| Study Abstract |
The eastern tropical North Atlantic (ETNA) is characterized by a highly productive coastal upwelling system and a moderate oxygen minimum zone with lowest open ocean oxygen (O2) concentrations of around 40 μmol kg-1. The recent discovery of reoccurring mesoscale eddies with exceptionally low O2 concentrations (<1 µmol kg-1) just below the mixed layer challenged our understanding of biogeochemical processes in this area. Here, we present the first metagenomic dataset from a deoxygenated anticyclonic modewater eddy in the open waters of the ETNA. We observed a significantly lower bacterial diversity in the eddy compared to surrounding waters, along with a marked community shift. We detected enhanced primary productivity in the surface layer of the eddy indicated by elevated chlorophyll concentrations and increased carbon uptake rates up to three times as high as in surrounding waters. Carbon uptake correlated to the presence of a specific high-light ecotype of Prochlorococcus, which is usually underrepresented in the ETNA. Our data indicate that high primary production in the eddy fuels export production and a microbial community responsible for enhanced respiration at intermediate depth. Progressively decreasing O2 concentrations in the eddy promote even denitrification in the O2-depleted core waters, a process usually absent from the open ETNA waters. This is supported by the qPCR analysis of nirS as key gene for denitrification in that area show unexpected high abundances inside the eddy, with ~3000 copies L-1 at the O2 minimum (~100 m) more than one order higher than outside the eddy (~100 copies L-1). As current studies suggest that these eddies occur more frequently than previously expected, respiration, accompanying acidification and potential fixed nitrogen loss may severely impact surrounding waters. |
