Nitrogenases in Oxygen Minimum Zone Waters

Biological dinitrogen (N-2) fixation is the pathway making the large pool of atmospheric N-2 available to marine life. Besides direct rate measurements, a common approach to explore the potential for N-2 fixation in the ocean is a screening-based targeting the key functional marker gene nifH, coding for a subunit of the nitrogenase reductase. As novel sequencing techniques improved, our understanding of the diversity of marine N-2 fixers grew exponentially. However, one aspect of N-2 fixation in the ocean is often underexplored, which are the two alternative types of the key enzyme of N-2 fixation, the nitrogenase. Altogether there are three isoenzymes, the most common Mo-Fe nitrogenase Nif, the Fe-Fe nitrogenase Anf, and the V-Fe nitrogenase Vnf, which differ regarding their genetic organization, as well as their metal co-enzymes. While Mo is only available in the presence of at least traces of oxygen (O-2), V and Fe are available if O-2 is absent. Therefore, low O-2 and anoxic ocean environments could be an ideal place to explore the diversity of the different isotypes of the nitrogenases. Most phylogenetic studies, however, were only based on the functional marker gene nifH, encoding for a subunit of the Nif nitrogenase, and thus limited in representing the diversity of alternative nitrogenases. Here, we screened metagenomes and -transcriptomes from O-2 minimum zones off Peru, from the Bay of Bengal, and the anoxic Saanich Inlet to explore the diversity of genes involved in N-2 fixation. We identified genes related to all three nitrogenases, and a generally increased diversity as compared to our previous nifH based on studies from OMZ waters. While we could not confirm gene expression of alternative nitrogenases from our transcriptomic, we detected diazotrophs harboring the genetic potential for alternative nitrogenases. We suggest that alternative nitrogenases may not be used under conditions present in those waters, however, depending on trace metal availability they may become active under future ocean deoxygenation.

Automated summary

This study explored the diversity of genes involved in N-2 fixation by screening metagenomes and -transcriptomes from different ocean regions, identifying a generally increased diversity compared to previous studies based on the nifH gene. While gene expression of alternative nitrogenases could not be confirmed, diazotrophs harboring the potential for alternative nitrogenases were detected.