<i>Ariadnema polymorpha</i> gen. et sp. nov. (Scytonemataceae, Cyanobacteria): A new heterocytous cyanobacterium with true and false branching from ruins in the Brazilian Atlantic Forest

A novel heterocytous Cyanobacterium, Ariadnema polymorpha gen. et sp. nov., was observed in the northern Brazilian Atlantic Forest (BAF), a globally significant biodiversity hotspot that has microbiome diversity yet to be explored. Isolated from ancient sugarcane mill ruins within the BAF, the new genus (strain CCAPE130) exhibits a unique morphology, including inverted Y true branching, coupled with scytonematoid and tolypothricoid false branching within the same trichome, an unusual morphology among heterocytous cyanobacteria. A polyphasic taxonomic approach, integrating detailed morphological characterization, transmission electron microscopy (TEM) ultrastructure analysis, 16S rRNA gene sequencing, and 16S-23S ITS rRNA region sequencing, was employed. Phylogenetic analysis using maximum likelihood and Bayesian inference placed CCAPE130 as a sister clade to the Symphyonema and Mastigocladopsis genera, which were previously associated with Symphyonemataceae, a family originally diagnosed by true Y-branching but now merged into the false-branched family Scytonemataceae. Structural comparisons of the 16S-23S ITS rRNA region D1-D1' showed variations, and the Box B region exhibited structural similarities with related lineages despite nucleotide differences. This research represents a phylogenetic analysis of Scytonemataceae specimens with true and false branching from the Americas, validating A. polymorpha as a new genus and species. Our results significantly enhance our understanding of Scytonemataceae phylogeny and underscore the rich, often overlooked, cyanobacterial diversity present in tropical forest regions, such as the Brazilian Atlantic Forest.