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s novel hypotheses based on unpublished observations in culture and in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basis of existing microscopic descriptions complemented by unpublished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.