Photosynthetic Responses Of Sargassum Horneri To Ultraviolet Radiation Under Different Nitrogen And Phosphorus Conditions

In recent years,the golden tide caused by drifting Sargassum horneri has caused serious ecological disasters in coastal areas of China.The drifting S.horneri on the ocean surface will be exposed to more ultraviolet radiation（UVR）than solid population.Additionally,eutrophication has increased the concentrations of inorganic nitrogen and phosphorus in seawater,which may significantly affect on the photosynthetic physiology response of S.horneri to UVR.However,its mechanism remains unclear.In this study,seawater with different nitrogen and phosphorus nutrient conditions was set up for continuous culture of drifting S.horneri for 6-7 days.The cultured algae were irradiated continuously for two hours exposure with three different light treatments（PAR,400-700 nm;PAR+UVA,320-700 nm;PAR+UVA+UVB,280-700 nm）,which were obtained by the full-wavelength solar simulator and different filter plates,and the photosynthetic parameters of algae were measured respectively.The results showed that:1.N concentration and UVR:After 2 h of radiation treatment,UVR significantly reduced the maximum photochemical quantum yield（F_v/F_m）andψ_O under LN and HN conditions.In addition,UVR also promoted the non-photochemical quenching（NPQ）and increased V_J and M_O.Compared with the initial state,PA treatment significantly increased the contents of chlorophyll a,Carotenoid and UV-absorbing compounds（UVACs）in the alga cultured in LN.The contents of chlorophyll a,carotenoid and ultraviolet absorbent（UVACs）in the alga cultured in LN were significantly increased under PA treatment compared with the initial state.And PAB also promoted the synthesis of two pigments,and increased V_J,M_O,ABS/RC and DI_O/RC.After continuous culture,HN environment significantly increased the pigment content in the algae.Compared with LN culture,the alga cultured in HN environment showed higher relative electron transfer rate(r ETR_max)after PAR irradiation,and PA radiation also had a higher promotion effect on pigment synthesis under HN culture.2.P concentration and UVR:UV radiation significantly reduced the F_v/F_m,ψ_O,electron transport efficiency（α）,r ETR_max and photosynthetic efficiency of the algae,with the effect of PAB treatment being more significant.Additionally,radiation treatment also increased NPQ,V_J,M_O,ABS/RC,DI_O/RC,ET_O/RC and respiration efficiency of algae,but had no significant effect on the pigment content and UVACs.Compared with LP environment,respiration efficiency decreased and photosynthesis efficiency increased under HP environment.HP environment reduced the production of NPQ in the algae treated with PA,and the algae cultured with HP exhibited higherαand r ETR_max and photosynthetic efficiency after treatment with PAB,while the algae cultured with HP showed lower efficiency for V_J,ABS/RC and DI_O/RC.3.N form and UVR:Compared with the other two radiation treatments,PAB radiation has greater damage on F_v/F_m of algae,and could promote the algae to synthesize more UVACs.In natural seawater and nitrate-enriched seawater,PAB radiation increased the content of soluble protein in algae,but the effect was not significant.Compared with ammonia nitrogen enrichment,nitrate nitrogen enrichment culture was more beneficial to improveαand r ETR_max,especially in the case of coupling with PAB.However,when coupled with PA radiation,the activity of PSII system is significantly changed,and V_J,M_O,ET_O/RC,ABS/RC and DI_O/RC were increased,whileψo is decreased.It was disadvantageous to the accumulation of soluble protein and soluble sugar.In addition,nitrate-enriched culture significantly increased NRA,while ammonia-enriched culture significantly increased NPQ.In conclusion,UV radiation can reduce the F_v/F_m and inhibit the photosynthetic physiology of algal,especially PAB radiation.The higher concentration of nitrogen and phosphorus nutrients can improve the photosynthetic capacity of algal and change the activity of PSII system,so as to weaken the inhibition of photosynthesis by radiation and improve the protection ability of S.horneri under UV stress.Different forms of nitrogen enrichment had different effects on the photosynthetic physiology of alga.Ammonia nitrogen enrichment culture can help S.horneri resist ultraviolet stress by promoting the induction of NPQ,while nitrate nitrogen enrichment culture has a more significant effect on the photosynthetic ability of algae,which is conducive to enhancing the photosynthetic ability of algae and weakening radiation damage to a greater extent.