Holo-heterococcolithophore life cycles: ecological implications

摘要

Many coccolithophores have complex life cycles with haploid and diploid stages bearing structurally different coccolith types (holococcoliths and heterococcoliths, respectively). Laboratory studies suggest that holo- and heterococcolithophores may occupy distinct ecological niches, but the potential ecological implications of the existence of haploid and diploid stages are poorly known. We present here a study of holo- and heterococcolithophore distributions in the Catalano-Balearic Sea, during 2 cruises (MESO-96, from 18 June to 3 July, and FRONTS-96, from 16 to 21 September) that covered 2 intervals of the stratification period of 1996. We define a holo-coccolithophore prevalence index (HOLP index), calculated for each coccolithophore-containing sample, as the percent ratio between the number of holococcolithophores and the total number of holo- and heterococcolithophores belonging to families with alternation of holo- and heterococcolithophore life stages (coccolithophores having HOL-HET life cycles; Total_HHLC). In both cruises, the distribution of holo- and heterococcolithophores and the HOLP index indicated a preference of the holococcolithophores for shallower waters and of the heteroccolithophores for deeper layers. This segregation may be linked to a differentiation of ecological niches, with the haploid holococcolithophores occupying the more oligotrophic upper layers and the diploid heterococcolithophores inhabiting relatively rich deeper waters.