Fine structure and phylogeny of green algal photobionts in the microfilamentous genus Psoroglaena (Verrucariaceae, lichen-forming ascomycetes)

According to the literature the microfilamentous thalli of lichen-forming ascomycetes of the genus Psoroglaena are assumed to harbour vivid green "prochlorophyte" cyanobacterial photobionts. As this would be the first report of terrestrial "prochlorophytes" we investigated the fine structure and two molecular markers (SSU rDNA and rbcL) of the photobionts of P. stigonemoides (Orange) Henssen and P. epiphylla Lucking. Both Psorogiaena spp. had unicellular green algal photobionts, representatives of the Trebouxiophyceae. The photobionts of P. stigonemoides is closely related to the non-symbiotic Auxenochlorelia protothecoides and to a Chlorelia endosymbiont of the freshwater polyp Hydra viridis. The putative photobiont of P. epiphylla may be related to Chlorella luteoviridis, C. saccharophila, and a Pseudochlorelia isolate. In contrast to other microfilamentous lichens, which derive their shape from filamentous green algae or cyanobacterial colonies overgrown and ensheathed by the fungal partner, Psoroglaeno mycobionts position their unicellular photobiont in uni- or multiseriate rows which strongly resemble the situation in filamentous cyanobacterial colonies.