THE INITIATION OF PHAEOCYSTIS COLONIES

This study was designed to elucidate the sequence of events that leads to the formation of new colonies of Phaeocystis sp. (strain PCC 540) starting from single cells released from mature colonies. Colonies wer e first isolated by filtration onto a 10 mum mesh. Colonial cells were then liberated by shaking and inoculated into individual culture well s containing medium with a PO42-concentration of approximately 1 muM. Cell size and shape were determined daily by image analysis. while chl orophyll and DNA distributions were estimated by flow cytometry. Relea sed cells were non-flagellated and mostly located in the G1 phase of t he cell cycle. They developed flagella and up to 90% became motile wit hin 24 h. Swarmers lost motility rapidly, became elongated, began to c ycle again, excreted a mucilaginous compound and divided leading to ne w colonies within a few days. During this reproducible process, no cha nge of ploidy could be observed. Colonies initially adhered to the bot tom of culture wells. Frequent mixing drastically reduced the fraction of colonies produced and their volume. High initial pO42- concentrati ons (5 muM) delayed colony appearance. whereas low concentrations (0.3 muM) prevented colony formation. The two main conclusions of this stu dy are: (i) under favorable conditions (approximately 1 muM PO42-, no mixing), a large percentage of released colonial cells give back colon ies after going through a flagellated stage; (ii) sexuality does not a ppear to be involved in this process.