominated by known chemoautotrophs Sulfurimonas, SUP05, and BS-GSO2. We suggest chemoautotrophy and competition with chemoautotrophs for ammonium as reasons for N2 fixation in the presence of ammonium. Profiles of N2 gas unequivocally show the importance of N loss in the suboxic zone of the Black Sea; however, our data suggest a role for N2 fixation. These results suggest that N cycling is seldom unidirectional.

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