Effects of Ocean Acidification and UV Radiation on Marine Photosynthetic Carbon Fixation

Abstract

The oceans absorb anthropogenically released CO2 at a rate of more than one million tons per hour, which causes a pH decrease of seawater and results in ocean acidification (OA). The effect of OA and absorption of CO2 via the biological carbon pump driven by marine photosynthesis has drawn increasing attentions. As a consequence, there are numerous studies on influences of OA on primary producers, and the effects on photosynthetic carbon fixation are still under debate. OA can promote the growth of diatoms at low PAR irradiances and inhibit it at high PAR. Besides, OA may influence metabolic pathways of phytoplankton, upregulating β-oxidation, and the tricarboxylic acid cycle, resulting in increased accumulation of toxic phenolic compounds. In parallel, phytoplankton cells in the upper mixed layer are affected by intense PAR and UV radiation (UVR). The calcareous layers of calcified algae, which have been shown to shield the organisms from UVR, are thinned due to OA, exposing the cells to increased UVR and further inhibiting the calcification. Therefore, effects of OA and UV on marine photosynthetic carbon fixation could be compounded. While the photosynthetic carbon fixation is controlled by other environmental stressors in addition to OA and UV, such as nutrients limitation and warming, combined effects of OA and UV have been less considered. In this review, we synthesize and analyze recent advances on effects of OA and UV and their combined effects, implying that future studies should pay special attentions to ecological and physiological effects of OA in the presence of solar UV irradiance to reflect more realistic implications. The ecophysiological effects of OA and/or UV and their mechanisms in complex environments should be further explored.