Life cycle association of the coccolithophore Syracosphaera gaarderae comb. nov. (ex Alveosphaera bimurata): Taxonomy, ecology and evolutionary implications

The coccolithophores are an important group of marine phytoplankton bearing external calcite plates called coccoliths, and displaying complex sexual life cycles, composed of two main cell phases (haploid and diploid) that in most species can produce distinct coccolith types. Typically, diploid cells produce heterococcoliths, whereas haploid cells produce holococcoliths. An important source of evidence for the ability of coccolithophores to undergo life cycle transitions comes from observations of combination coccospheres. These combination cells bearing both hetero- and holococcoliths are interpreted as cells undertaking sexual transitions and enable linking morphologically distinct life phases, thus expanding our knowledge of coccolithophore life cycle diversity, but more widely on coccolithophore ecology and evolution. Here we report on a newly discovered life cycle in the genus Syracosphaera, linking the heterococcolithophore previously named Alveosphaera bimurata Okada & McIntyre, erroneously placed within the Calciosoleniaceae, and the holococcolithophore Poricalyptra gaarderae Borsetti & Cati, resulting in a new combination, Syracosphaera gaarderae (Borsetti & Cati) comb. nov. This life cycle association is supported by several combination coccospheres collected in the Northern Red Sea and is the first involving both the genus Poricalyptra and a Syracosphaera belonging to the lamina type subgroup. Biennial survey indicates that cells of both life cycle phases are only found during summer stratification periods and display clear depth zonation with S. gaarderae appearing in the lower photic zone and P. gaarderae in the upper photic zone. This marked vertical separation between life phases was further confirmed across other oceanic basins through bibliographic revision, confirming the prevailing view that coccolithophore life phases occupy different ecological niches.