Stylococcus aureus and S. brevis sp. nov. (Chrysophyceae) and their phylogenetic relationship to Lagynion

We isolated into culture, along with their host cells (Desmidium, Spondylosum and Geminella), Stylococcus aureus and a previously undescribed taxon we name Stylococcus brevis sp. nov. The loricas of S. aureus were attached to the host cell with a long, thin stalk, and cells produced a long pseudopodium that often branched at the tip. Cells divided transversely, followed by daughter cell rotation of 90°, giving the appearance of longitudinal division. One daughter cell was flagellate, escaped the mother lorica and swam to a host cell where it attached, producing a new lorica. The swimming cell had a long immature flagellum and short mature flagellum that were easily observed using a light microscope. Conversely, the loricas of S. brevis were anchored to the host cell with a short, thick stalk, and the cells produced a short pseudopodium. Stylococcus brevis cell division was the same as for S. aureus; however, its swimming cells had an extremely short mature flagellum that was visible by light microscopy only after chemical fixation. Molecular phylogenetic analyses using the 18S rRNA and rbcL genes showed that the two Stylococcus species were distinct species. Surprisingly, Stylococcus branched with Lagynion scherffelii (generitype) and L. ampullaceum, suggesting that these algae are congeneric, which supports the idea that the typical Stylococcus lorica morphology and stalk formation are a response to the host cell. We found no evidence suggesting that Stylococcus and Stylochrysalis are synonyms.