Thermoacidophilic cyanidia (Cyanidiales) are the primary photosynthetic eukaryotes in volcanic areas. These red algae also serve as important model organisms for studying life in extreme habitats. The global biodiversity and community structure of Cyanidiales remain unclear despite previous sampling efforts. Here, we surveyed the Cyanidiales biodiversity in the Tatun Volcano Group (TVG) area in Taiwan using environmental DNA sequencing. We generated 174 rbcL sequences from eight samples from four regions in the TVG area, and combined them with 239 publicly available rbcL sequences collected worldwide. Species delimita-tion using this large rbcL data set suggested at least 20 Cyanidiales OTUs (operational taxono-mic units) worldwide, almost three times the presently recognized seven species. Results from environmental DNA showed that OTUs in the TVG area were divided into three groups: (i) dominant in hot springs with 92%–99% sequence identity to Galdieria maxima; (ii) largely distributed in drier and more acidic microhabitats with 99% identity to G. partita; and (iii) primarily distributed in cooler microhabitats and lacking identity to known cyanidia species (a novel Cyanidiales lineage). In both global and individual area analyses, we observed greater species diversity in non-aquatic than aquatic habitats. Community structure analysis showed high similarity between the TVG community and West Pacific-Iceland communities, reflecting their geographic proximity to each other. Our study is the first examination of the global species diversity and biogeographic affinity of cyanidia. Additionally, our data illuminate the influence of microhabitat type on Cyanidiales diversity and highlight intriguing questions for future ecological research.
