The ultrastructure of zoospores of Asterococcus superbus (Cienk.) Scherffel was studied to provide ultrastructural data relevant to the systematic position of the genus. Our results demonstrated that the motile cells of A. superbus were similar to those of the tetrasporalean algae, such as Tetraspora sp. and Tetrasporidium javanicum Moebius. The flagellar apparatus of A. superbus had the same clock-wise orientation of basal bodies and the V-shaped alignment of basal bodies as Tetraspora cylindrica (Wahlb.) Ag. and T. lubrica (Roth) Ag., but differed by having rhizoplasts. The motile cells of A. superbus displayed chlamydomonadal ultrastructure, similar to Chlamydomonas reinhardtii Dangeard, including the absolute configuration of the flagellar apparatus. The pyrenoid matrix in A. superbus, however, showed a large lateral invagination occupied by chloroplast stroma, a characteristic that has never been observed in Chlorophyta.
Keywords: Asterococcus superbus; Asterococcus; Chlamydomonadales; Tetrasporales; Volvocales; absolute configuration; fine structure; flagellar apparatus; pyrenoid; zoospores.