Ocean Acidification Increases The Formation Of Resting Spore In Diatom Ditylum Brightvellii Under Nitrogen Limitation

The marine environment is facing a variety of changes,such as ocean acidification,warming,UV radiation and stratification enhancement,etc,which will have important impacts on phytoplankton,zooplankton and the ecological environment in the ocean.Ocean acidification has recently received a lot of attention,the increase of CO2 concentration and the decrease of pH in seawater will have direct and indirect effects on the primary productivity of marine phytoplankton.Diatom is an important component of marine phytoplankton,which contributes c.40%of the total primary productivity in marine ecosystem.At the same time,diatom is an important vector of marine material cycle.Resting stage is an important phase in the life cycle of some diatoms,since it is related to the survival and development of diatom populations.Studies focus on the effects of ocean acidification on the physiological characteristics of phytoplankton has been widely carried out.However,we still have a lack of understanding of the effects of ocean acidification on diatom resting spore.Therefore,in this study,we chose the diatom,D.brightwellii,as experimental objective and established the ocean acidification cultural condition based on laboratorial culture system.Then,combined with nitrogen limitation,we measured the effects of ocean acidification and nitrogen limitation on the growth,photosynthesis,resting spore formation,elemental composition and biochemical composition,etc.This thesis analyzes the reason that ocean acidification increase the formation of resting spores under nitrogen limitation,and the ecological significance of these concomitant results.The main results of this study are as follow:(1)Under nitrogen limitation,ocean acidification had no significant effects on the total cell density,but significantly increased the formation of resting spores.At the end of study,total cell densities of the high CO2 group(HC)and low CO2 group(LC)both reached about 2500 cells mL-1,but resting spore formation rate of HC and LC were 52.4%and 26.8%respectively.These results show that ocean acidification could significantly increase the formation of resting spore under nitrogen limitation.(2)Ocean acidification promoted the absorption and assimilation of nitrogen in D.brightwellii,and further changed POC,PON and POC/PON.There were 2 stages in culture period,the first stage was from day 0 to day 2,absorption and assimilation of nitrogen in HC was stronger than that of LC.Compared to LC,HC cultures had lower nitrogen concentration,high POC and PON,and no significant differences in POC/PON.The second stage was from day 3 to day 7,nitrogen concentration of LC and HC were kept at a low level,and the difference of PON was not significant,but the POC and POC/PON of HC was higher than LC.Therefore,the enhancement of nitrogen stress(higher POC/PON)may be the main reason for the increase of resting spore formation under HC condition.(3)Under nitrogen limitation,ocean acidification changed the distribution of intracellular PON in D.brightwellii.During the culture time,Day 4 and Day 7 could represent two different stages.On day 4,the first stage,the early phase of nitrogen limitation,PON of HC and LC were not significantly different,but the protein and amino acids content of HC was lower than that of LC.It showed that the part of organic nitrogen,except protein and amino acids,of HC was higher than that in LC.It may be speculated that this part of organic nitrogen was involved in the regulation and expression of genes.On day 7,the second stage,after a period of nitrogen limitation,PON in HC was significantly lower than that in LC,but the content of protein and amino acids in HC was higher than that in LC.These results showed that organic nitrogen of HC was distributed more in protein and amino acids.A part of organic nitrogen may be transferred back to protein and amino acids to take part in downstream metabolism.Therefore,the distribution of intracellular PON in the D.brightwellii under ocean acidification was different from that under the current CO2 condition.(4)Under nitrogen limitation,the effects of ocean acidification on the growth,photosynthesis,carbonhydrate and fatty acid composition in D.brightwellii were not significant.In this study,Chl a content,Fv/Fm,carbonhydrate and fatty acid composition of HC were not significantly different from that of LC.Therefore.under nitrogen limitation,ocean acidification mainly through enhancing nitrogen stress to affect nitrogen metabolism in D.brightwellii and increases resting spore formation rate,but the effects on physiological characteristics of resting spores itself and its carbon metabolism were not significant.