Ecology of oceanic coccolithophores.: I.: Nutritional preferences of the two stages in the life cycle of Coccolithus braarudii and Calcidiscus leptoporus

Coccolithus braarudii and Calcidiscus leptoporus are 2 coccolithophores (Prymnesio-phyceae: 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 +/- 0.04 d-1, K/25: 0.29 +/- 0.01 d(-1), K/100: 0.16 +/- 0.01 d(-1) for the haploid stage of C. 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.