Limnofasciculus delicatus (Coleofasciculaceae, Coleofasciculales), a new mat-forming cyanobacterium from Shenandoah River, Virginia, USA

Benthic cyanobacterial mat proliferations in flowing waters are increasing worldwide, threatening ecosystem stability as well as wildlife and domestic animals due to their high biomass and toxin production. These complex communities typically comprise a mixture of taxa from the orders Coleofasciculales and Oscillatoriales, many of which have unresolved taxonomic positions and unknown toxic potential. This taxonomic uncertainty limits our ability to fully understand their diversity, distribution, and ecological roles. We collected field mats of a benthic non-heterocytous filamentous cyanobacterium from the North and South Forks of the Shenandoah River in Northern Virginia (USA) that were dominated by a novel morphotype of cyanobacteria. Phylogenetic analysis of the 16S rRNA gene placed the isolate within the recently described genus Limnofasciculus (Coleofasciculaceae), sharing 99.3% nucleotide identity with Limnofasciculus baicalensis, the only formally described species in the genus to date, but exhibiting marked structural differences in the Box B helix of the 16S - 23S ITS region. Whole-genome phylogenetic analysis confirmed its placement within Limnofasciculus, while genome-based species delimitation metrics fell below accepted thresholds for bacterial species delineation with L. baicalensis, supporting recognition of the isolate as a distinct species. Comparative genomic analyses revealed a genome size similar to L. baicalensis and a conserved core gene repertoire, alongside substantial divergence in biosynthetic gene cluster composition. Neither species contain biosynthetic gene clusters associated with the production of known cyanotoxins. Based on morphological, phylogenetic, and genomic evidence, we describe Limnofasciculus delicatus sp. nov., supported by light microscopy and whole-genome characterization. This work expands the known diversity of Limnofasciculus and improves taxonomic resolution of benthic cyanobacteria in riverine ecosystems.