High Diazotrophic Diversity but Low N2 Fixation Activity in the Northern Benguela Upwelling System Confirming the Enigma of Nitrogen Fixation in Oxygen Minimum Zone Waters

Oxygen minimum zones (OMZs) have been suggested as a suitable niche for the oxygen-sensitive process of biological fixation dinitrogen (N 2 ) gas. However, most N rates reported from such waters are low. This low activity has proposed to result unusual community fixers, in which cyanobacteria were typically underrepresented. The Northern Benguela Upwelling System (North BUS) is part one productive marine ecosystems and hosts well-developed OMZ. Although previous observations indicated absent rates, composition diazotrophs needed understand North BUS not described. Here, we present first detailed analysis diazotrophic diversity OMZ Angola tropical zone (ATZ), based on genetic data isotope speciation. Consistent with study, detected slight deficit OMZ, but did indicate any active or past fixation. diazotroph was dominated by non-cyanobacterial microbes clustering members gamma-proteobacteria, typical other regions. found strikingly high Cluster III yet described OMZs. In contrast observations, could also identify clades Trichodesmium sp., UCYN-A Cyanothece surface connected above potentially shown presence genes transcripts key functional marker gene fixation, nifH . While detection absence (based isotopic speciation) consistent regional variation that still far understanding role OMZs, which, however, relevant cycle waters, well predicting future development biogeochemistry changing ocean.