The effects of nitrate, ammonium and urea on the growth and colony formation of three strains of Phaeocystis globosa were investigated. Although ammonium and urea supported growth, nitrate was the favoured nitrogen source for the growth of solitary cells for all three strains. Phaeocystis globosa CCMP 1528 and 629 formed colonies in all cultures where nitrate was the sole nitrogen source, but only a few colonies were observed in ammonium and urea treatments. Ammonium and urea were far less effective in supporting growth, biomass generation and colony formation in all three strains. Once colonies developed, colonial cells accounted for at least 15% of the total cells when grown with nitrate; colonial chlorophyll also contributed up to 60% to the total chlorophyll. The growth rates of colonial cells when using nitrate were greater than solitary cells. Changes in colony size, colonial cell abundance and total P. globosa abundance as affected by the nitrogen source may influence the carbon flux within the pelagic food web
