The Effect Of Seawater Carbonate System On Diatoms And Its Mechanisms

Since the industrial revolution,the ability of humans to interfere with nature has increased greatly,resulting in a series of global environmental problems.Global warming caused by excessive human-caused CO2 emissions has become a global research hotspot.The ocean is an important global carbon storage,absorbing about one-third of the artificially emitted CO2,These CO2 dissolved in seawater,resulting in dissolved CO2 in seawater rises and the pH drops,in other words,ocean acidification.Diatoms are important carbon-fixing organisms in the ocean,contributing 40%of the oceans and 1/4 of the world's primary productivity.It is crucial to the global carbon cycle.In addition,diatoms have high bio-pump efficiency and contribute more than half of the flux of marine organic carbon downwards,which is of great significance for alleviating global warming.At present,most researches on acidification of diatoms are merely explanations of phenomena rather than mechanisms.However,two factors(CO2 and pH)must be considered from the mechanism of how acidification affects diatoms.Since the effect of ocean acidification on diatoms should be a net effect of the two factors(C02 and pH).However,traditional research methods for acidification studies cannot distinguish these two factors and thus limit the mechanism studies of acidification on phytoplanton.Based on the above,the purpose of this study was to distinguish the dual effects of ocean acidification.We developed a special method to regulate the carbonate system of culture media:based on artificial seawater SOW,adding different concentrations of NaHCO3,adjusting the pH with strong acid and alkali,and using organic acid EPPS to stabilize the pH.After doing a series of operations we achieve the purpose of distinguishing the two factors(CO2 and pH).The results of the first part of the experiment showed that the CCMs of th e three diatoms under acidified condition(400-8.1 vs7.8 900)all down-regulated.Moreover,CO2 is the main regulatory factor.The increased CO2 concentration has a certain effect on the down-regulation of the three diatom CCMs,but p-H has no effect on the diatoms CCMs.Crosswise comparisons of three species of diatom CCMs suggest that Chaetoceros muelleri may have the lowest CC-M efficiency(max K1/2),so it may much more benefit from acidification(incre-ased CO2 concentration),which can be demonstrated by the result of increased carbon source(CO2)under acidification condition(400-8.1 vs800-7.8)(Chaetoce ros muelleri:38%,Thalassiosira pseudonana:16%,Phaeodactylum tricornutum 20%).The results of the second part of the experiment showed that acidification(400-8.1vs800-7.8)promoted the growth of Chaetoceros muelleri(13%),and the increased CO2 was the main factor.Acidification(400-8.1vs800-7.8)has a significant effect on the total carbon sequestration rate(short term 14C uptake rate)and net carbon uptake rate(Net C uptake rate),and the increased CO2 and decreased pH both have a certain contribution.However,these two factors have almost no effect on Respiration rate.The experimental results also show that the nitrogen metabolism and carbon metabolism of Chaetoceros mueller are synchronously changed.The rate of net nitrogen assimilation was also promoted by acidification(400-8.1vs800-7.8),This was confirmed by the simultaneous change in the activity of the nitrogen assimilation key enzyme nitrate reductase and the net nitrogen assimilation rate.Therefore,for Chaetoceros muelleri,the increased CO2 and the decreased pH lead to enhanced absorption of carbon,which leads to increased assimilation and assimilation of nitrogen.Eventualy,accelerated the growth of Chaetoceros muelleri.However,not all diatoms responded in a consistent response to acidification.In combination with the first part of the experiment and unpublished data from our laboratory,acidification(400-8.1vs800-7.8)does not necessarily lead to accelerated growth of diatoms.The growth of Thalassiosira pseudonana had no effect to acidification no matter increaded CO2 or decreased pH.While the increased CO2 had a certain effect on the growth of Phaeodactylum tricornutum,which eventually led to faster growth(3.3%)under acidification(400-8.1vs800-7.8).Different diatoms have distinct differences in response to ocean acidification,which may be due to the net effect of different physiological and biochemical processes(inorganic carbon capture,respiration,etc.)regulated by different regulators(CO2 or pH).The inorganic carbon capture of Chaetoceros muelleri is regulated by CO2,which saves energy and promotes growth under acidified condition(400-8.1vs800-7.8).The respiration of Thalassiosira pseudonana is regulated by pH.Under acidification,the respiration is strengthened.The positive effect of CO2 is offset by the negative effect of respiration,resulting in the non-response to acidification.The inorganic carbon of Phaeodactylum tricornutum is controlled by CO2,which saved energy and promoted the growth slightly under acidification condition.In short,different diatoms respond differently to ocean acidification,either with or without effect.From a preliminary physiological point of view,it may be the net effect of physiological processes such as inorganic carbon acquisition(CO2 regulated)and respiration(pH regulated).