Effects Of Diverse CO₂ On The Physiology Of Microalgae

Increasing anthropogenic CO₂ emissions,which increased dissolved CO₂ in seawater and lowered the pH value in ocean.This change in seawater may result in a decrease in marine biodiversity,a change in phytoplankton community structure and a frequent occurrence of harmful algal blooms.We investigated the effects of CO₂ variation on the growth and elemental composition and sinking rate of cultures of the diatom Skeletonema dohrnii and the raphidophyte Heterosigma akashiwo,bloom-forming microalgae.The growth rates of S.dohrnii increased with CO₂ increases up to 400ppm,then were CO₂-saturated.For S.dohrnii.there was a significant interactive effect in which the higher temperature decreased the CO₂ half-saturation constant for growth.In contrast to S.dohrnii,the growth rates of S.dohrnii increased with CO₂ increases up to 1000ppm.then were CO₂-saturated.The growth rates of S.dohrnii at all CO₂ concentrations（except 1000ppm）tested were significantly higher than H.akashiwo.In addition,the pCO₂ half-saturation constant of S.dohrnii was lower than H.akashiwo.At low temperature 15℃ and high temperature 25℃,the cellular elemental composition of S.dohrnii has a different response tendency to the increase in PCO₂.At 25℃.as the pCO₂ increases from 400 ppm to 2000 ppm,C:P,C:Si,C:Chi a,C:N and N:P of S.dohrnii rise first and then decrease,and have a maximum at 1000 ppm.N:P of S.dohrnii at 15℃ is consistent with the trend at 25℃.At 15℃.as the pCO₂ increases,the C:N of S.dohrnii increases.At 15℃,as the pCO₂ increased from 400 ppm to 2000 ppm,C:P and C:Chi a of S.dohrnii increased continuously.The effects of CO₂ increase on the elemental composition of H.akashiwo were minimal.At 25℃,H.akashiwo have higher N:P,higher C:P and higher C:Chl a than S.dohrnii.As thepCO₂ continues to rise to 2000 ppm,the sinking rate of S.dohrnii increased.The ascent rate of H.akashiwo increased first and then decreased with the increase ofpC02,and the ascent rate fastest at the current pCO₂ level.The results of this study suggest that there may be advantageous to the S.dohrnii when water temperatures are high in the summer,especially in the late stage of algal blooms,when pCO₂ is low in water.In the future,ocean acidification will be more favorable to H.akashiwo,This could increase the frequency and intensity of the red tide induced by H.akashiiwo,as well as change the structure of phytoplankton communities off the coast of China,which could affect the structure of the food web in Chinese waters and the biogeochemical cycling of carbon and nutrients in the oceans.