Effects of pH/pCO₂ fluctuation on photosynthesis and fatty acid composition of two marine diatoms, with reference to consequence of coastal acidification

Abstract. Coastal waters are impacted by a range of natural and anthropogenic factors, which superimpose on effects of increasing atmospheric CO₂, resulting in dynamically changing seawater carbonate chemistry. Research on influences of dynamic pH/pCO₂ on marine ecosystem is still in its infancy, although effects of ocean acidification have been extensively studied. In the present study, we manipulated the culturing pH/pCO₂ to investigate physiological performance and fatty acid (FA) composition of two coastal diatoms Skeletonema costatum and Thalassiosira weissflogii in both steady and fluctuating pH/pCO₂ regimes. Generally, seawater acidification and pH variability showed neutral or positive effects on specific growth rate, chlorophyll a, and biogenic silica contents of two species. Elevated pCO₂ inhibited net photosynthetic rate by 27 % and enhanced mitochondrial respiration rate of S. costatum by 36 % in the steady pH regime, while these rates were unaltered by elevated pCO₂ in the fluctuating regime. Elevated pCO₂ leaded to 21 % lower saturated FA and twofold increase in polyunsaturated FA proportions of T. weissflogii. Our results indicate that costal acidification could affect primary production in a different way from ocean acidification. Together with the altered nutritional quality of prey for higher trophic levels, coastal acidification might have far-reaching consequence for marine ecosystem functioning.