Abstract
Abstract. Diversity and community composition of nitrogen (N) fixing microbes in the three main oxygen minimum zones (OMZs) of the world ocean were investigated using operational taxonomic unit (OTU) analysis of nifH clone libraries. Representatives of three of the four main clusters of nifH genes were detected. Cluster I sequences were most diverse in the surface waters, and the most abundant OTUs were affiliated with Alpha- and Gammaproteobacteria. Cluster II, III, and IV assemblages were most diverse at oxygen-depleted depths, and none of the sequences were closely related to sequences from cultivated organisms. The OTUs were biogeographically distinct for the most part – there was little overlap among regions, between depths, or between cDNA and DNA. In this study of all three OMZ regions, as well as from the few other published reports from individual OMZ sites, the dominance of a few OTUs was commonly observed. This pattern suggests the dynamic response of the components of the overall diverse assemblage to variable environmental conditions. Community composition in most samples was not clearly explained by environmental factors, but the most abundant OTUs were differentially correlated with the obvious variables, temperature, salinity, oxygen, and nitrite concentrations. Only a few cyanobacterial sequences were detected. The prevalence and diversity of microbes that harbor nifH genes in the OMZ regions, where low rates of N fixation are reported, remains an enigma.
Highlights
Nitrogen fixation is the biological process that introduces new biologically available nitrogen (N) into the ocean and, constrains the overall productivity of large regions of the ocean where N is limiting to primary production
Because diazotrophs have an ecological advantage in Ndepleted waters, and because those conditions occur in the vicinity of oxygen minimum zones, due to the loss of fixed N by denitrification, it has been proposed that N fixation should be favored in regions of the ocean influenced by OMZs (Deutsch et al, 2007)
Phylogenetic analysis of the sequences from the Arabian Sea (AS), eastern tropical North (ETNP), and eastern tropical South Pacific (ETSP) have been reported separately in previous publications (Jayakumar et al, 2012; Jayakumar et al, 2017; Chang et al, 2019), but the sequences have been combined for additional global analyses here
Summary
Nitrogen fixation is the biological process that introduces new biologically available nitrogen (N) into the ocean and, constrains the overall productivity of large regions of the ocean where N is limiting to primary production. The symbiotic genus Candidatus Atelocyanobacterium thalassa (UCYN-A) These cyanobacterial species are widespread and have different biogeographical distributions (Moisander et al, 2010), they are restricted to sunlit surface waters, mainly in tropical or subtropical regions. It has been suggested that the energetic constraints on N fixation might be partially alleviated under reducing, i.e., anoxic, conditions (Großkopf and LaRoche, 2012). In response to these ideas, the search for organisms with the capacity to fix nitrogen has been focused recently in regions of the ocean that contain OMZs. That search usually takes the form of characterizing and quantifying one of the genes involved in the fixation reaction, nifH, which encodes the dinitrogenase reductase enzyme.
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