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.
(© 2022. The Author(s).)