Bacterial quorum sensing signals reshape phycosphere functions to regulate colony morphology in Phaeocystis globosa

Phaeocystis globosa exhibits a complex life cycle alternating between solitary cells and colonial forms. However, the factors that mediate bacterial behaviour to influence colony formation, as well as how bacterial quorum sensing signals regulate colony morphology and density in P. globosa , remain poorly understood. In this study, we used metagenomic approach to investigate bacterial QS profiles and metabolic potential in intra- and extra-colonies of P. globosa and we detected notable enrichment of acyl-homoserine lactone (AHL)-based QS genes and intensified intra-specific bacterial communication within colonies. To test whether these field observed QS signals causally regulate colony development of P. globosa , we performed controlled AHL exposure experiments. Exogenous exposure of P. globosa to AHL signal induced a strategic shift in colonial development, resulting in significantly larger colonies; however, with a reduced colony number (p < 0.05). Metagenomic analysis revealed that AHL reshaped the bacterial community by enriching the populations of polysaccharide degraders and vitamin producers. In addition, exposure to AHL upregulated fatty acid and terpenoid synthesis, carbon fixation, nitrogen recycling, and phagosome ability in the host algae. Collectively, these bacterial QS induced metabolic shifts enhance resource recycling and biosynthetic capacity within colonies, facilitating colony expansion despite reduced colony frequency.