Studies On Environmental Change Physiology Of Calcifying Phytoplankton(Coccolithophores)

Rising atmospheric CO2 concentration results in global and ocean warming;at the same time,more and more CO2 has been absorbed by the oceans,decreasing seawater pH and leading to ocean acidification(OA).These physical and chemical environmental changes directly or indirectly affect phytoplankton living in upper mixing layer.In this study,we investigated the physiological responses of the cosmopolitan coccolithophore species,Emiliania huxleyi and Gephyrocapsa oceanica,to nitrogen limitation,OA(1000 ppmv atmoshperic CO2),different nitrogen sources(NO3 and NH4)and warming.We also investigated the evolutionary responses of G.oceanica to OA.The main results are as follows:1.Physiological responses of Gephyrocapsa oceanica to varying growth irradiance were affected by different nitrogen sourcesUnder growth-saturating PAR(190 ?mol m-1s-1),when NO3(5 ?mol L-1)was replaced by NH4+(5 ?mol L-1)as nitrogen source,growth rate of G.oceanica increased,while cellular chlorophyll a(Chl a)content didn't change.Under unsaturating(50 ?mol m-1s-1)and inhibiting levels of PAR(400 ?mol m-1s-1),cellular Chl a increased in NO3--grown cells.Although different nitrogen sources had no significant effects on G.oceanica's growth rate.-Non photochemical quenching(NPQ)was higer in NO3 than in NH4+,indicating that the energy dissipation increased in NO3-grown cells when they suffered more stress in high light situation.Cellular particulate inorganic carbon(PIC)decreased with increasing light intensity,and it was further reduced by NH4+ under light inhibited condition.2.Effects of ocean acidification on coccolithophoresIn nitrogen replete condition,the acidification reduced G.oceanica's growth rate,E.huxleyi's cell size,and cellular particulate inorganic carbon(PIC)content in both species,while their cellular particulate organic carbon(POC)content was not affected.In nitrogen limited(4 ?mol L-1 NO3-)condition,the OA treatment reduced cellular POC content in both coccolithophore species,and stimulated G.oceanica's growth,but didn't affect E.huxleyi's growwth rate.When NO3 was replaced by NH4+ as nitrogen source,growth rate increased while cellular POC and PON(particulate organic nitrogen)decreased in the high CO2-grown G.oceanica and E.huxleyi cells.In addition,the high CO2 or OA-resultant decrement of PIC was independent of nitrogen sources3.The effect of OA on coccolithophres was mediated by temperatureAt the optimum growth temperature(20 ?),OA treatment did not affect the growth of E.huxleyi,but reduced it at 15 ? or 24 ? by about 17%.OA increased cellular POC and PON content at 24 ?,and enlarged the protoplast size by nearly 20%.OA decreased cellular PIC content,which was more pronounced at 15 ? and 20 ?.When the cells that had been acclimated in different temperature and CO2 levels were exposed to UV(ultraviolet)radiation(for 3h),UVA and UVB induced inhibition of photosynthesis was negatively related to the thickness of coccolith layer.The repair rate on UV-induced damage was positively related to cellular PON content.4.Evolutionary responses of Gephyrocapsa oceanica to OAAfter 1000 generations of adaption to OA,G.oceanica showed evolutionary responses to OA,.Its growth rate was lower in high CO2(pCO2,1000?atm;pH7.8)compared to in low CO2(pCO2,400?atm;pH8.16)condition,while cellular POC and PON were higher in high CO2.OA decreased cellular PIC content and PIC/POC ratio,and the decrement extent increased with time.When the high CO2 adapted populations were shifted back to low CO2 and acclimated for 10 generations,the downregulated calcification ability couldn't be restored,suggesting the high CO2 adapted populations had lost their phenotypic plasticity.5.Responses of phytoplankton assemblages to OAWe added one species of coccolithophores and two species of diatoms(the same initial cell concentration)into mesocosm bags(about 4 tons)and monitored their growth under two CO2 concentrations(high pCO2,1000?atm,pH7.8;low pCO2,400?atm,pH8.16).We found that coccolithophores disappeared soon and diatoms became the predominant phytoplankton species.In the logarithmic growth phase,there was no significant difference in POC and PON per volume of seawater between the low and high CO2 treatments;In the plateau phase,the cumulative POC(per volume of water)increased by 46-102%in high C02.In addition,in plateau phase,POC/PON ratio increased the initial low value of 5.3 to the highest value of 18.9,with no significant difference in POC/PON between the high and low CO2.As for the ship-board study in central South China Sea during a summer cruise,under low light(10%solar radiation)and high(100%solar radiation)incident solar radiation,the growth rate(based on Chl a per unit water,the dominant phytoplankton species were synechococcus,prochlorococcus and haptophyte)was not affected by elevated CO2 after 6 days of acclimation,but the effective absorption cross section of photosystem ?(aps?)decreased under the high CO2 condition,indicating a high CO2-enhanced photo-stress.In conclusion,coccolithophore's growth,PIC content and NPQ were all affected by different nitrogen sources.OA inhibited the growth of G.oceanica,but this was relieved when NO3-was replaced by NH4+ as nitrogen source.Increasing temperature and OA synergistically reduced E.huxleyi's cellular PIC,which exacerbated UV impacts on photosynthesis and calcification.G.oceanica showed significantly evolutionary responses to OA.