| Human activities have caused the continuous increase in atmospheric carbon dioxid(CO2)level,leading to a tendency of global warming.Oceans act as a sink for increasing anthropogenic CO2.This process changes seawater carbonate chemistry,including an increase in the partial pressure of CO2(pCO2)and a decrease in pH,which are commonly referred to as ocean acidification.It has been shown that ocean acidification will decrease nitrification.Warming will increase stratification,resulting in less injection of NO3-into surface ocean.Together with more atmospheric anthropogenic ammonium deposition,the chemical form of nitrogen supplied to phytoplankton will be shifted with a tendency of more NH4+ and less NO3-in surface ocean.The physiological responses of marine phytoplankton to ocean acidification have aroused widespread concerns.Previous studies have shown stimulative,neutral or inhibitory effects of acidification on the growth of marine phytoplankton.As increasing pCO2 and decreasing pH happen concomitantly,the effects of ocean acidification on the growth and photosysthesis of phytoplankton should be net effects of elevated pCO2 and decreasing pH as well as other environmental factors.The often-used methodologies for ocean acidification studies do not generally distinguish the individual effects of pH and pCO2.Therefore separating the effects of pCO2 and pH on the phytoplankton is largely unknown.Moreover,recent studies have mostly focused on the interactive effect of ocean acidification and other environmental factors,such as light,temperature and nutrients limitation.The effect of ocean acidification combined with the shift of the chemical form of nitrogen nutrients(NO3-vs.NH4+),however,has received little attention.Diatoms contribute about 40%of marine primary production and play a key role in carbon export to deep seawater.How diatoms respond to the change in pH and pCO2 individually and to what extent the shift in nitrogen form(NO3-vs.NH4+)interacts with ocean acidification basically remain unknown.Here we examined separate effects of pCO2 and pH on diatoms,using synthetic ocean water(SOW)as the culture media in which pCO2 and pH were varied independently by adjusting dissolved inorganic carbon concentration and keeping pH within a range using the organic buffer EPPS,with the addition of different nitrogen sources(NO3-or NH4+).Growth,chlorophyll fluorescence,carbon metabolism,nitrogen assimilation,elemental and biochemical compositions of two diatom species,i.e.,Phaeodactylum tricornutum and Thalassiosira pseudonana,which are of similar cell size but different morphologies were systematically assessed.Although different responses to ocean acidification were observed in P.tricornutum and T.pseudonana,common phenomenon still presented:NH4+stimulated the growth of P.tricornutum and T.pseudonana(3-7%and 14-20%).Activities of nitrate reductase in both diatoms were entirely inhibited by the presence of NH4+ and were inhibited to some extent due to ocean acidification(33%and 41%).Acidification,however,did not affect the uptake and utilization of nitrogen.Effects of acidification on carbon metabolism such as photosysthetic carbon fixation and respiration were not significant when T.pseudonana and P.tricornutum were grown in NH4+ Cellular β-1,3-glucan in both diatoms remained rather constant despite of the environmental change(elevated pCO2,decreasing pH or shift of nitrogen form).Effects of elevated pCO2 or decreasing pH on carbon metabolism may explain the different responses of P.tricornutum and T.pseudonana to ocean acidification under NO3-conditions.Although ocean acidification(decreasing pH)increased the gross photosynthetic rate(13%),it enhanced the respiration rate(34%)in the mean time,consequently resulting in neutral net effect on net carbon fixation and growth in T.pseudonana.The stimulative effect of ocean acidification on the growth(5%)of P.tricornutum was largely due to the enhancement in gross photosynthetic rate under acidification(decreasing pH)(16%)in combination with no significant difference in respiration rate.The processes of photosynthesis and respiration in T.pseudonana may be more easily influenced by ocean acidification than in P.tricornutum,because it was more sensitive to environmental stress and had higher light use efficiency.When cells were grown in NH4+,the uptake and utilization of carbon and nitrogen was stimulated by the presence of NH4+ in T.pseudonana but was inhibited in P.tricornutum.T.pseudonana had positive response of growth and the uptake and utilization of nitrogen to acidification under NH4+ conditions.Positive response of growth to acidification was also shown in P.tricornutum,but the response was similar to the response under No3-conditions.The different mechanisms of NH4+ on two diatom species need further study.Separating the elevated pCO2 and decreasing pH effects of ocean acidification on two diatom species p.tricornutum and T.pseudonana provided new insights into the mechanisms of physiological and biochemical responses of marine phytoplankton to ocean acidification. |
