124 LIFEHAB
LIFE CYCLES IN HAPTOPHYTA. C. LANCELOT AND V. ROUSSEAU
The microalgal class Prymnesiophyceae Hibberd in the division Haptophyta includes some well-known HAB species, all of which occur in European waters. Among the haptophytes implicated in HAB events are ichthyotoxic species (Chrysochromulina polylepis, C. leadbeateri, Prymnesium parvum, Phaeocystis pouchetii) and high- biomass colonial Phaeocystis (P. globosa in nutrient-enriched coastal areas). The Prymnesiophyceae also includes the coccolithophorids (haptophytes covered with calcified scales) which are generally not considered as HAB species but are seen as important agents in climate regulation (e.g. Emiliana huxleyi). Some non-blooming coastal coccolithophorid species, including members of the genera Pleurochrysis and Ochrosphaera, are however suspected to have the capability of producing toxins (I. Probert, unpublished results).
Although knowledge of the haptophytes has expanded with recent focus on HAB species and the coccolithophorids, information is still needed on the basic biology and particularly the life cycles of this group as a whole. The discussion focussed on three main areas: life cycle mechanisms, haptophyte toxins, and the ecological relevance of haptophyte life cycles.
Life cycle mechanisms
Despite the fact that very few complete life cycles have been entirely elucidated in haptophytes, there is growing evidence that haplo-diploid life cycles with alteration of morphologically distinct stages are widespread (possibly ubiquitous) in the Prymnesiophyceae. Current knowledge of haptophyte life cycles results mainly from culture studies in which ploidy levels have been demonstrated by various methods (chromosome counting, flow cytometry, etc.), but the processes of meiosis and syngamy have rarely been observed. In some cases, field observations complement our knowledge, particularly in Phaeocystis, with observations of stages which have not been seen in culture. The life cycles of known haptophyte HAB species vary in their complexity, involving alternation of two or more morphologically distinct stages. In this volume, the current state of knowledge in the genera Phaeocystis (V. Rousseau, L. Peperzak), Chrysochromulina and Prymnesium (B. Edvardsen) are reviewed. The coccolithophorid genera Pleurochrysis and Ochrosphaera alternate haploid scale-bearing cells and diploid coccolith-bearing cells. In the coccolithophorid genus Pleurochrysis, meiosis and syngamy have been directly observed (Gayral & Fresnel, 1983). Also, alternance of holococcolith-heterococcolith has been demostrated in several cocoloithophors (Cross et al. 2000).
The main points to arise from this part of the discussion were: • Identification of complete life cycles, including ploidy analysis, is required in more members of the Prymnesiophyceae in order to determine whether patterns exist across group. Improvements in culture techniques are required to be able to maintain and manipulate a wider range of haptophytes in culture. GAPS IN KNOWLEDGE 125
• Direct evidence for a haplo-diploid sexual life cycle involving meiosis and syngamy is needed. These processes are rare and probably short-lived events. Knowledge of the triggering factors for phase changes would increase the likelihood of observing these phenomena in culture (at present we have very little information on potential phase change triggering factors in haptophytes). • Are complex mating systems involved in gamete recognition? Clonal cultures of haploid stages of Prymnesium and some coccolithophorids have been observed to undergo the transition to the diploid stage (i.e. homothallism can occur), but information is at present too scarce to form any solid conclusions. • Field and culture observations need to be cross validated (e.g. can the different flagellate stages be produced in cultures of Phaeocystis? Do Chrysochromulina and Prymnesium benthic stages actually exist in nature?).
Haptophyte toxins The toxins involved in haptophyte HAB events have only been elucidated extensively in P. parvum. Further studies are required to determine the nature of toxins in other haptophyte species: is one toxin type common to all toxic haptophytes, or are different toxins involved as in the dinoflagellates? Preliminary tests showing that Pleurochrysis and closely related littoral coccolithophorid genera are toxic to Artemia require validation.
Evidence from some cultured clones of Chrysochromulina reveals that toxin production may differ between life cycle stages (B. Edvardsen, this volume). Pure cultures of different ploidy stages of clonal cultures are required to determine whether this is a common phenomenon. Little information is available on the genotypic vs phenotypic control of toxin production in haptophytes and the extent to which cultivated cells maintain their ability to produce toxins over time.
Life cycles and ecology The relevance of the haplo-diploid life cycle to haptophyte ecology was discussed. Unlike diatoms (diploid life cycle) and dinoflagellates (haploid life cycle), both stages of the haptophyte haplo-diploid life cycle are capable of independent asexual division. Generally a haplo-diploid life cycle is considered as an adaptation to an environment which is seasonally variable or contains two different niches. Preliminary evidence from autecological studies of pure cultures of haploid and diploid stages of certain oceanic coccolithophorid species suggests an ecological differentiation between stages. The diploid stage adopts a strategy similar to diatoms (r-selected: high growth rates, use of inorganic nutrients, resistance to turbulence...), while the haploid stage, like dinoflagellates, is better adapted to stable nutrient-poor conditions (K-selected: low growth rates, motility, mixotrophic nutrition...). Some time-series distribution studies of coccolithophorids in nature do provide support for such ecological niche separation. Do life cycles play key roles in the ecology of haptophyte HAB species? No autecological differences have been observed in cultures of the two phases of Prymnesium parvum, and in our experience no clear niche separation is evident in nature. The two flagellate stages reported in Chrysochromulina polylepis have different environmental 126 LIFEHAB
requirements and could be adapted responses to changing conditions in the water column. Systematic autecological studies on both stages of haptophyte HAB species would not only provide useful ecological information towards interpreting / predicting natural bloom events, but may also provide evidence on the factors inducing phase transitions. From our discussion, the factors suspected to be of potential relevance for phase transitions are: inorganic and organic nutrient depletion, light quality and intensity, infochemicals (diatoms, grazers), turbulence and endogenous factors / biological clock (it was noted that phase changes in cultures maintained in constant conditions throughout the year often occur at similar times of the year).
One intriguing question about HAB haptophytes is the extent to which toxin production, colony formation (see Lancelot et al., this volume), or coccolith production correspond to an adaptive or defence strategy for developing blooms.
Summary of priorities for future research • The identification of all life stages (including benthic stages) in the field is a prerequisite for a complete construction of the life cycle of the different species. The combination of flow cytometric, electron microscopic and genetic techniques applied to samples taken at high temporal and depth resolution (including sediment) is necessary to ensure the identification of the life stages and avoid taxonomic confusion. • Cultures of the identified life cycle stages would provide material for the identification of the processes leading to the reconstruction of the whole life cycle and the understanding of its controlling mechanisms. These include syngamy, meiosis and mating systems (never studied in most species), the formation and germination of Prymnesium and Chrysochromulina cysts and possibly Phaeocystis amoeboid stages, the process of Phaeocystis colony formation and termination. The factors triggering the switch between the different life stages have also to be identified. • These laboratory cultures would provide material for ecophysiological characterization of the different stages. This would contribute to an understanding of possible ecological niche separation of the different stages in the natural environment and of how the harmful events associated with these species (toxicity, foam, anoxia) are related to their life cycles.