Effects Of Nitrogen Supply And Solar Radiation On Photophysiological Responses Of Diatoms

As a result of rising sea temperature,the upper mixed layer(UML)becomes shallower.Phytoplankton living in the upper mixed layer are stressed by multiple environmental factors such as insufficient supply of nutrients and increased solar radiation.At the same time,the enhanced ultraviolet radiation caused by ozone depletion also changes the light exposure environment of phytoplankton.Diatoms are the main primary producers in the marine environment,and participate in a variety of biogeochemical cycles and play an important role in ecology.At present,most studies were concentrated on the effects of single environmental factor on the photosynthesis of diatoms and the systematic and thorough research of coupling effect between nutrient restriction and solar radiation was not enough.In this study,we incubated two diatom species,Thalassiosira pseudonana and Thalassiosira weissflogii,to investigated the photosynthetic response of diatoms under the interaction of nitrogen limitation and high visible light or ultraviolet radiation,and provided a theoretical basis to explain how multiple marine environmental pressures affect phytoplankton photosynthesis.The main results of research are as the follows:Compared with T.pseudonana,T.weissflogii could maintain photosynthetic activity for a longer time under N starvation.The electron transfer of both diatoms was inhibited and the rapid closure of reaction centers promoted the production of Q_Bnon-reducing PSII centers,which promoted the rapid induction of NPQ,but the degree of N starvation decided the induction of NPQ.N starvation intensified the photoinhibition of high visible light.T.pseudonana had a higher?i value and was more sensitive to high visible light,but its PSII repair rate was also higher.On the contrary,T.weissflogii had a lower?i value and was more tolerant to high visible light,but the tolerance of high visible light was significantly reduced under N starvation.Compared with visible light,UV radiation inhibited the photosynthetic activity of two diatoms more seriously and the Fv/Fm of T.pseudonana and T.weissflogii respectively decreased by 44.5%and 23.3%,which indicated T.pseudonana was more sensitive to UV radiation.Coupled with N limitation,UV photoinhibition of both species of diatoms was further intensified,and the Fv/Fm of T.pseudonana and T.weissflogii respectively decreased by 64.1%and 63.9%,which indicated the tolerance of T.weissflogii to UV radiation was significantly reduced by N limitation.NPQ was induced by UV radiation,which was further increased when coupled with N limitation and the NPQ of T.pseudonana was higher than T.weissflogii.The UV radiation increased the net closure rate of reaction centers,absorbed light energy and dissipated energy per reaction center.Furthermore,the electron transfer from Q_A^-to Q_Bwas inhibited and the electron transfer rate decreased,and N limitation further intensifies the inhibition effect of UV radiation on electron transport of two diatoms.The decline of rETR_max,?and E_k further indicated that the number of reaction centers,efficiency for solar energy utilization and acceptable light radiation decreased significantly.