Raphidophyceae (raphidophyta)

Abstract

The Raphidophyceae are flagellated unicellular algae that live in diverse marine, brackish, and freshwater habitats. Ten genera are currently recognized: Gonyostomum, Merotricha, Vacuolaria, Chattonella, Chlorinimonas, Fibrocapsa, Haramonas, Heterosigma, Psammamonas, and Viridilobus (the first three are freshwater representatives). They are wall-less heterokonts, i.e., the forward flagellum possesses tubular mastigonemes, and both flagella arise from a shallow pit at or near the apex of the cell. All known raphidophytes are photosynthetic and bear multiple plastids containing chlorophylls a and c1and/or c2. With the exception of Chlorinimonas sublosa, marine species possess fucoxanthin as a major carotenoid, while freshwater representatives lack this pigment. Marine raphidophytes are widely recognized as ichthyotoxic organisms; species such as Chattonella spp., Fibrocapsa japonica, and Heterosigma akashiwo have been associated with finfish kills. Knowledge of the raphidophyte life cycle, cyst formation, and vertical migratory behavior is important for understanding mechanisms of bloom formation. Molecular phylogenetic analyses suggest that (1) the greenish colored freshwater species diverged from brownish colored marine raphidophytes, (2) all three species of the genus Haramonas and a species of Psammamonas are sand-dwelling and evolved from a marine plank- tonic ancestor by acquiring characters of benefit to benthic habitats, (3) Chlorinimonas is also sand-dwelling, a characteristic that must have been acquired independently from Haramonas and Psammamonas, and (4) basal lineages of the Raphidophyceae, Fibrocapsa, Haramonas, and Psammamonas, possess unique carotenoids such as fucoxanthinol (F. japonica and P. australis) and 19'-butanoyloxyfucoxanthin (H. dimorpha), but the significance of the pres- ence of these pigments is currently unknown.