THE LIFE-CYCLE OF PHAEOCYSTIS (PRYMNESIOPHYCEAE) - EVIDENCE AND HYPOTHESES

The present paper reviews the literature related to the life cycle of the prymnesiophyte Phaeocystis and its controlling factors and propose s novel hypotheses based on unpublished observations in culture and in the field. We chiefly refer to P. globosa Scherffel as most of the ob servations concern this species. P. globosa exhibits a complex alterna tion between several types of free-living cells (non-motile, flagellat es, microzoospores and possibly macrozoospores) and colonies for which neither forms nor pathways have been completely identified and descri bed. The different types of Phaeocystis cells were reappraised on the basis of existing microscopic descriptions complemented by unpublished flow cytometric investigations. This analysis revealed the existence of at least three different types of free-living cells identified on t he basis of a combination of size, motility and ploidy characteristics : non-motile cells, flagellates and microzoospores. Their respective f unction within Phaeocystis life cycle, and in particular their involve ment in colony formation is not completely understood. Observational e vidence shows that Phaeocystis colonies are initiated at the early sta ge of their bloom each by one free-living cell. The mechanisms control ling this cellular transformation are still uncertain due to the lack of information on the overwintering Phaeocystis forms and on the cell type responsible for colony induction. The existence of haploid microz oospores released from senescent colonies gives however some support t o sexuality involvement at some stages of colony formation. Once colon ies are formed, at least two mechanisms were identified as responsible of the spreading of colony form: colony multiplication by colonial di vision or budding and induction of new colony from colonial cells rele ased in the external medium after colony disruption. The latter mechan ism was clearly identified, involving at least two successive cell dif ferentiations in the following sequence: motility development, subsequ ent flagella loss and settlement to a surface, mucus secretion and col ony formation, colonial cell division and colony growth. Aggregate for mation, cell motility development and subsequent emigration from the c olonies, release of non-motile cells after colony lysis on the other h and, were identified as characteristic for termination of Phaeocystis colony development. These pathways were shown to occur similarly in na tural environments. In the early stages of the bloom however, many rec ently-formed colonies were found on the setae of Chaetoceros spp, sugg esting this diatom could play a key-role in Phaeocystis bloom inceptio n. Analysis of the possible environmental factors regulating the trans ition between the different phases of the life cycle, suggested that n utrient status and requirement of a substrate for attachment of free-l iving cells would be essential for initiation of the colonial form. Ph ysical constraints obviously would be important in determining colony shape and fragmentation although autogenic factors cannot be excluded. Some evidence exists that nutrients regulate colony division, while t emperature and nutrient stress would stimulate cell emigration from th e colonies.