Enhancement of diatom growth and phytoplankton productivity with reduced O2 availability is moderated by rising CO2

Sun et al. investigate the effects of current ambient and potential future oxygen levels on phytoplankton growth and photosynthesis with field observations and mesocosm and lab experiments. Their results demonstrate positive effects of low O-2 on phytoplankton growth, photosynthesis, and inorganic carbon acquisition at current and future high levels of CO2. 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 CO2 concentrating mechanisms (CCMs), effective quantum yield and improved light use efficiency, which was apparent under both ambient and elevated CO2 concentrations leading to ocean acidification (OA). While the elevated CO2 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.

Automated summary

Sun et al. investigate the effects of current and potential future oxygen levels on phytoplankton growth and photosynthesis. Their results demonstrate positive effects of low oxygen on phytoplankton growth, photosynthesis, and inorganic carbon acquisition.