New holococcolith-heterococcolith life-cycle associations are documented based on observations of combination coccospheres. Daktylethra pirus is shown to be a life-cycle phase of Syracosphaera pulchra and Syracolithus quadriperforatus a life-cycle phase of Calcidiscus leptoporus. In addition, new observations from cultures confirm the life-cycle associations of Crystallolithus braarudii with Coccolithus pelagicus and of Zygosphaera hellenica with Coronosphaera mediterranea. In all four cases previous work has shown that the heterococcolithophorid species is associated with another holococcolithophorid. Two other examples of a heterococcolithophorid being associated with two holococcolithophorids have previously been identified, so this seems to be a common phenomenon. The six examples are reviewed to determine whether a single underlying mechanism is likely to be responsible for all cases. It is concluded that there is no single mechanism but rather that the six examples fall into three categories : (a) in two cases the holococcolith types are probably simply ecophenotypic morphotypes ; (b) in two other cases the holococcolith types are discrete and are paralleled by morphometric differences in the heterococcolith types ; (c) in the final two cases the holococcolith types are discrete but are not paralleled by any obvious morphological variation in the heterococcolith morphology. We infer that cryptic speciation may be widespread in heterococcolithophorid phases and that study of holococcolithophorid phases can provide key data to elucidate this phenomenon.
Several coccolithophore species are known to exhibit heteromorphic life cycles. In certain species, notably Emiliania huxleyi, the heterococcolithbearing phase alternates with a non-calcifying stage, whereas in others the heterococcolith-bearing phase alternates with a holococcolith-bearing phase. Heterococcolithophore-holococcolithophore life cycles have previously been observed for only one species in culture, but have also been inferred from an increasing number of observations of combination coccospheres. 18S rDNA sequences from pure cultures of both the heterococcolith-bearing and holococcolith-bearing phases of Coccolithus pelagicus were identical, providing an additional indication of their identity as different life cycle stages of the same species. Flow cytometric analyses have been undertaken on SybrGreen-stained nuclei isolated from pure cultures of the two phases of four coccolithophore species (Coccolithus pelagicus, Calcidiscus leptoporus, Coronosphaera mediterranea and Emiliania huxleyi) in order to determine relative DNA content. Results confirm the hypothesis that holococcolithophoreheterococcolithophore life cycles are haplo-diploid in nature. Light microscope observations of the processes of sexual fusion and meiosis are reported for two of the experimental species. The results are discussed in the context of the evolution of biomineralization in the coccolithophores and the possible ubiquity of haplo-diploidy in the haptophytes.
Coccolithus braarudii and Calcidiscus leptoporus are 2 coccolithophores (Prymnesiophyceae: Haptophyta) known to possess a complex heteromorphic life cycle, with alternation between a motile holococcolith-bearing haploid stage and a non-motile heterococcolith-bearing diploid stage. The ecological implications of this type of life cycle in coccolithophores are currently poorly known. The nutritional preferences of each stage of both species, and their growth response to conditions of turbulence were investigated by varying their growth conditions. Of the different culture media tested, only the synthetic seawater medium did not support the growth of both stages of C. braarudii and C. leptoporus. With natural seawater-based media, the growth rate of the haploid phase of both coccolithophores was stimulated by the addition of soil extract (K/2: 0.23 ± 0.02 d -1 and K/2 with soil extract 0.35 ± 0.01 d -1 for the C. braarudii haploid stage), while the diploid phase was not, indicating that the motile stage is capable of utilizing compounds present in soil extract or ingesting bacteria that are activated in enriched media. The addition of sodium acetate to the medium also stimulated the haploid phase of C. braarudii, and further experiments using labeled bacteria demonstrated the capacity for phagotrophy of this motile stage. The effect of nutrient concentrations on the growth rates of both species was evaluated, showing clear differences between the 2 phases of the life cycle, with higher growth rates for the diploid stage in nutrient-rich media (K/2: 0.34 ± 0.
leptoporus).Responses of the 2 phases of both coccolithophores to physical turbulence were also different, with a haploid flagellate stage sensitive to mixing and a more resistant non-motile diploid stage. The results of these experiments strongly indicate that each morphological stage of C. braarudii and C. leptoporus corresponds to a different ecological niche, the motile haploid stage exploiting a more stable oligotrophic niche than the diploid non-motile stage, in accordance with field observations. The 2-dimensional phytoplankton niche space model of Margalef (1978; Oceanol Acta 1:493-509), defined by nutrients and turbulence, was amended to integrate this hypothesis.
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