Reduced salinity interacts with ultraviolet radiation to alter photosystem Ⅱ function in diatom Skeletonema costatum

To investigate the effect of reduced salinity on diatoms’ capacity to cope with changing ultraviolet radiation(U VR) and photosynthetically active radiation(PAR),Skeletonema costatum was grown in a range of salinity(15,25,and 35).The photo system Ⅱ(PSⅡ) function was analyzed by increasing PAR and UVR to mimic a mixing event in turbulent waters.The re sults show that high UVR exposure significantly reduced PSII activity,especially in cells grown at low salinity.UVR,but not salinity,stimulated the ’removal’ rate of PSII protein PsbA.Salinity alone,in the range of 15 to 35,did not regulate PSⅡ acceptor region;however,the low salinity+UVR treatment decreased the energy flux for electron transport per PSⅡ reaction center in S.costatum.It showed that low salinity exacerbated the damaging effect of UVR on PSⅡ function in S.costatum by suppressing Psb A protein synthe sis and modifying the photochemistry of PSⅡ.Although higher catalase(CAT) activity and NPQs were induced,they were unable to prevent the combined damage effect of low salinity+UVR.Our findings indicate that reduced salinity and increased UVR potentially affect the abundance and distribution of S.costatum with the escalation of climate disturbances.