Specificity of Chroomonas (Cryptophyceae) as a source of kleptochloroplast for Nusuttodinium aeruginosum (Dinophyceae)

The unarmoured dinoflagellate Nusuttodinium aeruginosum retains a kleptochloroplast, which is a transient chloroplast stolen from members of the cryptomonad genus, Chroomonas. Both N. aeruginosum and the closely related N. acidotum have been shown to restrict their diet to a limited number of species of this blue‐green genus of cryptophyte. However, it is still unclear how flexible the predators are with regard to the ingestion and utilization of Chroomonas spp. as a source of kleptochloroplast. To address specificity of cryptomonad in N. aeruginosum, we collected the cells of N. aeruginosum from several ponds in Japan, and analysed the phylogeny of the kleptochloroplasts based on their plastidial 16S rDNA sequences. All sequences obtained in this study were restricted to only one (the subclade 4) of four subclades known to comprise the Chroomonas/Hemiselmis clade. Therefore, N. aeruginosum is specific in its dietary requirements, selecting their prey within the subclade level.     

INGESTION AND RETENTION OF CHROOMONAS SPP. (CRYPTOPHYCEAE) BY GYMNODINIUM ACIDOTUM (DINOPHYCEAE)1

Gymnodinium acidotum Nygaard, a blue-green dinoflagellate previously shown to contain cryptophycean chloroplasts and other organelles, was observed from water and soil samples and in culture. The dinoflagellate excysts from soil samples as a mononucleated colorless cell that is positively phototactic. Colorless cells in unialgal culture remain colorless and can only be maintained less than one week whereas pigmented cells cultured unialgally grow for 10 days but then decline rapidly. Colorless cells cultured with Chroomonas spp. regain chloroplasts and have been maintained in mixed cultures for nine months. Fifty-seven percent of the dinoflagellates from mixed cultures are bi-nucleated, and three cells have been observed possibly ingesting cryptophytes. We suggest that cryptophycean chloroplasts are retained and possibly utilized by G. acidotum for at least ten days and then digested.