Phaeothamniophyceae Classis Nova: A New Lineage of Chromophytes Based upon Photosynthetic Pigments, rbcL Sequence Analysis and Ultrastructure.

A new algal class, the Phaeothamniophyceae classis nova, is established from genera formerly classified in the Chrysophyceae (e.g., Chrysapion, Chrysoclonium, Chrysodictyon, Phaeobotrys, Phaeogloea, Phaeoschizochlamys, Phaeothamnion, Selenophaea, Sphaeridiothrix, Stichogloea, Tetrachrysis, Tetrapion and Tetrasporopsis) as well as one genus previously assigned to the Xanthophyceae (Pleurochloridella). HPLC analysis revealed the presence of fucoxanthin, diadinoxanthin, diatoxanthin, β-carotene and heteroxanthin, in addition to chlorophylls a and c, in four genera (Phaeoschizochlamys, Phaeothamnion, Stichogloea, Pleurochloridella). The combination of fucoxanthin and heteroxanthin is known only for these organisms. The rbcL sequences of the same four genera, along with representatives of other chromophyte classes, were analyzed phylogenetically and provided independent support for recognition of the Phaeothamniophyceae as a distinct taxon. These data indicate that the Phaeothamniophyceae are more closely related to the classes Xanthophyceae and Phaeophyceae than to the Chrysophyceae. Electron microscopy revealed that Phaeoschizochlamys, Phaeothamnion and Stichogloea possess electron opaque vesicles at the cell periphery, have a cell wall that often appears laminate, form new daughter cell walls via eleutheroschisis, and have plastids with girdle lamellae and a ring-shaped genophore. The flagellar apparatus of Phaeothamnion zoospores (described in a previous study) is chosen as representative of the new class. The flagella are inserted laterally, basal bodies form an angle of ca. 145° or more, a multi-gyred flagellar transitional helix is present and tripartite flagellar hairs lack lateral filaments. Genera placed in the Phaeothamniophyceae are assigned to the orders Phaeothamniales and Pleurochloridellales, each with a single family.