Enhancement of diatom growth and phytoplankton productivity with reduced O 2 availability is moderated by rising CO 2 .

Many marine organisms are exposed to decreasing O 2 levels due to warming-induced expansion of hypoxic zones and ocean deoxygenation (DeO 2 ). Nevertheless, effects of DeO 2 on phytoplankton have been neglected due to technical bottlenecks on examining O 2 effects on O 2 -producing organisms. Here we show that lowered O 2 levels increased primary productivity of a coastal phytoplankton assemblage, and enhanced photosynthesis and growth in the coastal diatom Thalassiosira weissflogii. Mechanistically, reduced O 2 suppressed mitochondrial respiration and photorespiration of T. weissflogii, but increased the efficiency of their CO 2 concentrating mechanisms (CCMs), effective quantum yield and improved light use efficiency, which was apparent under both ambient and elevated CO 2 concentrations leading to ocean acidification (OA). While the elevated CO 2 treatment partially counteracted the effect of low O 2 in terms of CCMs activity, reduced levels of O 2 still strongly enhanced phytoplankton primary productivity. This implies that decreased availability of O 2 with progressive DeO 2 could boost re-oxygenation by diatom-dominated phytoplankton communities, especially in hypoxic areas, with potentially profound consequences for marine ecosystem services in coastal and pelagic oceans.