Effects Of Solar UV Radiation, Ocean Warming And Nitrogen Limitation On Photophysiology Of Diatom Species

Ocean water temperature rise caused by global warming,leads to the upper mixed layer becomes shallow,nutrient limitation increase,and an increased level of phytoplankton by solar radiation.And because the ozone depletion caused by ultraviolet radiation enhance further change of phytoplankton by light environment.These changes in environmental factors will be various influence on phytoplankton.Diatom,a key taxon of the unicellular photosynthetic eukaryotes,contribute to 40%of the marine primary production,influencing greatly the biogeochemical cycling of carbon,nitrogen and silicon.The coupling effect of UV radiation and temperature of sea water rise and nitrogen nutrient limitation will be what kind of effect on diatom photosynthesis,has not yet been fully understand.Therefore,this study choose two diatom species,Thalassiosira pseudonana and Thalassiosira weissflogii as the research object,and discusses the photosynthetic responses of these diatoms to the coupling effect of UV radiation and temperature of sea water rise and nitrogen nutrient limitation.The main results are as the follows:UV radiation significantly inhibit the photosynthetic activity of T.pseudonana,increase of sea water to 24^oC relieves the inhibition of UV radiation on F_v/F_m in T.pseudonana,but not for T.weissflogii.Non photochemical quenching?NPQs?was induced by UV radiation in T.pseudonana,but was decreased by elevated temperatue of24^oC.In contrast,it did not show significant induction by UVR and PAR in T.weissflogii.The PsbA removal rate(K_PsbA,s^-1)was increased significantly in both diatom species under UV radiation,T.weissflogii showed much higher KPsbA than T.pseudonana.Although the temperatue rise and UV radiation increased the repair rate(K_rec,s^-1)of T.pseudonana,it is still lower than that in T.weissflogii.Under nitrogen limitation conditions,the photosynthetic activity in both T.pseudonana and T.weissflogii were decreased significantly by PAR and UVR,while the combination of nitrogen limitation and UV radiation intensified the photoinhibition of the two diatom species.The PsbA removal rate(K_PsbA,s^-1)in both species are increased significantly under UV radiation,when coupled with nitrogen limitation,K_PsbAsbA decreased significantly in T.weissflogii.T.weissflogii has higher K_rec value in nitrogen repletion conditions,it was reduced by UV radiation,while K_rec value decreased significantly under nitrogen limitation conditions for both diatoms.T.pseudonana has higher SOD activity but T.weissflogii has higher CAT activity when treated with UV radiation.Under different mixing rates,F_v/F_m was decreased significantly in T.pseudonana by UV radiation,the synthesis of PsbA and PsbD protein were inhibited.Increased irradiance levels led to furher inhibition.However the Rubisco content was not changed significantly.In summary,diatoms response differently to the individual and interactive effect of UV radiation,ocean warming and nitrogen limitation.Elevated temperatue alleviates the inhibition of UV radiation on photosynthesis in T.pseudonana;And nitrogen limitation exacerbated the inhibition of UV radiation to both diatom species,especially for T.weissflogii.However,further research is required to investigate the comprehensive influences of various environment factor on diatoms in the natural marine environment.