Coccolithophore Diversity And Calcification Rate In The Eastern Indian Ocean(EIO)

Living coccolithophores are calcified unicellular(4-24?m)nanoflagellates occupying in marine euphotic zone,which are one of major phytoplankton groups.The surface area of organic cell is covered by different layers of calcite scales(i.e.coccoliths),then these interlocked coccoliths form coccospheres.Dense coccoliths have ballasting effects on POC.Coccolithophores are capable of organic carbon fixation and inorganic biomineralization,these double mechanisms endowing them a unique role in marine biogeochemical cycle and global carbon sink.Modern oceans are facing interference of human activity,such as ocean acidification,ocean warming and eutrophication have profound impacts on marine ecosystem service and biodiversity,especially threaten the physiological metabolism and ecological functions of calcifying organisms.Influenced by reversing Asian monsoon systems and Tibet Plateau,Indian ocean formed unique habitats and circulation systems differed from other oceans.Indian Ocean is vulnerable to climate change and anthropogenic forcing,which exert complex effects on biological activity therein.Many physical and biological processes in Indian ocean are relatively unknown,in particular the natural coccolithophore community structure and calcification rate are scarcely investigated.The aim of this study is to explore the current status of calcifying coccolithophores and calcification rate and their response mechanisms to multiple biogeochemical factors in eastern Indian Ocean(EIO).The major contents include(1)interannual coccolithophore(2011-2018)community structure and abundance and their relationship with global climate anomaly incidents;(2)major species-specific contributors to CaCO3 standing stocks,calcification rate and primary productivity in the water column;(3)calcification-related eco-physiological traits in the coastal coccolithophore species(Chrysotila dentata);(4)simulated CO2 elevation and N-enrichment effects on natural phytoplankton community structure shifts and metabolic processes.During 7-year field investigations,33 coccolithophore taxa were observed,among which 30 taxa belong to heterococcolithophores.Coccolithophore populations were dominated by Gephyrocapsa oceanica,Emiliania huxleyi,Florisphaera profunda and Umbellosphaera irregularis,etc.Generally,coccolithophore cell abundance and organic carbon concentration showed annually increasing trend,accompanied by certain fluctuations due to special climate anomaly.Specifically,warm El Nino year(positive IOD)enhanced coccolithophore abundance,in turn,cold La Nina year(negative IOD)decreased coccolithophore abundance.Based on coccolithophore biogeography and environmental variables,we modeled the realized niches of dominant coccolithophore species in EIO.In our findings,coccolithophore calcification contributed 8.5%of total carbon fixation(organic carbon?inorganic carbon),and numerically dominant coccolithophore species are major calcite inventory contributors.Coccolith calcite had enhanced ballast effects on POC as depth increasing.The detected decoupling phenomenon between calcification and photosynthesis indicated calcification may be more nitrate-dependent and less light-limited than photosynthesis.To explore coccolithophore response to future ocean acidification and warning,indoor experiment of Chrysotila dentata response to temperature and irradiance was simulated.Furthermore,the natural coccolithophore assemblages,phytoplankton group shifts and metabolic rates were investigated under CO2 elevation and N-enrichment.We showed evidence that future ocean acidification(by the end of this century)and N-limitation caused by water stratification would inspire coccolithophore abundance and calcification rate,while inhibit microphytoplankton abundance(e.g.diatoms and dinoflagellates)and primary productivity.We make the following prediction that coccolithophores will become"winners" and microphytoplankton(diatoms and dinoflagellates)will become "losers"in the pelagic ocean.