Colony sizes, cell abundances, carbon contents, and intra-colony nutrient concentrations of Phaeocystis globosa from Mirs Bay, China

Phaeocystis globosa blooms have frequently occurred in the coastal waters of South China, although information about the structure and composition of their colonies is scarce. Colony cell abundances, as well as concentrations of mucous particular organic carbon (POC), cellular chlorophyll a, and nutrients in intra-colony fluids were investigated during a P. globosa bloom that occurred in late January 2021 in Mirs Bay, China. Giant colonies exhibited unique characteristics that considerably differed from previous observations of P. globosa. Colonies ranged from 0.47 to 1.83 cm in diameter, with up to 4.0 x 106 cells distributed within mucilaginous matrices. Colony cell densities and mucous carbon densities linearly increased with colony diameters, suggesting that colony envelopes become thicker and more structurally stable as colonies grow bigger. The contributions of mucous carbon relative to total carbon ranged from 47% to 84% and were positively associated with colony size. In contrast, chlorophyll a concentrations per colonial cell decreased as colony size increased. Intra-colony fluids contained greater nitrate, phosphate, and silicate concentrations, but less ammonium and nitrite concentrations relative to ambient seawater, suggesting constrained nutrient flux across the mucous layer. The occurrence of blooms caused by giant P. globosa colonies may influence food web structures and carbon cycling in sub-tropical Chinese waters.