The ultrastructure and life cycle of the coastal coccolithophorid Ochrosphaera neapolitana (Prymnesiophyceae)

Abstract

Unialgal cultures of the widespread tremalith-bearing coccolithophorid Ochrosphaera neapolitana Schussnig were investigated using light and electron microscopy. O. neapolitana exhibits a heteromorphic life cycle identical to that described for the closely related tremalith-bearing genus Hymenomonas, involving an alternation between (diploid) coccolith-bearing cells and a (haploid) non-calcifying phase, characterised by the production of layers of mucilage. Differing scale ornamentation between the two phases together with chromosome counts confirms the diplo-haplontic nature of this cycle. The coccolith-bearing phase may be non-motile or possess flagella. The coccoliths of all strains studied exhibit the typical tremalith structure, but variation in size and shape is observed (vari-monomorphism). Unusual among coccolithophorids, the coccosphere of this species is not continually renewed, and progressive extracellular calcification of the coccoliths is observed in old cultures that may result in the formation of a calcareous ‘pseudocyst’. At the ultrastructural level, the flagellar apparatus is of the Pleurochrysis type, consisting of two flagellar basal bodies linked by proximal, intermediary and distal bands, a vestigial haptonema consisting of five microtubules, and four asymmetrically arranged microtubular roots. Two roots are compound, each possessing a secondary crystalline microtubular component, and two are simple. The peripheral endoplasmic reticulum plays a role in coccolithogenesis, which occurs in the proximity of the plasma membrane. The taxonomy of the genus Ochrosphaera is discussed and suppression of the two other species in the genus, O. verrucosa Schussnig and O. rovignensis Schussnig, is suggested.