Abstract

The Oxygen Minimum Zones (OMZs) along the Pacific tropical coasts of the Americas are characterized by a 20 to 120 m thick surface layer with high primary productivity and an up to 600 m thick anoxic core. It is generally assumed that primary production and nitrification occurring in the upper oxic zone fuel the NO3 and NO2-dependent anaerobic metabolisms in the anoxic core, which results in massive nitrogen loss by denitrification and anammox. A Secondary Chlorophyll Maximum (SCM) is commonly found in these OMZs, located in the upper part of the anoxic core and composed mainly of Prochlorococcus sp. During two cruises to the Pacific OMZs, water from the SCM was collected and incubated in full glass vessels under conditions similar to those found in situ: dim bluish light and low-oxygen conditions. O2 production was monitored by high resolution STOX sensors and optodes at resolutions of 1-10 nM. Carbon fixation rates were simultaneously measured by stable or radioactive carbon isotopes. Our results show that oxygenic photosynthesis in the SCM supplies significant amounts of O2 to the otherwise anoxic environment. However, O2 concentrations remain below detection in situ, presumably due to a tight coupling between production and consumption by aerobic processes. Indeed, transcriptomic analysis of the microbial community in the SCM revealed the enhanced expression of genes for aerobic processes, such as NO2- oxidation. The photosynthetic rates in the SCM were similar to measured rates of nitrification in these OMZ regions, as well as rates of dissimilatory nitrate reduction in the OMZ core, suggesting a significant impact of SCM photosynthesis on OMZ biogeochemical cycling