Diversity in Cyanidiophyceae: Cyanidiofrigus pintoensis gen. et sp. nov. within polyextremophilic red algae (Rhodophyta)

The polyextremophilic red algal class Cyanidiophyceae inhabits a wide range of extreme habitats (e.g. acidic geothermal areas and dim-light caves), and shows various cell morphologies (e.g., subspherical and club-shape) and trophic strategies (e.g., photoautotrophy, heterotrophy and mixotrophy). Historically, inconsistent observations of morphology and physiology have hindered taxonomic advancements in Cyanidiophyceae despite indications of species diversity from molecular studies. Here we revisited this taxonomic conundrum and explored their cryptic speciation based on multi-locus and morpho-physiological evidence examined from expanded taxa sampling, including 20 cultures and one environmental sample. Our multi-locus phylogeny revealed two well-supported monophyletic lineages: the C-lineage that is generically rich (including Cavernulicola, Cyanidiococcus, Cyanidiofrigus, Cyanidioschyzon, Cyanidium, Gronococcus, and Sciadococcus) and the G-lineage that currently only contains the genus Galdieria. Species delimitation analyses revealed at least 21 species in Cyanidiophyceae. Although our morpho-physiological observations failed to differentiate closely related species apart within a genus, we propose Cyanidiofrigus pintoensis gen. et sp. nov. as this species is in a highly divergent lineage with much lower thermal tolerance than other recognized species. Overall, our study fills the taxonomic knowledge gaps and expands the number of known species in Cyanidiophyceae.