Effect of solar UV radiation on photosystem Ⅱ function in Thalassiosira pseudonana under different mixing rates

The marine diatom Thalassiosira pseudonana was selected as experimental material to investigate the impact of photosynthetically active radiation (PAR) and PAR plus ultraviolet radiation (UVR) on the function of photosystem Ⅱ(PSⅡ) with increasing irradiance levels at two different mixing rates. Results showed that, at slow mixing speed, the maximum quantum yield of PSⅡ(F_v/F_m), photosynthetic efficiency parameter (α), and maximum relative electron transport rate (rETR_max) were progressively inhibited with increasing irradiance levels in both PAR and PAR + UVR treatments. The decreased PSⅡ D1 protein content, which indicates less active PSⅡ reaction centers, led to the increase in energy fluxes for absorption (ABS/RC) and dissipation (DI₀/RC) per reaction center. In contrast, at fast mixing speed, F_v/F_m increased under PAR compared with that at slow mixing and D1 protein content was also enhanced. However, similar changes were shown in various photosynthetic parameters under UVR. These results indicate that fast mixing could alleviate the photoinhibition caused by high PAR but could not modify the effects of UVR.