Traditionally, brackish and marine coccoid green algae are placed as isolated species of common trebouxiophycean or chlorophycean genera (e.g. Chlorella, Chlorococcum); exclusively the marine genera Chlorocystis and Halochlorococcum are characterized by quadriflagellated zoospores and a ‘Codiolum-stage’ (characteristic for the Ulvophyceae) in their life histories. In Chlorocystis cohnii, sexual reproduction could only be induced by presence of the tube-dwelling diatom Berkeleya rutilans. Phylogenetic analyses of the SSU rDNA sequences of 32, mostly marine, coccoid or sarcinoid strains confirmed the monophyly of the Ulvophyceae. The class contain 11 independent well-supported lineages; of which eight contain coccoid species. (1) The basal branch of the ulvophytes is the Oltmannsiellopsis clade (in agreement with the order Oltmannsiellopsidales emend.) which contains the quadriflagellated monads (O. viridis and O. unicellularis) and the type species of the coccoid genus Halochlorococcum (H. marinum). (2) The other species of Halochlorococcum (except H. saccatum) formed together with Chlorocystis and Chlorella salina a significant clade corresponding to the order Chlorocystidales in emended form. (3) Gomontia polyrhiza is closely related to Urospora penicelliformis within the Acrosiphonia group (Acrosiphoniales). (4) The sarcinoid Trichosarcina polymorpha and T. mucosa, and the freshwater planctonic Helicodictyon planctonicum are member of the Ulothrix clade together with filamenteous taxa. (5) The type species of Pseudoneochloris (P. marina) form together with three unidentified marine coccoid strains an independent lineage within the Ulvophyceae (called Pseudoneochloris clade). (6–8) Ignatius tetrasporus (isolated from soil), the strain CCMP 250 originating from desert soil and the marine strain CCMP 1293 isolated from Palau Islands represent three single lineages within the ulvophytes. From our results, the class Ulvophyceae can be subdivided taxonomically into at least 11 orders (including the Ulvales, Cladophorales and Dasycladales) with the largest biodiversity at the coccoid level.

Present address: Department of Molecular and Cell Biology, Brown University, Providence, RI 02912, USA.