Freshwater ‘microcroissants’ shed light on a novel higher-level clade within Trebouxiophyceae and reveal the genus Chlorolobion as a trebouxiophyte

Trebouxiophyceae is a widespread and species-rich green algal class encompassing mostly coccoid algae with a simple ovoid or ellipsoidal outline. However, some poorly-sampled lineages have evolved more elaborate shapes or even complex thalli, adding to the class’s morphological diversity. Led by new and previously established strains, this study additionally uncovered a clade of croissant-like trebouxiophytes. Phylogenetic analyses inferred from nuclear 18S rDNA and chloroplast rbcL sequences confirmed the monophyly of the ‘microcroissant’ clade, which we propose to be classified as a new family, Ragelichloridaceae. This family includes two novel genera, Ragelichloris and Navichloris , and the previously described Thorsmoerkia . The position of Ragelichloridaceae within Trebouxiophyceae stayed unresolved but chloroplast phylogenomics showed that the family belongs to the broader incertae sedis group, which also includes Xylochloris and Leptosira . In addition, our study showed that the microcroissant-like genus Chlorolobion , previously classified within Chlorophyceae, is a genuine trebouxiophyte, potentially related to Ragelichloridaceae.