Influence of phosphate depletion on the growth and colony formation of Phaeocystis pouchetii

The haptophycean alga Phaeocystis pouchetii (Hariot) Lagerheim, a bloom-forming species in the North Sea, formed massive colonies in batch cultures only when the phosphate concentration in the medium was below 1 mol l^-1. In general, colony cells and single cells showed a similar response to phosphate depletion: decreased cellular phosphate (up to a factor of 32), decreased chlorophyll-a concentration (up to a factor of 4.2) and high activities of alkaline phosphatase (APA). However, the growth rate of colony cells was reduced at low phosphate concentrations (<1 mol l^-1) in contrast to that of single cells. Colony cells tended to have a slightly higher phosphate concentration than single cells, while the increase in APA was delayed. The differences between single cells and colonies in phosphate utilization, cellular phosphate content, and growth rate cause, at low phosphate levels, a shift towards the formation of colonies. Similar changes seem to occur in nature during transition of nutrient-sufficient to nutrientlimited conditions.This investigation was supported by the Foundation for Fundamental Biological Research (BION), which is subsidized by the Dutch Organization for the Advancement of Pure Research (ZWO). Publication No. 8 of the project Ecological Research of the North Sea (EON)