Effect Of Thermal Variation And Elevated CO₂ On The Typical Phytoplankton

Global warming and ocean acidification are the most serious marine environmental problems currently.Global warming will be combined with predicted increases in thermal variability in the future surface ocean,but how temperature dynamics will affect phytoplankton biology and biogeochemistry is largely unknown.The effect of high CO2 on phytoplankton physiological characteristics has been widely reported,but only few studies focus on the mechanism of how phytoplankton gene expression responds to high CO2 stress.In the present study,three important and typical marine phytoplankton species were examined,including open ocean coccolithophore species Emiliania huxleyi,California coastal diatom species Thalassiosira weissflogii(=Conticribra weissflogii)and widely-distribution diatom species Chaetoceros muelleri.The effects of different frequency of thermal fluctuation,random thermal fluctuation and high CO2 stress on each microalgae were studied,respectively.The main results and conclusions were as follows:1.We examined the responses of the globally important marine coccolithophore Emiliania huxleyi to thermal variations at two frequencies(one-day and two-day)at low(18.5?)and high(25.5?)mean temperatures.Elevated temperature and thermal variation decreased growth,calcification and physiological rates,both individually and interactively.The negative effects of thermal variation on E.huxleyi growth rate and physiology were especially pronounced at high temperatures.Warming decreased the growth rate of one-day and two-day variation about 38%and 72%,respectively.The growth rate of one-day thermal variation was 0.52 ± 0.02 d-1,which was higher than two-day variations of 0.20±0.02 d-1 under high temperature,indicating that high frequency thermal fluctuations may reduce heat-induced mortality and mitigate some impacts of extreme high temperature events.Cellular elemental composition and calcification was significantly affected by both thermal variation treatments relative to each other,and to the constant temperature controls.The calcification rate of one-day and two-day variation decreased 46%and 52%respectively compared with constant high temperature.2.The effects of constant and random temperature fluctuations on growth and physiology of Thalassiosira weissflogii at current temperature(16?)and future ocean warming(20?)were investigated.It was found that future warming could promote the growth rate of Thalassiosira weissflogii,but decreased the cellular elemental composition,such as cellar POC?PON?POP?BSi and Ch1 ?.Temperature fluctuation decreased the growth rate of Thalassiosira weissflogii,and random temperature fluctuation decreased the cellar elemental composition as well,while constant temperature fluctuation did not significantly affect the cellar elemental composition of Thalassiosira weissflogii.Warming can offset the negative effect of temperature fluctuation on the growth rate of Thalassiosira weissflogii.Especially,there was no significant difference between the growth rate of random temperature fluctuation treatment and that of constant temperature.Warming and temperature fluctuation interactively affected the growth and physiology of Thalassiosira weissflogii.The interactive effect of warming and random temperature fluctuation was mainly on the growth rate and cellar elemental composition(POC,PON,POP and BSi),while the interactive effect of warming and constant temperature fluctuation had less effects on the physiology of Thalassiosira weissflogii.The interactions of constant temperature and random temperature fluctuation with warming have no significant effect on Photosynthetic rate.3.The expression of RNA-seq related genes in the transcriptome of Chaetoceros muelleri under high CO2 concentration was analyzed.It was found that high concentration of CO2 could up-regulate the genes related to metabolic pathways such as Calvin cycle,tricarboxylic acid cycle,glycolysis and pyruvate metabolism,fatty acid de novo synthesis and Glycerolipid biosynthesis.However,it down-regulated the genes related to CO2 fixation in CCMs and C4 pathways,such as eCA and PEPC.Up-regulation of genes related to Calvin cycle and tricarboxylic acid cycle can promote the growth of Chaetoceros muelleri.The up-regulation of genes related to glycolysis,pyruvate metabolism,fatty acid and Glycerolipid biosynthesis pathway would increase the metabolic products in Chaetoceros muelleri.During the exponential growth phase,high concentration of CO2 promoted the Glycerolipid biosynthesis,but down-regulates the expression of a key enzyme gene(DGAT)in TAG biosynthesis,suggesting that the main gene expression during exponential growth phase was to promote the growth and to the adaption to high CO2 concentration in Chaetoceros muelleri.However,the expression of a key enzyme gene in SQDG biosynthesis-SQD was up-regulated,indicating that the lipid synthesis of Chaetoceros muelleri tends to favor SQDG,which is biologically functioned in cells,showing its unique biological characteristics in response to high CO2.