Hidden biosphere in an oxygen-deficient Atlantic open-ocean eddy: future implications of ocean deoxygenation on primary production in the eastern tropical North Atlantic

摘要

The eastern tropical North Atlantic (ETNA) is characterized by a highlyproductive coastal upwelling system and a moderate oxygen minimum zone withlowest open-ocean oxygen (O₂) concentrations of approximately 40 μmol kg?1. The recent discovery of re-occurring mesoscale eddies withclose to anoxic O₂ concentrations (< 1 μmol kg−1)located just below the mixed layer has challenged our understanding ofO₂ distribution and biogeochemical processes in this area.Here, we present the first microbial community study from a deoxygenatedanticyclonic modewater eddy in the open waters of the ETNA. In the eddy, weobserved significantly lower bacterial diversity compared to surroundingwaters, along with a significant community shift. We detected enhancedprimary productivity in the surface layer of the eddy indicated by elevatedchlorophyll concentrations and carbon uptake rates of up to three times ashigh as in surrounding waters. Carbon uptake rates below the euphotic zonecorrelated to the presence of a specific high-light ecotype ofProchlorococcus, which is usually underrepresented in the ETNA. Our data indicate that highprimary production in the eddy fuels export production and supports enhancedrespiration in a specific microbial community at shallow depths, below themixed-layer base. The transcription of the key functional marker gene for dentrification, nirS, further indicated a potential for nitrogen loss processes in O₂-depleted core waters of the eddy. Dentrification is usuallyabsent from the open ETNA waters.In light of future projected ocean deoxygenation, our results show that evendistinct events of anoxia have the potential to alter microbial communitystructure with critical impacts on primary productivity and biogeochemicalprocesses of oceanic water bodies.