The roles of light–dark cycles in the growth of Phaeocystis globosa from the South China Sea: The cost of colony enlargement

The allocation of materials to build the mucilaginous matrix of colonies does not impair the growth rate of colonial cells in Phaeocystis, and the matrix provides protection against grazing. We conducted experiments to investigate the effects of darkness on the growth rates of solitary and colonial cells in Phaeocystis globosa from the South China Sea, as well as the potential costs associated with colony enlargement. The growth rates of solitary and colonial cells, colony concentrations and fraction of cells in colonial form were all suppressed under continuous light. Colonial cells had higher growth rates compared to solitary cells only within longer dark periods, and the difference became insignificant under short durations of darkness. In all cases, colonial cell density showed a negative relationship with colony size for P. globosa. We also found clear negative relationships between initial colony diameters and growth rates of individual colonies. The reduced growth rate and colonial cell density would result in a fragile colonial envelope and represent potential cost in colony enlargement for Phaeocystis.