Physiological And Molecular Mechanism Involved In Marine Diatom Skeletonema Dohrnii In Response To Long-term Ocean Warming

Diatoms are one of the most important phytoplankton groups,contributing to～40%of global marine primary productivity and playing a key role in marine food webs and biogeochemical cycles.Temperature is an important environmental factor that affects the growth,metabolism and productivity of phytoplankton.With the increase of human emissions of CO2 and other greenhouse gases,the ocean has absorbed a large amount of heat emitted by greenhouse gases,and the seawater temperature has been enhanced,causing ocean warming.It is predicted that the seawater temperature will rise by 2.6 to 4.8℃ by the end of this century,and the change will have a significant impact on diatoms and their ecosystem.At present,most warming experiments have partially been concentrated on the short-term effects and rarely on the long-term adaptability;the research on ocean warming mainly focuses on the physiological response of phytoplankton to warming,but the exploration of its mechanism behind is still lacking;particularly,there is very scarce mechanistic information on the development process of their thermal adaptability under different time scales of warming.Together,it is very important to study the long-term effects of warming on marine diatoms and their adaptive mechanisms,as it can provide accurate prediction for the response of marine phytoplankton to future ocean warming.In this study,Skeletonema dohmii,which has important ecological significance,was cultured to 700 generations in a semi-continuous batch at the ambient(20℃)and two different warming(24 and 28℃)temperatures.Six important physiological parameters(Fv/Fm,growth rate,chlorophyll a,particulate organic carbon,particulate organic nitrogen,and biogenic silicon)of S.dohrnii were regularly measured throughout the long-term experiment.More importantly,proteomic approach was used to compare the differentially expressed proteins of the 300-and 700-generation S.dohrni under warming conditions,as well as the biological processes involved,revealing the mechanism concerning the development process for thermal adaptation of marine diatom under long-term warming.The main findings are described as follows:(1)After 700 generations of growth under different temperature treatments,the growth rates of the two warming populations were significantly increased throughout the selection,indicating that warming promoted the growth and reproduction of cells;however,the chlorophyll a content of both warming cells was strikingly decreased,but the Fv/Fm value and biogenic silicon content insignificantly affected by warmer temperature.Interestingly,in the early period,particulate organic carbon and particulate organic nitrogen contents were decreased in both warming populations,but their contents tended to recover to normal levels in the late period.Together,S.dohrnii had ultimately made to develop adaptive response to long-term warming selection;(2)Proteomic analysis in S.dohrnii of 300-and 700-generation demonstrated that both warming populations enhanced various energy metabolism in order to maintain higher growth rate;and due to trade-off,the elevated metabolic cost could lead to a decrease of carbon and nitrogen storage in the cells of 300-generation.Intriguingly,energy supply was dominated by lipid catabolism after 700 generations in both warming populations,enabling the cells to adapt to warmer temperature and so restoring cellular carbon content.Moreover,protein synthesis was likely enhanced,and in turn,probably increased cellular nitrogen content.Collectively,the proteomic differences have provided a mechanistic understanding about its adaptive response development of this diatom under long-term warming selection;that is,S.dohrnii had primarily switched diverse energy metabolism to lipid catabolism for energy supply,ultimately rendering its development of thermal adaptation;(3)The effect of long-term warming on S.dohrnii was a kind of developmental process probably from acclimatization response to adaptation.In conclusion,our paper provided mechanistic insights into the adaptation response development of diatoms to warming,thus helping to accurately project the marine phytoplankton’ response to the future warming oceans.