Photosynthetic Responses Of Diatoms To Solar UV Radiation And Increased CO₂ Concentration

The increasing atmospheric CO2 and subsequent rise of pCO2,resulted in the warming of ocean and lower pH in seawater,which is called ocean acidification(OA).On the other hand,the depletion of ozone layer led to the increase of solar UV radiation reaching the earth.These environmental variations would bring about various potential effects on phytoplankton.Diatoms contribute to 40% of the marine primary producion and play a central role in the biogeochemical cycling of carbon and silicon.The photophysiological responses of diatoms to ocean acidification and UV radiation need to be explored.In this study,we used pH/pCO2 perturbation technique to cultivate diatom species Thalassiosira pseudonana and Thalassiosira weissflogii under two different pCO2 levels,representing the atmospheric pCO2 of present and the year around 2100(pCO2:1000 ppmv),to evaluate the impact of ocean acidificaton and/ or solar UV radiation on the photophysiological characteristics and growth of the two diatoms.The main results are as follows:The growth rate of T.pseudonana and T.weissflogii,particularly for T.weissflogii was enhanced by OA.OA did not change the PSII maximal quantum yield(Fv/Fm)and photosynthetic efficiency(?),but decreased the NPQ.OA also resulted in the increase of light saturation parameter(Ik)and maximum relative electron transport rate(rETRmax)in both species,and enhanced the content of PsbA and Rubisco in T.weissflogii.When cells were exposed to high light radiation,OA acclerated the photoinhibition and induced high NPQs.The small T.pseudonana showed high functional absorption cross section driving the photoinactivation of PSII(?i),indicating that it was more sensitive to high light.Different OA cultivation methods(400-1000ppmv;400-600-800-1000ppmv)did not bring about significant different effects on diatom cells.Increase of sea water to 24 oC enhanced the photoinactivation constant(Kpi)in both species,but OA alleviated this effect.Both species possesed high PsbA removal rate(KPsbA)under OA conditions,especially in T.weissflogii.Molar changes in PsbD content were similar in magnitude and in rate to changes in PsbA content when cells were shifted to high light,and under high CO2 conditions,both species showed a 1:1 ratio of subunits changes regardless of temperature.The diatoms induce NPQs when photoinactivation outruns removal of PsbA protein.Cells grown under OA conditions induced much more NPQs at given low ratio of Kpi to KPsbA than that cells under low CO2 conditions.High temperatue stimulated the superoxide dismutase(SOD)and catalase(CAT)activity,they were highly induced in T.weissflogii than that in T.pseudonana.The combination of UV radiation and OA decreased the photosynthetic activity of diatom cells to a great extent.Under visible light,both diatom species possesed higher PSII repair rate(r)than the damage rate(k),which contributed to the relatively high Fv/Fm value.In the presence of UV radiation,particularly for T.pseudoanna,k was larger than r,indicating significant photoinhibition.The ratio of r to k was decreased under the combination of OA and UV radiation.UV radiation induced higher NPQs and decreased the content of PsbA and PsbD when diatom cells were exposed to high irradiance level.In contrast,PsbA content was kept stable in T.weissflogii in the absence of UV radiation,although the Fv/Fm was declined.The combination of UV and OA accelerated the PsbA removal rate in both diatom species.In summary,OA or/and UV radiation brought about different effects on T.pseudonana and T.weissflogii.Diatom cells were more sensitive to photoinhibition under OA.As it was shown in the present study,the larger diatom T.weissflogii would outcompete the smaller diatom T.pseudonana in a warmer and CO2-enricher ocean combined with increase UV exposure.More research using more species with various sizes needs to do to confirm this conclusion.