Piromyces was technically the second anaerobic fungal genus to be described in the literature, as it was first reported by Liebetanz in 1910 as a flagellated protozoan called Piromonas communis. However, it was the third anaerobic fungal genus to be described by Orpin (preceeded by Neocallimastix and Sphaeromonas (=Caecomyces)). In 1988, Gold et al proposed to rename Piromonas to Piromyces. This was subsequently done by Barr et al (1989), who also documented a comprehensive description of the genus.
Piromyces is a monocentric fungus with a filamentous rhizoid system, however, it was clearly morphologically distinct from the previously described Neocallimastix genus as it has monoflagellated rather than polyflagellated zoospores. These morphological characteristics were considered to be a defining feature of the genus, despite it being reported during the 1990s that the genus was polyphyletic, based on phylogenetic analysis of ITS1 sequence data. However, it has since been realised that having a monocentric thallus, filamentous rhizoid system and monoflagellated zoospores is characteristic of numerous different anaerobic fungal genera. The first of these to be described were Buwchfawromyces and Oontomyces in 2015. Since then, a total of eight more genera have been described that share the same main morphological features as Piromyces: Pecoramyces, Liebetanzomyces, Agriosomyces, Aklioshbomyces, Capellomyces, Joblinomyces, Khoyollomyces and Tahromyces. As such, caution needs to be taken with literature predating 2015 that refers to ‘Piromyces’ categorized solely based on its morphological characteristics.
To date, several different Piromyces species have been described in the scientific literature. However, in light of the above, there is a need to revisit these in order to understand the species diversity that actually exists within the Piromyces genus. This work is currently being undertaken by Dr Gareth Griffith and collaborators.
Within the family Neocallimastigaceae, Piromyces is phylogenetically most closely related to Caecomyces and Cyllamyces (when using LSU as a taxonomic marker). Interestingly, these two closely related genera do not have a filamentous rhizoid system but a bulbous rhizoid system.
Images are shown above of a recently isolated strain of Piromyces communis (strain jen1).
On solid roll agar media, it produces small circular colonies with a dark centre to its sporangial structure (Image 1). It exhibits thin biofilm-like growth in liquid medium containing a soluble subtrates, and this firmly attaches to the glass walls of the tube (Image 2).
Microscopically, Piromyces produces mainly monoflagellated zoospores (Image 3), although bi-flagellated zoospores are occasionally produced (Image 4).
Piromyces has a monocentric thalli with both endogenous (Image 5) and exogenous (Image 6-8) sporangia.
Three sequenced genomes are currently publicly available for this genus: Piromyces sp. E2, Piromyces finnis and Piromyces sp. UH3-1.
For the Piromyces communis strain Jen1 that is pictured above, sequence information is available in the NCBI database (accession number MT085716: ITS1, 5.8S, ITS2 and D1/D2 LSU sequence data).