The Taxonomic Position of Phaeodactylum tricornutum

SUMMARY : A revised taxonomic description of Phaeodactylum tricornutum Bohlin is presented, with special emphasis on the details of the silica valve, a structure hitherto unobserved. The features distinguishing P. tricornutum from Nitzschia closterium (Ehr.) W. Sm., with which it has frequently been confused, are summarized. In view of the unique characters of P. tricornutum, it is proposed to establish within the Bacillariales a new suborder, the Phaeodactylineae, to which Phaeoductylum is assigned as the only known genus. In 1897 Bohlin described a new genus of unicellular algae, Phaeodactylum, with a single species, P. tricornutum. The organism was stellate with three narrow arms, possessed yellow-brown chromatophores and was weakly silicified. On the basis of the chemical nature of the cells and their characteristic plane of division, parallel with that of the arms, Bohlin considered that P. tricornutum was closely related to the diatoms. Wilson (1946), in a study of the various cell forms of ' Nitxschia closterium W. Sm. forma minutissima ', recognized the triradiate form of this organism as identical with P. tricornutum Bohlin. Since Hendey (1954) found no evidence for siliceous structures in the cell walls of this organism, he felt that it could not be classed as a diatom, and the pre- sence of leucosin, as shown by Parke (recorded by Hendey, 1954), suggested to him a possible relationship to the Chrysophyceae. On the basis of the absence of movement, the absence of a raphe, and the absence of siliceous structure, Bourrelly & Dragesco (1955) also concluded that P. tricornutum could not be regarded as a diatom, but was a 'primitive Chrysococcalian'. Since the oval cells of P. tricornutum have now been shown to possess a silica valve (Lewin, Lewin & Philpott, 1958), it seems necessary to describe the organism more precisely on this basis, and to assign it to an appropriate taxonomic position. Though the specific epithet tricornutum might appear inappropriate, having been based on an atypical form, it has to be retained according to the rules of the International Code of Botanical Nomenclature (Lanjouw et al. 1956).