Program

Fixed nitrogen (N) is a limiting nutrient in many marine environments. The ability to fix N is confined to a subset of the microbial community, historically known in oceanic systems primarily as phototrophs. Here we document the activity of N fixing bacteria in the dark suboxic and anoxic layers of the Black Sea, known mainly as a site of N loss due to denitrification and anammox. Sampling in the suboxic zone (O₂ < 10 mM, H₂S below detection) and deeper sulfidic waters yielded mRNA transcripts of nitrogenase reductase (nifH), even though fixed dissolved inorganic N was detected. Multiple phylogenetic groups expressed nifH. Cluster III type transcripts related to sulfur cycling Deltaproteobacteria were detected as well as Cluster I type sequences related to Arcobacter and Halorhodospira. Profiles of dissolved inorganic N unequivocally show the importance of the suboxic zone for overall N loss; however, a mass and isotope balance suggests a role for N fixation even in the presence of fixed N. These results suggest that N cycling is seldom unidirectional.