A massive and simultaneous sex event of two Pseudo-nitzschia species

Sex is an obligatory event in the life cycle of many diatoms, being the means of restoration of maximum cell size that counteracts the progressive miniaturization induced by asexual divisions. Nonetheless, sexual stages and evidence for sexual reproduction in diatoms have rarely been directly reported from the marine environment. On September 20, 2006, different stages of the sexual cycle of two species, Pseudo-nitzschia cf. delicatissima and P. cf. calliantha, were detected in surface waters of the Gulf of Naples, at the Long Term Ecological Research Station MareChiara. The sampling date corresponded with a seasonal peak of abundance of the two species, which reached maximum concentrations of 9.1×106 and 7.0×105 cells L−1, respectively. Gametangia, auxospores of different size, and initial cells within and outside the perizonium were observed. Up to 9.2% of P. cf. delicatissima and 14.3% of P. cf. calliantha cells were undergoing sexual reproduction. The sexual event was mainly concentrated in surface waters and no sexual stages were observed a week before or after the date. The low percentage of sexual stages and the short duration of such events are probably the reasons for the scarcity of sexual stage observations in nature.