Physiological Responses Of Diatoms To Seawater Acidification Under Influences Of Multiple Drivers

Diatoms are responsible for about 20%of global primary productivity and about 50%of organic carbon burial in marine sediments.Moreover,diatoms are key species influencing the biogeochemical cycle of silicon.Increasing atmospheric CO2 concentration leads to increased dissolved CO2,ocean acidification(OA)and rise of sea-surface temperature.These changes will definitely affect the physiology of phytoplankton.Numerous studies about the impacts of seawater acidification on diatoms have been documented to date.However,most of these studies focused on short-term effects of seawater acidification without interactions with other environmental drivers.In the present study,we investigated physiological responses of coastal and oceanic diatoms to diel fluctuating or constant pH/pCO2 regimes under ambient and elevated CO2 concentration.In addition,interactive effects of multiple drivers(temperature,CO2 and nitrogen concentration)and the long-term effects of seawater acidification on the centric diatom Thalassiosira pseudonana and pennate diatom Phaeodactylum tricornutum were studied.The main results are as follows:1.Differential physiological responses of coastal and oceanic diatoms to fluctuations of light intensity and pH/pCO2The coastal diatom Thalassiosira weissflogii and oceanic diatom Thalassiosira oceanica responded either similarly or differentially in different aspects to changes in light intensity and fluctuation frequency.While pigment contents decreased with increasing growth light intensity,the ratios of carotenoid to chlorophyll a contents and the photosynthetic light saturation point(Ik)increased in both species.Comparative analysis showed T.weissflogii always had significantly higher Ik and photochemical efficiency and lower damage rate of PSII than T.oceanica.Physiological responses of T.weissflogii and T.oceanica to seawater acidification and fluctuations in seawater carbonate chemistry were markedly different.While seawater acidification enhanced mitochondrial respiration and production rates of intracellular organic matter in T.weissflogii,no significant effects on its growth rate were found.The growth rate of T.oceanica was significantly inhibited under the elevated pCO2(1000 ppmv).The diurnal fluctuation in seawater pH showed no significant effects on T.weissflogii.However,decreased growth rate and production rates of intracellular organic matter were observed in T.oceanica.Fluctuations in seawater carbonate chemistry could alleviate the negative effects of seawater acidification on growth and photosynthetic carbon fixation rate of T.weissfogii under N-limited condition(3.5 ?mol L-1).Both seawater acidification and fluctuations in seawater carbonate chemistry influenced the allocation of absorbed light energy in PS? under both steady and fluctuating light regimes.Cells acclimated to steady pH under the elevated CO2 always showed higher quantum yield of dissipation associated with non-photochemical quenching(?NPQ)and lower quantum yield of electron transport in PS?(?PS?)and less repair rate of PS? than cells acclimated to the fluctuating pH regime.2.Combined effects of seawater acidification,elevated temperature and nutrient limitation on growth and photosynthesis of diatomsElevated pCO2(1000 ppmv)showed no significant effects on growth and photosynthesis of T.pseudonana under nitrate-replete conditions(102.5 ?mol L-1).However,it significantly inhibited growth,cell size,pigment content,photochemical efficiency of PSII and photosynthetic carbon fixation rate of T.pseudonana under N-limited conditions(2.5 ?mol L-1).Elevated temperature(from 20 to 24 ?)exacerbated the negative effects of seawater acidification on growth and pigment synthesis of T.pseudonana under N-limited conditions.In addition,negative and positive correlations of specific growth rate and mitochondrial respiration were evident under N-limited and replete conditions,respectively.While elevated pCO2(1000 ppmv)showed no significant effects under N-replete conditions(102.5 ?mol L-1),it significantly inhibited the growth and photosynthetic carbon fixation of P.tricornutum under N-limited conditions(2.5 ?mol L-1).The elevated temperature mitigated the inhibition of seawater acidification on growth of P.tricornutum cells.Moreover,elevated temperature from 20 to 24 ? significantly increased the cell size,pigment content and mitochondrial respiration rate of P.tricornutum cells.3.Evolutionary responses of T.pseudonana and P.tricormutum to seawater acidificationThe growth rate,photosynthesis,photochemical efficiency of PSII and mitochondrial respiration of T.pseudonana were not different from those of short-term incubation(20 generations)in the process of long-term growth(3500 generations)under influence of OA(pCO2 1000 ppmv,pH 7.8)treatment.Based on these results,there was no evidence of evolutionary response of T.pseudonana to seawater acidification.On the other hand,P.tricornutum cells differed physiologically from each other after having been grown for differing spans of time under seawater acidification.While short-term(20 generations)seawater acidification enhanced mitochondrial respiration rate and showed no significant effects on growth and photosynthetic rates,long-term(1860 generations)acidification decreased growth,photosynthesis and mitochondrial respiration and increased the pigment contents of P.tricornutum cells.The growth rate of the long-term acidification population did not drop immediately,but was significantly reduced during the last 100 generations.The decrease in growth rate might be partly related to decreased respiration,as mitochondrial respiration provides ATP and converts carbohydrates to carbon skeletons used for growth.In summary,T.oceanica was more sensitive to seawater acidification and pH/pCO2 fluctuation relative to T.weissflogii,with decreased growth rate.Seawater acidification under increased temperature and limited nitrate conditions showed negative effects on photosynthetic carbon fixation of T.pseudonana and P.tricornutum.Responses of P.tricornutum to seawater acidification depend on the timescale for which they are exposed,cells grown under acidification for 1860 generations showed decreased growth,photosynthesis and mitochondrial respiration.