Proteomic responses of Phaeocystis globosa during colony development promoted by Skeletonema costatum culture filtrate.

Phaeocystis globosa has a polymorphic life cycle characterized by the alternation between free-living solitary cells and a colonial morphotype. Colony formation is pivotal to the initiation and development of Phaeocystis blooms, which in marine ecosystems often occur after diatom blooms. To investigate the relationship between diatoms and P. globosa colony formation, we exposed solitary cells of P. globosa to cell-free culture filtrates of Skeletonema costatum and analyzed the response using tandem mass tag (TMT)-based quantitative proteomics combined with physiological analysis. Both 15% and 25% culture filtrate treatments significantly increased colony density and colony diameter. Among the 3504 proteins identified, 282 and 403 were differentially expressed in the two treatments, respectively. Functional analysis of these differentially expressed proteins indicated activation of photosynthesis, carbohydrate metabolism, oxidative phosphorylation, DNA replication and repair, ribosome-related processes, ABC transporters, and pathways putatively associated with glycosaminoglycan biosynthesis. These changes indicate that filtrate exposure was accompanied by coordinated increases in energy production, carbon fixation, and metabolic processes related to extracellular-matrix precursor supply and transport, creating a physiological state favorable for colony development. Together, the physiological and proteomic data provide protein-level evidence that S. costatum culture filtrate promotes P. globosa colony formation and offer mechanistic insight into phytoplankton interactions during bloom succession.