Deconstructing the Decomposing

© Jennifer Kerekes

Original publication: Mycena News, March 2010

Saprotrophic, or decomposing, fungi play very important ecological roles in decomposition and nutrient recycling. Saprotrophic fungi are responsible for recycling the majority of carbon from dead organic matter and have the unique ability to break down and release nutrients that are then readily available for other organisms. The process of decomposition has been viewed by some as being just as important as the process of photosynthesis (Heal et al., 1997).

Clathrus
White-rot fungus Pleurotus ostreatus. Photo © Michael Wood.

There are two main types of fungal wood rot: brown and white rot. What is the difference? The “rot” types are characterized by the morphological appearance of the wood they leave behind rather than the role they play in decomposition. White-rot fungi specialize in lignin degradation that leaves behind cellulose anda distinctive white color in wood. This ability to degrade lignin among microbes is unique. White-rot fungi are typically found on hardwoods and softwoods. Examples of white wood rot fungi are found in the Euagarics, Polyporoid, and Hymenochaetoid clades.Common genera that often encountered in the woods containing white-rot fungi are Pleurotus, Phanerochaete, Ganoderma, Phlebia, and Phellinus.

Brown-rot fungi specialize in cellulose degradation, which leaves behind a lignin-rich substrate; the decayed wood has a brown color and often looks cracked into cubical pieces. They are typically found on softwoods such as conifers. Brown-rot fungi are found in the Euagarics, Boletales, and Polyporoid clades. Common genera containing brown-rot fungi are Paxillus, Laetiporus, Phaeolus, and Fomitopsis. A unique type of brown rot called “coniophoraceae-rot” is found among saprotrophic Boletales and primarily decays conifer wood (Binder and Hibbett, 2006). Through ancestral-state reconstruction, Binder and Hibbett hypothesized that the ancestor of the Boletales was a resupinate or polyporoid saprotrophic brown-rot fungus. Interestingly, brown- and white-rot fungi, like mycorrhizal fungi, are not monophyletic; or, in other words, they are not each other’s closest relatives.

Rot
Top: white rot on aspen; Bottem: brown rot on pine. Photos © Prof. Tom Volk.

Fungi are able to decompose lignin and cellulose by releasing extra cellular enzymes that degrade these polymers. White-rot fungi in particular produce several types of enzymes, including lignin peroxidases, manganese peroxidases, and laccases. These enzymes allow the fungus to break down and utilize the organic substrates as an energy and nutrient source (Osono, 2007). Lignin is a large, complex, aromatic polymer made up of phenylpropane-based monomers linked via a variety of bonds which bind cell-wall components together (Osono, 2007). Lignin provides strength and support to plant cells and bonds cellulose fibers. Lignin can make up 20-35% of wood, while cellulose can make up 40-50% and hemicellulose can make up 25-40% (Pointing e tal., 2003).

A third type of wood rot is soft rot. Soft-rot fungi are typically ascomycetes and degrade cellulose and hemicellulose. Wood inflicted with soft-rot fungi often appears soft in consistency in wet environments and brown and crumbly in dry environments. Soft-rot fungi are typically found on hardwoods and are often most active in moist environments. Some ascomycetes, primarily xylariaceous fungi (Pointing et al., 2003), are capable of degrading lignin, although they are less capable than the white-rot fungi; this was recently explored in a study by Shary and colleagues (2007).

In comparison to wood-decay fungi, there has been less of a focus on litter decomposers, which are also capable of producing extracellular enzymes such as laccases and manganese peroxidases. However, litter decomposers vary in their ability to decompose lignin in leaf litter. Basidiomycete genera such as Clitocybe, Collybia, Marasmius and Mycena have been studied for their bleaching activity and enzyme production (Osono, 2007). Bleaching of leaf surfaces and humus indicates ligninolytic activity of fungi, as lignin content has been foundto be lower in both bleached leaf surfaces and humus compared with unbleached surfaces.

Recent and current research is looking at the diversity and evolution of laccase and peroxidase-encoding genes across a diversity of species and habitats. As you know, fungi play an important role in wood and leaf litter decomposition. Hopefully you will continue to appreciate the ecological roles of these saprotrophic fungi, including those pesky LBMs and persistent inedible polypores present along your next walkthrough the woods!

Further reading:

Literature Cited

About the Author:
Jennifer Kerekes received a Ph.D. in Microbiology in December, 2011 from the University of California at Berkeley, where she worked with Dr.Tom Bruns. She is interested in the ecology and diversity of saprotrophic fungal communities.