Hidden but Important-
BESIDES THE GILLED, TOOTHED AND PORE MUSHROOMS that we can easily see when walking around the woods, there are among the saprophytes (see Fungi and Armillaria, elsewhere on this site), a variety of smaller, less conspicuous species that are important degraders of dead wood in forests. In fact, without them, dead wood would pile up, tying up minerals and organic matter and becoming susceptible to destructive fires. A surprising variety of these small fungi may be found in yards or other managed lands where dead branches, limbs or stumps are not removed after they appear, but are left on the ground in a shaded area.
This heterogeneous grouping of species from various families have in common the lack of a stalk and cap ( i.e., there is no “mushroom”) and the presence of a fruiting body that is appressed tightly against the bark or wood surface. They are collectively called the crust fungi , and the growth habit is termed resupinate. Attached to the fruiting body of the crust fungus and extending into the interior of the wood is the mat-like mycelium which secretes extracellular enzymes that decay the wood.
The rotting of wood is a biochemically complex undertaking that requires specific enzymes. Some crustose species are efficient rotters of wood and are inconspicuously but continuously going about it in forests and other wooded ecosystems of the world. Most are “white-rotters” –i.e., the wood is much lighter in color when the fungus has been at work, while the less common “brown-rotters” leave the wood darker. Wood consists mostly of big molecules- cellulose which is white and composed of thousands of sugar units linked in long chains that form fibrils, and lignin, a complex strengthening material which is brownish and accumulates in the walls of water-transporting cells (xylem) of trees. Phanerochaete crassa, the striking purplish crustose fungus in the photograph (growing over another unidentified fungus) is a white-rotter that secretes enzymes that break up the 3-dimensional structure of lignin, leaving the cellulose untouched, and, ultimately, whitening and weakening the wood. The taxonomic group to which this species belongs bears its spores directly on the exposed surface of the fruiting body. There are no pores, gills or teeth. The fungus was found in my yard on the underside of an oak branch left lying under a tree.
P. crassa is closely related to P. chrysosporium, a yellowish species that has been extensively investigated for its potential to improve the bleaching process that destroys the unwanted color in wood pulp caused by lignin. Bleaching is an essential and expensive step in the paper-making industry. C. chrysosporium is an efficient bleaching agent although a recent study has concluded that certain strains of P. crassa may be even more efficient in degrading lignin. C. chrysosporium also has shown promise in bioremediation- the use of organisms in the degradation of industrial pollutants such as organic pesticides found in soil and water. Gusse and colleagues have demonstrated that this species has the extraordinary ability to degrade phenolic resin plastics, industrial products previously believed to be nonbiodegradable. These resistant compounds are used extensively in laminating Formica, gluing plywood, and contribute to many other industrial products that have added volumes of unrecycled waste to our landfills. Perhaps something
tremendously important has been discovered here!
The second photo shows another crustose fungus, this one found growing on the underside of a fallen cassia branch left lying on the ground in my backyard. This appears to be Irpex lacteus , another white rot fungus. First inspection suggests a toothed fungus in which the teeth (or spines) are irregular and somewhat flattened. But this is actually a pore fungus which, when young, has typical pores, but the pore walls partially break down, leaving them to appear as flattened somewhat irregular “teeth”. Irpex, like some other white rot fungi investigated for practical application, produces several enzymes capable of degrading complex molecules. This species is also able to colonize soils and resist chemical secretions of soil bacteria. Thus besides its lignin breakdown abilities, it is another species of interest as a potential aid in bioremediation.
The following references are among those consulted for this entry: botit.botany.wisc.edu/toms_fungi/may97.html; C. Novotny, et al., 2000. Irpex lacteus, a white rot fungus applicable to water and soil remediation. Appl Microbiol Biotech, vol 54:850-3; mushroomexpert.com; M. Tanaku, et al. 2004. Mn-Peroxidase from Phanerochaete crassa WD1694. Bull. of FFRI vol. 3:7-13 ; A.C. Gusse, et al. 2006. White-rot fungi demonstrate first biodegradation of phenolic resin. Environ. Sci. Technol 40: 4196.