Chemical-mediated effects of Chaetoceros affinis on colony formation of Phaeocystis globosa: induced release of allelopathic compounds

Phaeocystis globosa is a globally significant cause of harmful algal blooms, with outbreaks typically occurring after spring diatom blooms. A previous study revealed that Chaetoceros affinis promotes the formation of P. globosa colonies. However, the composition of its extracellular metabolites and the underlying molecular mechanisms driving colony formation remain unclear. This study utilized C. affinis and its filtrate as stress conditions to stimulate P. globosa. Results indicated that both C. affinis and its filtrate induced a life-history transition in P. globosa. The abundance of solitary cells decreased, whereas colony abundance, colony diameter, and the number of colonial cells increased. Additionally, the filtrate induced significant alterations in the metabolic pathways of P. globosa cells. Photosynthesis was enhanced; genes related to DNA replication were downregulated, while those involved in fatty acid synthesis, amino acid synthesis, and polysaccharide synthesis were upregulated, and catabolic genes were downregulated.