Freshwater ‘microcroissants’ shed light on a novel higher-level clade within Trebouxiophyceae and reveal the genusChlorolobionas 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 chloroplastrbcLsequences confirmed the monophyly of the ‘microcroissant’ clade, which we propose to be classified as a new family, Ragelichloridaceae. This family includes two novel genera,RagelichlorisandNavichloris, and the previously describedThorsmoerkia. The position of Ragelichloridaceae within Trebouxiophyceae stayed unresolved but chloroplast phylogenomics showed that the family belongs to the broaderincertae sedisgroup, which also includesXylochlorisandLeptosira. In addition, our study showed that the microcroissant-like genusChlorolobion, previously classified within Chlorophyceae, is a genuine trebouxiophyte, potentially related to Ragelichloridaceae.