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Flagellates

The P. jahnii flagellate is round, 3.5-5.0 |m in diameter when observed live, and has two to four golden-brown parietal chloroplasts. It bears two unequally long flagella (8.5-12 |m and 5.56.5 |m), a non-coiling haptonema 3.0-4.5 |m in length, and two scale types (Zingone et al. 1999). Filaments were not observed.

Morphotype of P. cordata

Phaeocystis cordata has only been described as a flagellated cell (Zingone et al. 1999). Live cells are 3-3.5 |m long and 3-4 |m wide. They have two unequally long flagella of 5.5-7.5 |m and 4.5-6 |m in length, and a non-coiling haptonema 2.2-2.5 |m in length. They possess two scale types, both with faint radiating ridges. These cells produce filaments forming pentagonal figures.

Morphotype of P. scrobiculata

Phaeocystis scrobiculata, which has been observed occasionally in field samples and never in cultures, was only reported as a flagellate (Moestrup 1979; Hallegraeff 1983; Estep etal. 1984; Chretiennot-Dinet unpublished data). The

Fig. 3 Schematic representation of a haploid-diploid life cycle (from Valero et al. 1992)

cell, 8 |m in length when fixed with osmium tetroxide, bears two equal flagella and a non-coiling haptonema. It is covered by a periplast of two types of scales, and produces filaments longer than 50 |m which form a nine-ray figure rather than the five-ray star observed in the other Phaeocystis species (Moestrup 1979; Hallegraeff 1983). Large differences have been reported in the length of the flagella and haptonema, and in body scale sizes (Moestrup 1979; Hallegraeff 1983; Estep etal. 1984; Hoepffner and Haas 1990) depending on the preparation procedures.

Synthesis of the observed morphotypes

A careful examination of the literature suggests that four different cell types exist within Phaeo-cystis species (Table 1). In common to all six species, whether colonial or not, is the occurrence of scaly flagellates (Table 1) which are of two types. One produces star-forming filaments and has been reported for all species except P. jahnii. The other, deprived of filaments and stars, has been found in P. globosa (Parke et al. 1971; Peperzak et al. 2000a) and P. jahnii (Zingone et al. 1999). These two cell types are particularly important as they provide relevant taxonomic criteria to compare and distinguish the different species of the genus. These criteria include difference in size, structure and arrangement of scales and star-forming threads (Moestrup 1979; Moestrup and Larsen 1992; Baumann et al. 1994; Medlin et al. 1994; Zingone et al. 1999; Jacobsen 2002; Lange et al. 2002). These cells have been shown to be haploid in P. globosa (Vaulot et al. 1994) but diploid in P. pouchetii (Jacobsen 2002).

In common to three colony-forming species, P. globosa, P. pouchetii and P. antarctica, is the occurrence of three cell types: a flagellate with

Diploid flagellate

Diploid flagellate

Haploid flagellate

Colonial cell

End of bloom

Haploid flagellate

Colonial cell

End of bloom

Inter-bloom: haploidy

Bloom: diploidy

Fig. 4 The haploid-diploid life cycle of P. globosa. The haploid flagellates are characterized by stars, filaments, scales and have a size in the range 3.6-5.8 ^m when live. Colonial cells, in the size range 5.8-10.4 ^m when live, present two short appendages on their apical side, are deprived of haptonema, stars, filaments and scales. Diploid flagellates, of the same size range than colonial cells, have two flagella, a haptonema and lack the stars, filaments and scales scales and filaments, a colonial cell, and a flagellate devoid of scales and filaments (Table 1). The Xagellates with scales and Wlaments have been observed to increase in number before colonial cell blooms, to disappear when colonial cells are present, and are massively formed inside colonies at the end of the colonial stage. The presence of larger Xagellates is usually found restricted to that of colonial cells. In species that do not form colonies, P. scrobiculata and P. cordata, only Xagellates with scales and Wlaments have been observed (Table 1).

The Phaeocystis life cycle

The haploid-diploid life cycle of P. globosa

The existence of different morphotypes, two ploidy levels related to phase changes, and the ability of both haploid and diploid stages to divide mitotically (Kornmann 1955; Rousseau et al. 1994; Vaulot et al. 1994), support the existence of a haploid-diploid life cycle in P. globosa. In such life cycles, both haploid and diploid stages are related by sexual processes, meiosis and syngamy, and both are capable of mitotic division (Fig. 3;

Valero et al. 1992; Houdan et al. 2004). Based on available information from cultures and field studies presented in the previous section, the P. globosa haploid-diploid life cycle has been reconstructed (Fig. 4). 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.

Phaeocystis globosa colony blooms result from sexual processes

The occurrence of haploid flagellates in the water column between two blooms of diploid colonial cells as observed in the southern North Sea (Rousseau and Chretiennot-Dinet unpublished data) provides evidence that P. globosa colony bloom initiation and termination involve sexual processes, with the length of the diploid phase being restricted to the colony blooms. The formation of a diploid non-motile colonial cell from haploid flagellates implies that syngamy (fusion of the cytoplasm and nuclei of two haploids and subsequent zygote production) must occur at the time of colony bloom initiation. The observation of two morphologically distinguishable haploid flagellates (Parke et al. 1971; Peperzak etal.

2000a) suggests that anisogamy occurs in P. globosa, but this still awaits proof. Filaments characteristic of haploid flagellates could possibly play a role in mating. These structures have indeed been suggested to be related to sexuality (Vaulot et al. 1994) or to play a role in attachment (Chretien-not-Dinet 1999). The inability to regenerate dip-loid colonial cells from clones of haploid flagellates (Parke et al. 1971; Vaulot etal. 1994) suggests that different mating types, i.e., compatible gametes able to form zygotes, exist within P. globosa (Vaulot et al. 1994). It is not known if homothallism (self-fertile colonies) or heterothal-lism (self-sterile colonies) is the rule in P. globosa.

Conversely, meiosis must happen to form hap-loid flagellates from diploid colonial cells. This may well occur during the massive production of haploid flagellates within colonies often reported at the end of the P. globosa bloom, before disappearance of the colonial stage (Rousseau et al. 1994; Peperzak et al. 2000a). Haploid flagellates are however produced in a restricted number of colonies. Most of them senesce and subsequently aggregate, being progressively invaded by various heterotrophic organisms that develop complex microbial networks (Lancelot and Rousseau 1994; Lancelot et al. 2002).

Vegetative reproduction in P. globosa

The vegetative reproduction of the diploid stage occurs through two distinct pathways involving colonial cells and diploid flagellates (Fig. 4). One consists of mitotic division of colonial cells within the colony, i.e., colony growth (Kornmann 1955; Rousseau et al. 1994; Veldhuis et al. 2005). This process can lead to colony division and budding as observed in mesocosms (Verity et al. 1988a) and in the field (Rousseau et al. 1994) but seems to be of minor importance. The second pathway involves the transition through short-lived diploid flagellates that are released from colonies and are able to reinitialize the colonial stage within a day (Kornmann 1955; Cariou et al. 1994; Rousseau et al. 1994). Diploid flagellates therefore co-occur with colonial cells and propagate the colonial stage, a pathway commonly used to produce colony cultures in the laboratory. However, its significance in the natural environment is highly questionable and is probably reduced due to the short life span of diploid flagellates (Kornmann 1955; Cariou et al. 1994). The natural occurrence of diploid flagellates in the field is difficult to estimate. It results from disruption of large colonies when the sea is stormy and under turbulent conditions (Kornmann 1955, Peperzak et al. 2000a). Occurrence of these cells, in association with non-motile free-living cells, could however also result from sample manipulation during collection, size fractionation, and incubation. The minor role of the diploid flagellate in the natural environment is also suggested by the observation of massive and synchronous generation of small colonies in the early phase of blooms (Rousseau et al. 1990). A significant vegetative reproduction would instead result in more-continuous colony production. However, this pathway could not be excluded and has been suggested to provide the inoculum for colony blooms (Cadee 1991).

Factors inducing phase changes within the P. globosa life cycle

Several factors have been hypothesized to play a role in transitions between P. globosa morphotypes. The formation of colonies from free-living cells has been related to phosphate depletion (Veldhuis and Admiraal 1987; Cariou et al. 1994), light intensity (Kornmann 1955; Peperzak 1993; Peperzak et al. 1998), chemical substances produced by vernal diatoms, especially some Chae-toceros species (Weisse et al. 1986; Boalch 1987; Rousseau et al. 1994), and turbulence (Schapira 2005; Shapira et al. 2006). The requirement of a solid substrate for cell attachment has also been suggested from the observation of small colonies attached to Chaetoceros setae at the early stage of the bloom (Fig. 1; Boalch 1987; Rousseau etal. 1994; Chretiennot-Dinet et al. 1997).

However, a careful examination of literature shows that most studies on P. globosa colony formation have been performed in laboratory cultures using the vegetative reproduction pathway, i.e., with an inoculum of diploid flagellates originating from colony disruption (Table 2). These studies show that solid substrate, turbulence, and phosphate are factors promoting the vegetative generation of colonies (Table 2). From field

Table 2 Factors involved in the transition from free-living cell to colonial stage in P. glob.

osa

References

Factors

Exp.

Free-living cell origin

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