DAILY IRRADIANCE GOVERNS GROWTH-RATE AND COLONY FORMATION OF PHAEOCYSTIS (PRYMNESIOPHYCEAE)

Phaeocystis was cultured at a range of ecologically significant daily irradiances under nutrient-replete conditions. Below a threshold of 10 0 W h m-2 day-1, the cells were small and flagellated, and remained so litary. Above this threshold, the cells were larger and able to form c olonies. Growth rate and colony formation were maximum at sea surface irradiances (>700 W h m-2 day-1). Presumably, colonial growth is a str ategy to maintain optimum growth rates in the water column. Sinking, n utrient-stressed colonies reach low irradiances and colonial cells can transform into small solitary flagellated cells. These observations a re important in understanding the ecology and life cycle of Phaeocysti s.