Formation of very young colonies by Phaeocystis pouchetii from western Norway

The marine phytoplankton species Phaeocystis pouchetii often forms large blooms of gelatinous colonies during spring in north temperate, boreal, and Arctic waters. The macroscopic colonies in such blooms develop originally from free-swimming solitary cells that shed their flagella, secrete a gelatinous membrane, and undergo repeated cell divisions within an enlarging colony membrane. However, the essential first steps in this process have not been described in situ, and hence the factors controlling initiation of colony blooms are poorly understood. Studies were con- ducted in mesocosms in a western Norway fjord to document the development of small young colonies under simulated and ambient conditions, resulting in 2 novel perspectives: (1) The critical early stages of colony bloom formation may occur in a relatively brief window in time. The rate of for- mation of young colonies from solitary cells was faster in nutrient-fertilized compared to unfertilized mesocosms, but this only happened after a period of diatom growth in the mesocosms and subse- quent increases in P. pouchetii colony abundance. Then, small percentages of solitary cells were mathematically necessary to account for observed colony numbers. (2) Initial formation of new young colonies occurred during the dark period of the ambient springtime diel light cycle, and patterns of colony accumulation implied rapid multiple cell divisions within colonies during the dark period, a process termed phased ultradian growth. Such an evolutionary strategy of rapid change from single cells to multi-cell colonies would be advantageous as a means of minimizing grazing losses during the critical period of size transition from microscopic free-swimming cells that are susceptible to microzooplankton and viruses, to larger colonies that are susceptible to metazoan zooplankton but relatively impervious to viruses and microzooplankton. The combined results offer a new insight into colony proliferation: colony blooms do not necessarily require a long period of constant conversion of solitary cells into colonies, but rather may be the product of punctuated, rapid life cycle transforma- tions and ultradian growth of young colonies.