Combined Effects Of Ocean Acidification And Key Environmental Factors On Typical Diatoms

Diatoms,as photosynthetic eukaryotes,contribute up to 40%of marine primary productivity and play an important role in marine ecosystems and biogeochemistry.The oceans take up anthropogenically released CO2,leading to ocean acidification(OA),which affects the metabolisms of many organisms,including diatoms.Therefore,the physiological and ecological effects of OA are one of the key scientific issues in the field of marine environmental research.Based on an in-depth review of relevant research progresses,this thesis investigated ecophysiological and molecular responses of typical diatoms to OA under the influence of multiple environmental factors.The main results are as follows:Light intensity and light quality regulated the positive and negative effects of OA on growth in the diatoms grown at optimal growth temperature(20℃).The growth of Thalassiosira weissflogii was not significantly affected by OA under growth nonlimited light levels(220,140 μmol photons m-2 s-1),while OA significantly increased the specific growth rate(over 9%)under the growth limited light intensity(35,70μmol photons m-2 s-1).Under all light levels tested,OA decreased the activity of extracellular periplasmic carbonic anhydrase(eCA),down-regulated CO2 concentration mechanisms(CCMs),and increased mitochondrial respiration.Further studies showed that OA inhibited the growth of diatom in the darkness but promoted it under the light.For Phaeodactylum tricornutum,the specific growth rate was higher in the light period than in darkness under the growth-saturating light levels(220,120 μmol photons m-2 s1),but the results were opposite under the light limited condition(50 μmol photons m2 s-1).Similarly,OA increased the respiration of the diatom and inhibited its growth rate in the darkness,but promoted its growth during light period.To investigate the effects of OA below euphotic zone(dark ocean),the diatoms were exposed to elongated darkness for 2-3 days.OA treatment caused the growth rate and photosynthetic activity of the above two species of diatoms to decrease and accelerated their cell death during the elongated darkness treatment.When UV radiation(280-400 nm)was added in the presence of visible light,OA and UV synergistically inhibited the growth and photo synthetic activity of Phaeodactylum tricornutum,and caused damage to the photosynthesis system Ⅱ,decreased the chlorophyll content and accelerated cell death.Results derived from transcriptome sequencing of Thalassiosira weissflogii showed that OA increased the expression of photosynthesis-related genes and carboxylation protein Rubisco(ribulose-1,5-bisphosphate carboxylase/oxygenase)during the light period,and down-regulated the expression of carbonic anhydrase and other related genes.In the dark period,OA up-regulated the expression of fatty acids oxidation related genes,and increased the gene expression of respiration related proteins.These molecular data support the physiological characteristics that OA stimulated during light but inhibited in darkness the specific growth rates of the diatom,revealing the molecular mechanism for the physiological responses to OA.Temperature changes regulated OA effects on the growth of diatom.The specific growth rate,net photosynthetic and respiration rates of Thalassiosira weissflogii increased with the increase of culture temperature within a range of 5 levels(15,20,23,25,30℃)at 200 μmol photons m-2 s-1.OA increased the growth rate of the diatom at 23,25 and 30℃,but decreased it(by about 23%)at 15℃,and had no significant effect on it at 20℃.By comparing the OA effects during light and dark periods,it was obvious that OA promoted the growth rate in the light period,but reduced it at 15℃ and 20℃in darkness,and enhanced the growth rate at other three temperature regardless of the light periods.The effect of OA on respiration was negatively correlated with temperature,that is,the respiration rate increased with the increase of temperature,and the degree of OA impact on respiration diminished with increasing temperature.Under prolonged dark conditions(simulated deep ocean without light),the combination of OA and warming accelerated the diatom cell death.Nitrogen(nitrate)limitation(11 μM)exacerbated the negative effects of OA on the diatom.Under the nitrogen limited condition,OA reduced the photosynthesis,growth rate and electron transport rate in Thalassiosira weissflogii at 200 μmol photons m-2 s-1 and temperature of 20℃.However,OA had no significant effect on its growth under nitrate replete(113 μM)condition.Analysis of transcriptome data showed that under the nitrogen restriction,OA down-regulated the expressions of genes related to oxidative phosphorylation,resulting in a reduces energy supply and inhibiting growth of diatom.Compared with nitrogen sufficient conditions,nitrogen restriction downregulated genes related to photosynthesis metabolism,TCA cycle and oxidative phosphorylation related genes,leading to limited ATP generation.In conclusion,ocean acidification(OA)can bring about positive and negative effects on diatoms,and its effects are closely related to levels of light,temperature and nutrient.Under conditions of replete nutrients,optimal temperature and light,OA downregulated CO2 concentrating mechanisms(CCMs)of diatoms,decreased the expression of carbonic anhydrase(the key enzyme of CCMs)genes,promoted the respiration rate,and increased the expressions of Rubisco and carboxylation-related genes as well as mitochondrial respiration related genes.These physiological and molecular responses provided a mechanistic explanation for the experimental results that OA enhanced in the light period but inhibited during night the growth of diatoms.Under nitrogen limited conditions,OA suppressed the expressions of genes related to oxidative phosphorylation,restricting energy supply,with significantly reduced growth rates in the diatoms.Subsequently,the balance between the positive and negative effects of OA determined the overall outcome of physiological performance,leading to either enhanced,unaffected or reduced diatom growth rates.In addition,the observed enhancement of diatom growth rate was attributed to extra energy saved from downregulated operation of CCMs that must have aided for growth and/or resistance to acidic stress,disproving the documented theory that increased CO2 availability of OA plays a"CO2 fertilization" role.OA accelerated death rate of diatom cell under long-term dark conditions,and warming further exacerbated this phenomenon,implying that OA and warming would reduce diatom biomass below euphotic zones and affect vertical transport of particulate organic matter.