Diversity, distribution, and expression of diazotroph nifH genes in oxygen-deficient waters of the Arabian Sea

The Arabian Sea oxygen minimum zone (OMZ), the largest suboxic region in the world's oceans, is responsible for up to half of the global mesopelagic fixed nitrogen ( N ) loss from the ocean via denitrification and anammox. Dinitrogen ( N₂ ) fixation is usually attributed to cyanobacteria in the surface ocean. Model prediction and physiological inhibition of N₂ fixation by oxygen, however, suggest that N₂ fixation should be enhanced near the oxygen-deficient zone (ODZ) of the Arabian Sea. N₂ fixation and cyanobacterial nifH genes (the gene encoding dinitrogenase reductase) have been reported in surface waters overlying the Arabian Sea ODZ. Here, water samples from depths above and within the Arabian Sea ODZ were examined to explore the distribution, diversity, and expression of nifH genes. In surface waters, nifH DNA and cDNA sequences related to Trichodesmium, a diazotroph known to occur and fix N₂ in the Arabian Sea, were detected. Proteobacterial nifH phylotypes (DNA but not cDNA) were also detected in surface waters. Proteobacterial nifH DNA and cDNA sequences, as well as nifH DNA and cDNA sequences related to strictly anaerobic N -fixers, were obtained from oxygen-deficient depths. This first report of nifH gene expression in subsurface low-oxygen waters suggests that there is potential for active N₂ fixation by several phylogenetically and potentially metabolically diverse microorganisms in pelagic OMZs.