Physiological Responses Of Coccolithophores To Changes In Salinity And Carbonate Chemistry

Coccolithophores,belonging to the phylum Haptophyta,are unicellular calcifying microalgae,playing important roles in the global carbon cycles.With increasing atmospheric CO2 concentration,the ongoing ocean acidification(OA)is known to affect photosynthesis and calcification of coccolithophores.However,uncertainties still remain due to differential and regional results,and unknown effects of multiple drivers are the bottle neck for understanding mechanistic responses to OA.Therefore,Emiliania huxleyi(PML B92/11)was studied to look into its responses to changing salinity and pCO2 levels.The main results are as follows:1.In order to mimic a scenario of desalination and ocean acidification of coastal seawaters with changes of terrestrial inputs and hydrologic cycles,we set 3 salinity levels(25‰,30‰,35‰)and 2 pCO2 levels to study the combined effects of OA and salinity on E.huxleyi.Our results showed that there were no significant differences in the photochemical yield F,,/Fm of cells grown under 3 salinity levels,all of which showed a high value,indicating the photosystem ? of this strain can tolerate lowered salinity.Under ambient level of CO2(LC,400 ?atm),growth rate and F'v/F'm of the cells were significantly higher under 25‰ compared to that grown under 30‰ and 35‰.No significant differences were found in calcification rate.Under the elevated CO2(HC,1000 ?atm),the growth rate and F'v/F'm of cells grown under 25‰ were only significantly higher than 35‰ and the highest calcification rate was observed under 35‰.The inhibition of calcification by OA treatment was only observed at salinity 25‰ while no significant effects were observed at 30‰ and 35‰.The photosynthetic carbon fixation rate decreased with increasing salinity irrespective of the pCO2 treatments while the highest ratio of calcification to photosynthesis(C/P)was found at salinity 35‰.The cells grown under LC and 25‰ treatment showed a higher repair rate for the damage induced by high light exposures,while no significant difference was found among salinity treatments under HC.2.With manipulation of carbonate system,we set up a diurnal fluctuating regime(600-1750 ?atm CO2,pH 7.48-7.93)and steady regimes(400 ?atm CO2,LCs,pH 8.12;1200 ?atm CO2,HCs,pH 7.65)to mimic coastal diel changes and pelagic diel constant mode of seawater carbonate chemistry,respectively.Our results showed that the fluctuating regime of pCO2 where the pH of cultures gradually increased during the daytime and dropped at night significantly elevated cells' growth rate and daily POC production comparing with those grown under stable carbon system(both LC and HC)but lowered the ratio of PIC/POC.The constant low pH(HCs)had no significant effect on cells' daily POC production but lowered PIC production,which was also observed under the diurnal fluctuating regime of elevated pCO2.Constantly lowered pH(without diel change)decreased cells' maximal(Fv/Fm)and effective quantum yields(F'v/F'm).Instead,diurnal variation of pH had no significant effect on the value of Fv/Fm.On the other hand,cells grown under the pH diel fluctuating regime showed a lower NPQ than those grown under stable pCO2 conditions.3.The main findings in this study are as follows:1)tolerance to changes in salinity of this coastal isolated E.huxleyi(PML B92/11)could be attributed to increased energy use efficiency and the adjusting ability of energy allocation between photosynthesis and calcification,which helps it adapt to the diurnal variation of carbonate system;2)OA decreased the calcium amount of cells even under the diel pH fluctuation,which inhibited calcification of E.huxleyi as well.Our findings imply that with progressive desalination and ocean acidification,the calcification of coccolithophores will decrease with increasing POC production.However,their evolutionary response to desalination and OA still needs further exploration.