Hylodesmus singaporensis gen. et sp nov., a new autosporic subaerial green alga (Scenedesmaceae, Chlorophyta) from Singapore

The algal flora of subaerial habitats in the tropics remains largely unexplored, despite the fact that it potentially encompasses a wealth of new evolutionary diversity. Here we present a detailed morphological and molecular characterization of an autosporic coccoid green alga isolated from decaying wood in a natural forest in Singapore. Depending on culture conditions, this alga formed globular to irregularly oval solitary cells. Autosporulation was the only mode of reproduction observed. The cell periphery was filled with numerous vacuoles, and a single parietal chloroplast contained a conspicuous pyrenoid surrounded by a bipartite starch envelope. The cell wall was composed of a thick inner layer and a thin trilaminar outer layer, and the cell surface was ornamented with a few delicate ribs. Phylogenetic analyses of 18S rRNA gene sequences placed our strain in the family Scenedesmaceae (Sphaeropleales, Chlorophyceae) as a strongly supported sister branch of the genus Desmodesmus. Analyses of an alternative phylogenetic marker widely used for the Scenedesmaceae, the ITS2 region, confirmed that the strain is distinct from any scenedesmacean alga sequenced to date, but is related to the genus Desmodesmus, despite lacking the defining phenotypic features of Desmodesmus (cell wall with four sporopolleninic layers ornamented with peculiar submicroscopic structures). Collectively, our results establish that we identified a novel, previously undocumented, evolutionary lineage of scenedesmacean algae necessitating its description as a new species in a new genus. We propose it be named Hylodesmus singaporensis gen. et sp. nov. A cryopreserved holotype specimen has been deposited into the Culture Collection of Algae of Charles University in Prague, Czech Republic (CAUP) as CAUP C-H8001.