Gymnodinium chlorophorum, a new, green, bloom-forming dinoflagellate (Gymnodiniales, Dinophyceae) with a vestigial prasinophyte endosymbiont

During the summer 1990, a green water discolouration off Helgoland, the North Sea, was caused by a dinoflagellate, here described as Gymnodinium chlorophorum sp. nov. The apoplastidic dinoflagellate harbours an endosymbiont presumably of prasinophycean lineage, separated from the host by two ribosome-free membranes. The endosympbiont cytoplasm contains ribosomes, small vesicles, and putative tiny nuclei, in addition to chloroplasts which contain chl a and b, prasinoxanthin, but lacks chl c and peridinin. Gymnodinium chlorophorum differs from the ultrastructurally very similar dinoflagellate Lepidodinium viride Watanabe et al. in the absence of a 'cytoplasmic projection' and body scales. It is placed in the genus Gymnodinium Stein and reasons are discussed, why in dinophytes, pigment composition and chloroplast ultrastructure are used as taxonomic criteria at the species level only, in contrast to common use in phycology.