Dictyosphaerium-like morphotype in terrestrial algae: what isXerochlorella(Trebouxiophyceae, Chlorophyta)?1

Several strains of terrestrial algae isolated from biological soil crusts in Germany and Ukraine were identified by morphological methods as the widely distributed speciesDictyosphaerium minutum(=Dictyosphaerium chlorelloides). Investigation of the phylogeny showed their position unexpectedly outside of Chlorellaceae (Trebouxiophyceae) and distantly fromChlorella chlorelloides,to which this taxon was attributed after revision of the genusChlorellabased on an integrative approach.SSU rRNAphylogeny determined the position of our strains inside a clade recently described as a new genus of the cryptic algaXerochlorella olmiaeisolated from desert biological soil crusts in the United States. Investigation of the morphology of the authentic strain ofX. olmiaeshowedDictyosphaerium-like morphology, as well as some other characters, common for our strains and morphospeciesD. minutum. The latter alga was described as terrestrial and subsequently united with the earlier described aquatic representativeD. chlorelloidesbecause of their similar morphology. The revision ofChlorellamentioned above provided only one aquatic strain (D. chlorelloides), which determined its position in the genus. But terrestrial strains of the morphospecies were not investigated phylogenetically. Our study showed that the terrestrialD. minutumis not related to the morphologically similarD. chlorelloides(=Chlorella chlorelloides, Chlorellaceae), and instead represented a separate lineage in the Trebouxiophyceae, recently described as genusXerochlorella. Therefore, revision ofXerochlorellais proposed, including nomenclatural combinations, epitypifications, and emendations of two species:X. minutaandX. dichotoma. New characters of the genus based on investigation of morphology and ultrastructure were determined.