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

Abstract

Phaeocystis globosa exhibits a complex life cycle alternating between solitary cells and colonial forms. Although phycosphere bacteria have been implicated in bloom dynamics, the factors that mediate bacterial behaviour to influence colony formation, as well as how bacterial quorum sensing (QS) signals regulate colony morphology and density in P. globosa, remain poorly understood. In this study, we found that QS genes and intra-specific signaling genes were markedly enriched in intra-colonies of P. globosa. To test whether these field observed enrichment of QS-related genes reflects a causal role in regulating colony development of P. globosa, we performed controlled AHL exposure experiments. Exogenous exposure of P. globosa phycospheric symbiont 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.