The important role of the antioxidant stress capacity in the response of Prochlorococcus to increased CO2 under varying iron and light conditions

• Low-light-adapted Prochlorococcus ecotypes have stronger low-iron adaptation capacity • Fe limitation in Prochlorococcus is enhanced under both low growth-limiting light and high photo-inhibitory light • High CO 2 promotes the growth of low-light-adapted Prochlorococcus ecotypes due to a reduction in cellular oxidation stress Ocean acidification caused by the ongoing increase in atmospheric carbon dioxide (CO 2 ) is expected to impact the growth of marine phytoplankton. Additionally, CO 2 -driven climate change influences light intensity and iron (Fe) availability in surface seawaters, two critical factors for marine phytoplankton carbon fixation and growth due to their central role in regulating photosynthesis. The cyanobacterium Prochlorococcus often dominates marine productivity in oligotrophic oceans with low but variable Fe concentrations and light intensities. However, the combined effects of light intensity, Fe availability and CO 2 concentration on the growth and photosynthesis of Prochlorococcus remain unclear. In this study, we found that the high-light-adapted Prochlorococcus strain MED4, isolated from shallower depths, required much higher Fe concentrations and light intensities to grow than the low-light-adapted strain NATL1A, isolated from deeper depths. Increased CO 2 had no effect on the growth of strain MED4 under any light or Fe conditions. In contrast, increased CO 2 caused a 29% increase in the growth of strain NATL1A under low Fe coupled with high photo-inhibitory light condition, owing to a reduction in cellular oxidative stress. The varying antioxidant stress capacities of different Prochlorococcus strains appeared to influence their responses to increased CO 2 . These results indicate complex interactions among light intensity, Fe limitation, and CO 2 concentration, which may affect the species distributions and productivities of marine phytoplankton, including Prochlorococcus , in a future high-CO 2 ocean.