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 were first isolated by filtration onto a 10 mum mesh. Colonial cells were then liberated by shaking and inoculated into individual culture wells containing medium with a PO42-concentration of approximately 1 muM. Cell size and shape were determined daily by image analysis. while chlorophyll and DNA distributions were estimated by flow cytometry. Released cells were non-flagellated and mostly located in the G1 phase of the cell cycle. They developed flagella and up to 90% became motile within 24 h. Swarmers lost motility rapidly, became elongated, began to cycle again, excreted a mucilaginous compound and divided leading to new colonies within a few days. During this reproducible process, no change of ploidy could be observed. Colonies initially adhered to the bottom of culture wells. Frequent mixing drastically reduced the fraction of colonies produced and their volume. High initial pO42- concentrations (5 muM) delayed colony appearance. whereas low concentrations (0.3 muM) prevented colony formation. The two main conclusions of this study are: (i) under favorable conditions (approximately 1 muM PO42-, no mixing), a large percentage of released colonial cells give back colonies after going through a flagellated stage; (ii) sexuality does not appear to be involved in this process.