The Formation Of Phaeocystis Colonies And Their Responses To The Environmental Factors

The genus Phaeocystis (Prymnesiophyceae) is one of the most widespread marine phytoplankton, and plays a significant role in the global carbon and sulfur cycles, food web structure and climate. It has the ability to form massive blooms that have substantial impacts on coastal systems, and has a complex polymorphic life cycle that involves both colonial and solitary forms. The solitary cells, either flagellated or non-flagellated, are typically 3-9μm in diameter. The colonial stage, with cells embedded in a polysaccharide gel matrix, may reach up to 3 cm in diameter. The organic matrix may provide an energy or nutrient source during periods of darkness or heterotrophic growth by the cells. The success of Phaeocystis is often attributed to its ability to form colonies, which potentially provides protection for the colonial cells against grazers, viruses and bacteria. Of the six described Phaeocystis species, three are wide-spread and regularly develop blooms in colonial form:P. globosa, P. pouchetii, and P. antarctica.Phaeocystis usually lacks the ability to form the colonies under lab-controlled conditions. In this work, we investigated the effects of air bubbling and turbulence on the growth and colony-forming of P. globosa. Both air bubbling and turbulence favored P. globosa to form more colonies. However, only turbulence greatly enhanced the colony size, colonial cell numbers and solitary cell numbers, whereas air bubbling did not exhibit significant impacts. Nonetheless, the colonies developed in the lab still had lower diameters than the natural ones.We also examined the effects of temperature on three colony forming species:P. globosa, P. antarctica and P. pouchetii. The temperature ranges over which P. globosa, P. antarctica and P. pouchetii grew were 16-32,0-6 and 4-8℃, respectively. Within these temperatures, P. pouchetii did not form colonies; P. globosa formed colonies at 16,20 and 24℃, whereas P. antarctica colonies were observed at all temperatures. More cells were partitioned into the colonial form at lower temperatures than at higher temperatures for P. globosa and P. antarctica. P. globosa colony size decreased with temperature, whereas P. antarctica colony size showed no distinct response to temperature. Numbers of cells per unit of colony surface area of P. globosa and P. antarctica were lowest at temperatures where highest growth rates and colonial abundances were observed; more organic carbon was partitioned into solitary cell biomass at higher temperatures, whereas the carbon concentration of colonies was not affected by temperature. Maximum quantum yields of P. antarctica and P. globosa exhibited subtle responses to temperature, whereas that of P. pouchetii was relatively invariant within the growth temperature range. Future changes in sea surface temperature will likely affect the ecology of systems dominated by Phaeocystis and the biogeochemical cycles in which it plays a central role.Recent investigations into the role of carbon dioxide on phytoplankton growth and composition have clearly shown differential effects among species and assemblages, suggesting that increases in oceanic CO2 may play a critical role in structuring lower trophic levels of marine systems in the future. Here we show that subtle biological responses occur in the HAB species Phaeocystis globosa as a result of CO2 enrichment induced by gentle bubbling. The alga, which has a polymorphic life history involving the formation of both colonies and solitary cells, exhibited altered growth rates of colonial and solitary forms at CO2 of 750 ppm, as well as increased colony formation. In addition, substantial modifications of elemental and photosynthetic constituents of the cells (C cell-1, N cell-1, potential quantum yield, chl a cell-1) occurred under elevated CO2 concentrations compared to those found at present CO2 levels. In contrast, other individual and population variables (e.g., colony diameter, total chlorophyll concentration, carbon/nitrogen ratio) were unaffected by increased CO2. Our results suggest that predictions of the future impacts of Phaeocystis blooms on coastal ecosystems and local biogeochemmistry need to carefully examine the subtle biological responses of this alga in addition to community and ecosystem effects.Massive blooms of giant Phaeocystis globosa colonies along the Chinese coast in recent years have caused serious environmental problems and economic loss. This Chinese strain is physiologically unusual, and ceases to form colonies in the laboratory. We conducted experiments to test whether the addition of grazers would re-initiate the formation of colonies. Our results showed that colonies were formed only in the presence of grazers. These colonies, however, had a very unique structure: Cells without flagella closely clumped together to form an approximately spherical structure, and the colonies lacked the mucous envelope typical of other P. globosa strains, hence they resembled aggregates of cells. Colony size and colonial cell number increased with time, and the development of colonies effectively protected the colonial cells from grazing. This is the first report of such unusual colony structure for P. globosa, and we speculate that the Chinese strain of P. globosa may be capable of behavioral cell congregation to increase their collective size under high grazing pressure.Buoyancy of Phaeocystis globosa (Prymnesiophyceae) colonies was investigated by measuring the vertical distribution of colonies in quiescent water where convection had been removed. Over 60% of the colonies exhibited negative buoyancy regardless of light condition or growth phase. Positively and neutrally buoyant colonies lost their buoyancy in the dark, but regained buoyancy upon return to the light. Colonies with closer cell packing; i.e., more cells per unit colonial surface area, had greater capability to remain buoyant. Our results suggest that colony buoyancy was not uniform within a P. globosa population, and that biological regulation of colony buoyancy required light energy.