Parachloroidium gen. nov (Trebouxiophyceae, Chlorophyta), a novel genus of coccoid green algae from subaerial corticolous biofilms

The Watanabea clade of the Trebouxiophyceae included mostly unicellular coccoid microalgae that thrived in various terrestrial microhabitats. The diversity of these morphologically uniform microalgae was little known, and several new genus-level lineages had recently been described on the basis of molecular data. In this study, we provided a taxonomic description of a new trebouxiophycean genus, Parachloroidium, found in the Mediterranean in corticolous phototrophic biofilms. Their simple chlorelloid morphology did not unambiguously distinguish the Parachloroidium strains from other similar green algae. However, ultrastructural characteristics and molecular phylogenetic analyses based on the 18S ribosomal (r) DNA, internal transcribed spacer region (ITS) and the chloroplast ribulose-bisphosphate carboxylase gene sequences provided a basis for the discrimination of Parachloroidium from related genera of the Watanabea clade. The four strains investigated formed two species, P. laureanum and P. lobatum, which differed in plastid morphology and in ITS and 18S rDNA sequences. All four strains were characterized by globular or ellipsoidal cell shapes, single parietal plastids and asexual reproduction by autospores. Their plastids lacked typical pyrenoids; however, plastids included peculiar thylakoid-free regions of irregular shape. On the basis of accumulating molecular data, we concluded that the Watanabea clade was a diverse phylogenetic lineage within the subaerial chlorelloid green algae.