AME 82:43-58 (2018)  -  DOI: https://doi.org/10.3354/ame01882

ABSTRACT: 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.

KEY WORDS: N₂ fixation · nifH · Black Sea · N cycling · Redox gradient

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Cite this article as: Kirkpatrick JB, Fuchsman CA, Yakushev EV, Egorov AV, Staley JT, Murray JW (2018) Dark N2 fixation: nifH expression in the redoxcline of the Black Sea. Aquat Microb Ecol 82:43-58. https://doi.org/10.3354/ame01882 Export citation Share:    Facebook - - linkedIn