Elevated CO₂ modulates the physiological responses of Thalassiosira pseudonana to ultraviolet radiation

Diatoms account for a large proportion of marine primary productivity, they tend to be the predominant species in the phytoplankton communities in the surface ocean with frequent and large light fluctuations. To understand the impacts of increased CO₂ on diatoms' capacity in exploitation of variable solar radiation, we cultured a model diatom Thalassiosira pseudonana with 400 or 1000ppmv CO₂ and exposed it to high photosynthetically active radiation (PAR) alone or PAR plus ultraviolet radiation (UVR) to examine its physiological performances. The results showed that the maximum photochemical efficiency (F_v/fm) was significantly reduced by high PAR and PAR + UVR in T. pseudonana, UVR-induced inhibition on PSII activity was exacerbated by high CO₂. PSII activity drops coincide approximately with PsbA content in the cells exposed to high PAR or PAR + UVR, which was pronounced at high CO₂. The removal of PsbD in T. pseudonana cells declined under high CO₂ during UVR exposure, limiting the repair capacity of PSII. In addition, high CO₂ reversed the induction of energy-dependent form of NPQ by UVR to the increase of Y_(No), indicating the severe damage of the photoprotective reactions. Our findings suggest that the adverse impacts of UVR on PSII function of T. pseudonana were aggravated by the elevated CO₂ through modulating its capacity in repair and protection, which thereby would influence its abundance and competitiveness in phytoplankton communities.