Abstract: Soil hosts diverse communities of photosynthetic eukaryotes (algae) that have not yet been fully explored. Here we describe an interesting coccoid green alga isolated from a soil sample from a forest-steppe in South Urals (Bashkortostan, Russia) that, based on a phylogenetic analysis of 18S rRNA gene sequence, appears to represent a new phylogenetic lineage related to the genus Leptosira within the class Trebouxiophyceae. This new alga is characterized by uninucleate cells with a shape ranging from spherical to ellipsoid or egg-like, occurring solitary or more often grouped in irregular masses or colonies. Remarkably, cells with a characteristic pyriform shape are encountered in cultures grown on a solid medium. The cells harbour a single pyrenoid-lacking parietal chloroplast with the margin undulated or forming finger-like projections; in mature cells the chloroplast becomes divided by deep incisions into more or less separate lobes. Transmission electron microscopy of vegetative cells revealed an unprecedented structure in the form of a cluster of microfibrils located in the cytoplasm near the plasma membrane, often appressed to the chloroplast. Reproduction takes place via autospores or biflagellated zoospores. The unique suite of characters of our isolate distinguishes it from previously described coccoid green algae and suggests that it should be classified as a new species in a new genus; we propose it be named Chloropyrula uraliensis.

Key words: Chloropyrula uraliensis, green algae, new species, Trebouxiophyceae, ultrastructure, Ural Mountains.