Differences Between Solitary Cells and Colonial Cells in the Heteromorphic Life Cycle of Phaeocystis globosa: Morphology, Physiology, and Transcriptome

Large-scale blooms of Phaeocystis globosa have caused serious damage to marine ecosystems in coastal waters of China. Phaeocystis blooms depend on the competitive advantage of their heteromorphic life history: colony formation has the benefit of resisting herbivory by zooplankton, and solitary cells can absorb nutrients rapidly. In order to better understand the mechanisms underlying the metabolic differences between the two types of cells, morphological observations, rapid light curve analysis, fatty acid profiling, and transcriptome assessment were conducted in the laboratory. The rapid light curve of colonial cells was higher than that of solitary cells, which indicated that colonial cells had higher CO2 fixation capacity. The fatty acid level of colonial cells was evidently lower than that of solitary cells, which is consistent with down-regulated synthesis of fatty acids and up-regulated degradation of fatty acids in the transcriptome. ATP-binding cassette transporters, the TCA cycle, and biosynthesis of exopolysaccharides (EPS) also displayed obvious differences. In summary, colonial cells have stronger carbon fixation capacity. They do not synthesize fatty acids as energy storage materials but secrete EPS, which might be one of the mechanisms of colony formation. Here we present a physiological and molecular overview of the differences between solitary cells and colonial cells and thereby provide further insight to help unravel the mechanisms that help Phaeocystis globosa adapt to different environments.