The life cycle of Phaeocystis: state of knowledge and presumptive role in ecology

Despite numerous investigations, the number and role of morphotypes involved in the life cycle of Phaeocystis species remain under debate. This is partly due to the application of different methodologies such as light, transmission, scanning electron microscopy and flow cytometry on specific samples. This heterogeneity of approaches results in the incomplete morphometric description of the different cell types existing within one species according to relevant criteria and the indetermination of the ploidy level of each observed stage. We review here the different morphotypes observed within each of the six Phaeocystis species recognized up to now. Four different cell types have been observed. In common to all six species is the occurrence of a scaly flagellate producing star-forming filaments (all species except P. jahnii) or not (P. globosa and P. jahnii). In three colony-forming species, P. globosa, P. pouchetii and P. antarctica, three morphotypes are observed: a flagellate with scales and filaments, a colonial cell, and a flagellate devoid of scales and filaments. In the non-colony-forming species, P. scrobiculata and P. cordata, only flagellates with scales and filaments have been observed. While suspected in P. pouchetii and P. antarctica, a haploid-diploid life cycle has only been evidenced for P. globosa. The two main prominent features of this cycle are that sexuality is prevalent in colony bloom formation and termination and that two types of vegetative reproduction exist. The ecological relevance of alternating haploid and diploid stages is not clearly apparent on the basis of existing ecological studies.