Effects Of PH Changes In Seawater On Photosynthetic Physiology And Nutritional Value Of Typical Diatoms

Diatoms provide 40%of the primary productivity of the ocean and play an important role in the cycle of carbon and silicon.Seawater p H is one of the important environmental factors affecting the growth and photosynthesis of marine organisms.The increase of atmospheric CO₂leads to ocean acidification.In addition,phytoplankton in coastal waters are more vulnerable to short-term p H fluctuations caused by multiple factors.Based on the photosynthetic physiology and nutritional value,the responses of Skeletonema costatum and Thalassiosira weissflogii to the changes of p H in typical diatoms were studied.The main results are as follows:In batch cultures,S.costatum bloom was characterized by fast accumulation,a rapid decline in biomass,and a shorter stationary phase;longer stationary phase was found in the T.weissflogii bloom and cell density remained at high levels.The major effects on physiological performance of two diatoms were caused by bloom phases.Nevertheless,we did find the modulating effects of initial p H/p CO₂ on biomass accumulation and bloom dynamics.Low initial p H/higher initial p CO₂enhanced the specific growth rate of T.weissflogii by 6%;In the exponential phase,leading to higher cell densities,while 86%higher decay rates were observed in Low initial p H/higher initial p CO₂ cultures of S.costatum.The effects of different cultured p H on two diatoms were studied.The growth rate of S.costatum and T.weissflogii decreased under p H9.0,and the maximum concentration of S.costatum was only half of that of other treatments,and the Effective photoquantum Yield（Yield）,nonphotochemical quenching（NPQ）and maximum relative electron transfer efficiency(r ETR_max)of PSII of S.costatum decreased.Yield,saturated light intensity（I_k）and r ETR_max of T.weissflogii decreased,light energy utilization efficiency（α）and NPQ,soluble protein and soluble carbohydrate increased.The proportion of saturated fatty acids in S.costatum cultured at different p H values is more than 90%;The p H7.0～8.0 was beneficial to the synthesis of unsaturated fatty acids of T.weissflogii When the initial p H>8.5,the amount of unsaturated fatty acids synthesized by T.weissflogii decreased.After clarifying the relationship between photosynthetic physiology and cultured p H,the effects of decrease of p H and its fluctuation on photosynthetic physiology,fatty acid and amino acid compositions and contents of two diatoms were studied.The results showed that low p H promoted the growth of T.weissflogii and increased I_k and Yield values,but low p H inhibited the net photosynthetic rate of S.costatum.The fluctuation of cultured p H influenced effects of low p H.At p H8.35,the light energy utilization efficiency of S.costatum was the lowest,and the dark respiration rate of the two diatoms reached the maximum under this condition.Low p H promoted the accumulation of palmitoleic acid（C16:1）,while T.weissflogii mainly promoted the accumulation of EPA and DHA.According to the analysis of amino acid composition,it was found that the two diatoms mainly compose of non-essential amino acids and OA decreased the content of most EAA.The fluctuation treatment with high p H decreased decreased the amino acid content of S.costatum,but the nutritional composition of the two diatoms was not affected by p H fluctuation under acidification conditions.