Effects of pH/ <i>p</i> CO ₂ fluctuations on photosynthesis and fatty acid composition of two marine diatoms, with reference to consequences 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 the influences of dynamic pH/pCO2 on marine ecosystems is still in its infancy, although effects of ocean acidification have been extensively studied. In the present study, we manipulated the culturing pH to investigate physiological performance and fatty acid (FA) composition of two coastal diatoms, Skeletonema costatum and Thalassiosira weissflogii, in both steady and fluctuating pH regimes. Generally, seawater acidification and pH variability showed neutral or positive effects on the specific growth rate, chlorophyll a, and biogenic silica contents of the two species. Decreased pH inhibited the net photosynthetic rate by 27 % and enhanced the mitochondrial respiration rate of S. costatum by 36 % in the steady pH regime, while these rates were unaltered by decreased pH in the fluctuating regime. Acidification conditions led to lower saturated FA and higher polyunsaturated FA proportions in both species, regardless of steady or fluctuating regimes. Our results indicate that coastal 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 consequences for marine ecosystem functioning.