Phaeocystis globosa is a significant contributor to harmful algal blooms, and its hemolytic toxins pose substantial risks to marine ecosystems and aquaculture. The increasing frequency of P. globosa blooms is associated with marine eutrophication and climate change. In this study, two morphologically distinct strains of P. globosa (single-celled and encysted forms) were analyzed. The growth cycle characteristics of P. globosa were assessed using optical microscopy and flow cytometry, and the community composition of its symbiotic bacteria was determined using high-throughput 16S rRNA amplicon sequencing.
The cell cycles for the single-celled and encysted forms were found to be 50 and 17 days, respectively. During the decline phase, the unicellular P. globosa doubled in size, whereas the encysted cells remained the same. The Shannon diversity indices of symbiotic bacteria increased during the decline phase, with the strain type being the dominant factor influencing bacterial community composition. Proteobacteria were the most dominant, constituting 72.13% of the community, followed by Cyanobacteria (14.12%) and Bacteroidetes (11.88%). the abundance of Balneola (747.74%), Dinoroseobacter (210.10%), and Fabibacter (1683.96%) significantly increased in the single-celled P. globosa.  Additionally, Alteromonas increased by 123.56% in the encysted form of P. globose. Function prediction using PICRUSt2 suggested that symbiotic bacteria enhance their growth by upregulating processes such as DNA replication, purine synthesis, and glycolysis, potentially competing with P. globosa for nutrients and inhibiting the its growth. Increased bacterial activity of enzymes like COX, GSHase, and BCAT may also affect algal oxidative stress and growth. This study underscores the potential of symbiotic bacteria associated with P. globosa as a biological control mechanism for algal bloom control.

Phaeocystis globosa, cell cycle, symbiotic bacteria, 16S rRNA, biocontrol