Sordariomycetidae: Soil Fungi A-Plenty

I'm pretty sure I've commented before that, although most of us tend to associate the word 'fungi' with mushrooms and other eye-catching fruiting bodies, the vast majority of fungal diversity is minute and tends to go unnoticed. Nevertheless, despite their obscurity, many of these microfungi are crucial to our own continued existence. These are the decomposers, the organisms that break down fallen plant matter and animal wastes in their own search for nourishment and so contribute to the release of locked-up nutrients back into the environmental cycle.

Neurospora growing on sugar cane waste, from here.

The group of fungi that I drew for today's post, the Sordariomycetidae, is primarily made up of these minute decomposers. Sordariomycetids have already made an appearance here at Catalogue of Organisms, in a post from ten years ago on black mildews. Depending on how broadly the group is circumscribed, the Diaporthales could also be included. Due to a simple morphology that provides few distinct characters, the Sordariomycetidae are primarily defined on the basis of molecular phylogenies. The difficulty of classifying microfungi by morphology alone is underlined by cases where species previously classified within the same genus have proven to belong to entirely distinct fungal lineages.

In general, the vegetative body of most Sordariomycetidae consists of little more than disassociated hyphae embedded in their substrate, with the only distinct structures being the reproductive fruiting bodies. These are perithecia: that is, globular or flask-shaped fruiting bodies with a single small opening or ostiole at the top through which the mature spores are released. In some cases, the internal structure of the mature perithecium will simply dissolve, freeing the spores to escape through the ostiole in the manner of a miniature puffball. In others, the spores become entangled in a long strand or seta that is then extruded through the ostiole like toothpaste being squeezed out of a tube.

Perithecium of Chaetomium extruding spore-bearing setae, from here.

Sordariomycetids are found in almost every habitat imaginable: as well as soil- and dung-dwelling forms, they may also be found in aquatic and even marine habitats. Perhaps the best-known sordariomycetid is Neurospora crassa, red bread mould, which is widely used in laboratories as a model organism for genetic research. Indeed, it was investigations into N. crassa in the 1950s that first led to the proposal of the 'one gene, one enzyme' model that became a cornerstone of molecular genetics.