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.