Sexual and vegetative phases in the planktonic diatom Pseudo-nitzschia multistriata

The planktonic diatom Pseudo-nitzschia multistriata (Takano) Takano is known to produce the toxin domoic acid and it is recorded during late summer and autumn in the Gulf of Naples (Tyrrhenian Sea, Italy). We describe the sexual cycle of this species and report information on the variability of growth and cell size reduction rates at different experimental conditions. We induced sexual reproduction by crossing monoclonal cultures of opposite mating type. P. multistriata has a heterothallic life cycle that follows the general pattern reported for other congeneric species. Sexual stages were detected in cultures with an average apical length between 55 and 39 μm. The size of the initial cells produced at the end of the sexual phase was comprised between 72 and 82 μm and the lower cell size detected in culture was 26 μm. Sexual reproduction was thus recorded within a size window corresponding to 39–71% of the maximum cell apical length. Both growth performances and cell size reduction rates depend on cell size. The largest cells showed slower growth rates and larger size reduction rates at each division, while the relationship was opposite for cells smaller then 60% of the maximum size.