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 100 W h m −1 day −1 , the cells were small and flagellated, and remained solitary. Above this threshold, the cells were larger and able to form colonies. Growth rate and colony formation were maximum at sea surface irradiances (>700 W h m −2 day −2 ). Presumably, colonial growth is a strategy to maintain optimum growth rates in the water column. Sinking, nutrient-stressed colonies reach low irradiances and colonial cells can transform into small solitary flagellated cells. These observations are important in understanding the ecology and life cycle of Phaeocystis .