Phaeothamniophyceae

Introduction

Included genera: Chrysapion, Chrysoclonium, Chrysodictyon, Phaeobotrys, Phaeogloea, Phaeoschizochlamys, Phaeothamnion, Sphaeridiothrix, Selenophaea, Stichogloea, Tetrachrysis, Tetrapion, Tetrasporopsis.

Characteristics

• Golden-brown colored photosynthetic eukaryotes that are filamentous, pseudofilamentous, coccoid or capsoid in nature.  Most described from fresh or brackish waters.
• Mitochondria with tubular cristae.
• Plastids (2-4 per cell) with girdle lamellae and ring-shaped nucleoids. Pyrenoids sometimes present and, if so, immersed.
• Photosynthetic pigments include chlorophylls a and c, fucoxanthin, heteroxanthin, diadinoxanthin, diatoxanthin, and beta carotene.
• Cell walls usually appear to be of laminate (layered) construction.
• Electron dense and electron transparent vesicles of unknown composition are present at the cells’ periphery.
• Asexual reproduction by cell division, nonmotile autospores, or biflagellate zoospores.  Sexual reproduction unknown.
• Reproductive cells are formed by eleutheroschisis, meaning that the daughter cells form entirely new cell walls inside the older mother cell wall.
• Zoospores with subapically to laterally inserted flagella; basal bodies forming an obtuse angle with respect to one another.
• Mastigonemes (flagellar hairs) lacking lateral filaments.
• Transitional helix present.
• Storage products (lipids, chrysolaminaran?), contractile vacuoles, and rhizoplast unknown.

Taxonomic History and Phylogeny

The Phaeothamniophyceae was established by Bailey et al. (1998) and includes organisms once classified in the Chrysophyceae or Xanthophyceae.  At this time the class includes 13 genera and 26 specific or subspecific taxa (varieties).  The class was originally erected based on ultrastructural and/or DNA sequence data for three principal taxa: Phaeoschizochlamys mucosa, Phaeothamnion confervicola, and Stichogloea doederleinii.  Ultrastructural observations of Phaeobotrys and Tetrasporopsis have also been made.  These three species form a monophyletic group that is not closely related to the chrysophytes or synurophytes.  Instead, Phaeoschizochlamys, Phaeothamnion and Stichogloea are most closely related to photosynthetic stramenopiles placed in the classes Phaeophyceae and Xanthophyceae. 

The remaining 10 genera assigned to the class have been placed in the class with uncertainty.  Comparative data for these phaeothamniophytes is extraordinarily limited because they are not available in culture.  In fact, most have not been mentioned in the literature since they were originally described.