O. mucronatum rhizoids (Figs a, e), inactive zoospores (Figs b, c), active zoospores (Fig d), and anucleate thallus after zoospore release (Fig e). Images by Joyce Longcore.

Obelidium mucronatum Nowakowski is a geographically widespread member of the Chytriomycetaceae (Chytridiales, Chytridiomycota) and has, since its description in 1876, been reported from wings and exuviae of aquatic insects and from chitin baits placed with collections from aquatic or moist sites. Unlike many members of this family, its single apical spine makes O. mucronatum reliably recognizable.

Isolate JEL802, pictured here in pure culture, was found on chitin bait placed with a collection from a mesotrophic lake in Penobscot County, Maine. Morphological features that indicate its placement in the Chytriomycetaceae are the fine, tapering rhizoids (Figs a, e), the zoospore release en masse with the spores remaining motionless for a few minutes before becoming active (Figs b, c), and the slightly eliptical shape of active zoospores (Fig d). After zoospore release through a sub apical pore, no nucleated cytoplasm remains in the fungal thallus (Fig e). Sparrow (1938) conjectured that “Similarity in body structure and method of development of Obelidium, Rhizoclosmatium, Siphonaria, and Asterophlyctis, all exuviae-inhabiting forms, suggests that they are closely related genera.” In this instance, morphology and habitat have inferred the same family grouping as molecular evidence.

Fungi are integral in nutrient cycling, particularly in converting refractory substances to nutrients for other biota, yet zoosporic fungi active in this role are rarely included in ecological studies and are poorly understood. The availability of the genome of O. mucronatum will allow identification of this taxon in eDNA studies, regardless of the genetic marker of choice used by researchers.

Information about enzymatic specialization, such as increased representation of proteinases found in the genome of Batrachochytrium dendrobatidis (Rhizophydiales), is poorly known for other orders and families of the Chytridiomycetes. This is the first sequenced genome of a chitinophilic member of the Chytridiales to be released and will inform comparative genomics of the zoosporic fungi and support more investigations into the ecological roles and evolution of these fungi.