Ocean acidification affects marine organisms and the structure of ecosystems. As the partial pressure of carbon dioxide (pCO₂) rises, marine microalgae will meet their energy or metabolic by regulating physiological activities. Nitrogen metabolism is one of the most crucial metabolic pathways in microalgae and directly affects organic growth. Phaeocystis globosa, a species with a heterotypic life cycle, can exist in two forms: colonies and solitary cells. It commonly causes harmful algal blooms in coastal areas of eastern China. However, the adaptive response of P. globosa to ocean acidification is not well understood. This study aimed to investigate the effects of seawater acidification on physiological parameters and key genes of nitrogen metabolism in P. globosa. Gene expression related to nitrogen metabolism was analyzed using transcriptome and real-time fluorescent quantitative PCR method. Upon exposure to acidification, P. globosa was less likely to form colonies, and the colonies that did form were smaller. This could be due to the down-regulation of glutamine synthetase (GS), which hinders the synthesis of large amounts of glutamine, leading to a decrease in glycosaminoglycan biosynthesis. This reduction in extracellular polysaccharide synthesis might be a strategy employed by P. globosa to adapt to ocean acidification, and the transition in the life cycle of P. globosa from colonies to solitary cells reduces the possibility of harmful algal bloom.
 

Ocean acidification; Phaeocystis globosa; Nitrogen metabolism