| Summary: | The anthropogenic increase in atmospheric carbondioxide already had severe consequences, including the melting of the polar caps, ocean acidification, and a decrease of dissolved oxygen (O2).The latter is most obvious in the tropics, where oxygen minimum zone (OMZ) waters were shown to expand and intensify. Still, the response of microbes to this progressive deoxygenation is not understood, yet. We explored microbialoxygen sensing, detoxification of reactive oxygen species, and formation of microbial consortia in the OMZ off Peru, which is the largest and one of the most intense OMZs, using a metagenomic approach. Our study revealed increased expression of the key functional marker for ROS scavenging, KatG., in the OMZ core. As only traces of oxygen were possibly left there, we interpret an extreme sensitivity of the respective organisms to O2 or other oxygen-containing molecules. More importantly we identified a dependency of glycosyltransferase activity as an indicator for formation of microbial consortia on O2 levels. Microbial consortia on marine snow are of critical importance for oxygen consumption, export and consumption of organic material in OMZ waters. Our results suggest however that microbial consortia in OMZs need O2 to develop. Given the ongoing deoxygenation of OMZ waters, a loss in marine snow-bound microbial consortia may occur, which in turn may have decrease nutrient remineralization and respiration in OMZs and could thus be regarded as a feedback mitigating OMZ expansion.
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