Fig. 7: Simplified illustration of low O2-enhanced CCMs activity and subsequently increased re-oxygenation due to global deoxygenation and/or in intruded hypoxic waters.
Reduced O2 levels enhance CO2 concentrating mechanisms (CCMs) and photosynthetic performance with increased light energy use efficiency in diatoms and phytoplankton assemblages (Figs. 2–6), promoting photosynthetic carbon fixation and O2 evolution. The enhanced photosynthesis and reduced mitochondrial and photorespiration (Figs. 5 and 6) can accelerate the re-oxygenation due to stimulated photosynthetic O2 evolution by up to 193–250% (based on net photosynthetic values of day 5 in Fig. 2 assuming that the photosynthetic quotient is 1.0). In natural environments, low O2-enhanced production of phytoplankton biomass makes them a more effective O2 source that may help to counteract the negative effects of hypoxia on heterotrophs. Black, red, and blue arrows indicate directions, increase and decrease, respectively.
