Ultrastructure of a new marine sand-dwelling dinoflagellate,Gymnodinium quadrilobatumsp. nov. (Dinophyceae) with special reference to its endosymbiotic alga

Abstract

A new sand-dwelling dinoflagellate, Gymnodium quadrilobatum sp. nov., is described from the south coast of Natal, South Africa. The organism contains 20–30 chloroplasts, which are peripherally arranged, and a conspicuous eyespot. The motile cell is directly transformed into a non-motile cell and cell division is restricted to the non-motile phase. The motile cell has a typical gymnodinioid organisation, while the non-motile cell possesses a thick cell wall and develops the characteristic cell shape, which is reminiscent of a four-leaved clover. The chloroplasts of G. quadrilobatum belong to an endosymbiotic alga the cytoplasm of which is separated from the host (dinoflagellate) cytoplasm by a single unit membrane. Other organelles contained in the endosymbiont cytoplasm are the nucleus, ribosomes and mitochondria. The endosymbiont is thought to be a chromophyte alga. The eyespot is bounded by a triple membrane and is situated in the host cytoplasm. On the basis of the similarities in the ultrastructure of G. quadrilobatum with those of four thecate dinoflagellates, viz. Peridinium balticum, P. foliaceum, P. quinquecorne and Peridinium sp., which are known to possess a similar endosymbiont, possible phylogenetic affinities between G. quadrilobatum and four thecate dinoflagellates are discussed.