Short-term elevated CO₂ exposure stimulated photochemical performance of a coastal marine diatom

Ocean acidification changes seawater chemistry, with increased CO₂ and decreased pH regarded as the most important factors that impact marine organisms. This study employed an unconventional methodology to distinguish the independent effects of pH versus CO₂. Changes in CO₂ dominated the photochemical responses of the coastal diatom Phaeodactylum tricornutum to short-term ocean acidification. Increased CO₂ lowered non-photochemical quenching of excitation and stimulated the electron transport rates of photosynthesis, with the largest effects on both parameters when CO₂ and pH were altered simultaneously. Changes in pH alone did not show significant effects upon non-photochemical quenching (NPQ) nor upon electron transport rates, but can synergistically amplify CO₂ effects under low light. Maximal induction of NPQ after illumination showed only a limited response to increasing CO₂ under stable pH, across a range of increasing light levels, but maximal induced NPQ declined rapidly with increasing CO₂ under variable pH, when measured under exposure to sub-saturating light, but not under saturating light. These findings show that aqueous CO₂ and pH affect different physiological processes independently or interactively, which should be taken into account in future research for better understanding of responses to ocean acidification at the mechanistic level.