Walking through any wooded area you can gain an understanding of the central role decay fungi play in the ecosystem. Often called rots, evidence of their work is everywhere. They degrade complex carbon compounds in wood and liberate nutrients that can then be taken up by other organisms. If fungi were unable to degrade wood, the ecosystem would collapse. With so much of the worlds carbon tied up in wood and other plant materials, the role fungi play in carbon cycling is central. The brown, white and soft rots have evolved different strategies to survive and reproduce using a food source denied to most other forms of life. These fungi degrade wood by secreting a group of biological compounds called enzymes. Enzymes are proteins that facilitate chemical reactions that would normally not take place by themselves. Every organism produces a complement of enzymes that allow it to break down and take up food. The process is analogous to what happens inside our own digestive tracts. When we eat, the digestive system secretes enzymes that break down complex food molecules into simpler pieces that our bodies can use as nourishment. Fungi do a very similar thing except the breakdown occurs in the areas surrounding the organism. When complex food molecules are sufficiently degraded, the fungi absorb them and use some of them. Certain fungi do this with wood and wood derived products.
Wood is composed
of primarily of cellulose, hemicelluloses and lignin. Cellulose is the most
abundant constituent, hemicelluloses the next most abundant, and lignin the
least. Cellulose is a polymer of glucose that very few organisms are able to
utilize as an energy source. Starch is also a polymer of glucose; but large
numbers of organisms are able to use it as an energy source. Why the difference?
The reason that one is so easily used as a nutrient source while the other is
not is due to the way the glucose molecules are arranged in the polymer. Both
starch and cellulose are classified as polysaccharides. Cellulose can be degraded
by fungi and a few other types of organisms such as certain bacteria. Certain
fungi produce an enzyme called cellulase, which is actually a family of closely
related enzymes. Very few organisms are able to produce these enzymes. These
enzymes are capable of breaking down cellulose and hemicelluloses. Hemicelluloses
are polysaccharides resembling cellulose but structurally the chains are shorter,
far more random and include a variety of monosaccharides other than glucose.
As a result, hemicelluloses are much weaker and more easily digestible than
cellulose. Lignin is the third most abundant constituent of wood. Lignin is
a polymer composed of aromatic hydrocarbons, not glucose or other carbohydrates.
Lignin gives the cellulose extra strength. Many fungi utilize lignin as well
as cellulose and hemicelluloses. As you would suspect, wood is composed of a
great many other constituents, such as pectin, simple sugars, proteins and a
variety of other potential food molecules.
The major players in decomposing wood are the basidiomycetes. Basidiomycetes are a large division of the fungi. These fungi typically produce macroscopic fruiting bodies. Examples of these macroscopic fruiting bodies are brackets and mushrooms. Many of these fruiting bodies are capable of producing enormous numbers of spores that are often collected in great quantities on non-viable spore trap samples outside. Some of these basidiomycetes have evolved into efficient wood decomposers. Both the white and brown rot fungi are basidiomycetes. The white-rots can degrade cellulose, hemicelluloses and lignin. The rotting wood appears white as decomposition proceeds. The brown-rot fungi degrade cellulose and hemicelluloses but not the lignin to any great extent. The wood looks brown because the pigmented lignin is brown and is left behind. Wood in the advanced stages of brown rot decay also displays cubical checking, which is when the wood becomes permeated with void areas, giving it a cubical appearance.
The lesser players in the wood decay hierarchy are the soft rots that are typically molds. These fungi are mostly hyphomycetes and ascomycetes. Their actions on wood are different from that of the basidiomycete rots. They work on specific parts of the cell walls of wood and not others.
The enzyme complements of brown rots are targeted for the decomposition of the cellulose and hemicelluloses components in wood. There are many brown rot fungi; some familiar examples are Serpula lacrymans and Meruliporia (Poria) incrassata. These two fungi are able to degrade wood with very low moisture contents because they can tap a moisture source and channel it to sites of active decay. They move the water through aggregations of hyphae called rhizomorphs, which act as pipes allowing fluids to flow between different parts of the organism. This is a remarkable advantage over other decay fungi that must exploit the moisture of the substrate at hand.
The enzyme complements of white rots are targeted at all three major components of wood, cellulose, hemicelluloses and lignin. White rot fungi are among the most visible basidiomycetes encountered in our environments. Their fruiting bodies can be seen on logs, twigs, fence poles and almost any wood out in exposed environments. Two very common examples are Trametes versicolor (turkey tails) and Ganoderma applanatum. These types of fungi, unlike the brown rots must have very wet conditions for decomposition to proceed.
The enzyme complements of soft rots do not attack in the same virulent manner as the white and brown rots. They selectively decay certain parts of the wood cell wall and leave others relatively untouched. Due to this fact, the soft rots dont have the same structural effect on wood and most damage is in the form of other non-desirable effects such as discoloration or as sources of allergenic spores. Some examples of such fungi are Chaetomium spp., Cladosporium spp. and Stachybotrys chartarum. Most soft rots are fungi every mold investigator deals with on a daily basis. Many molds have the ability to grow on cellulitic substrates other than wood. Drywall paper is a good example of a wood derived product whose crystalline structure has been disrupted and is therefore digestible by a wider range of organisms. The cellulose becomes more accessible to enzymatic attack by the less specialized cellulose degraders. The white and brown rots have evolved specifically to attack cellulose in the wood form. The soft rots can degrade wood to a certain extent but often do far better on modified wood products. They often grow very fast and have a broad spectrum of enzymes, and they can utilize many different substrates that may be present. This explains why so many fungi are found on the surfaces of wood. The question arises, are the fungi decomposing cellulitic substrates or is it surviving on other nutrients present? The relationships between fungi recovered on moldy building materials can be complex. Often several types of molds are found growing together and just how these fungi react with the substrate and each other is not well understood. An example, Stachybotrys chartarum (atra) and Acremonium spp. are often recovered growing together on drywall paper. Are these fungi working together? Is the sum of their enzymatic complements more effective than either can produce individually? Questions like these cloud the issue of just which fungi are doing what. Under different circumstances, a fungus may preferentially utilize one energy source or the other. They may go to work on the cellulose only after other nutrients have been exhausted. In the laboratory many molds grow great on malt extract agar while the same organism on a modified cellulose plate will grow only very weakly. The opposite is also true. Some molds do not do well at all on malt extract agar and do great on modified cellulose.
To help deal with some of the fungi recovered during the course of an investigation it is often helpful to know what they are. Certain groups of fungi lend themselves to a particular set of analysis. Below are some general strategies to help your lab identify your samples.
Identifying basidiomycete hyphae in and on wood using a microscope is sometimes possible. Their hyphae often have structures called clamp connections, which are unique to the basidiomycetes. This is complicated by the fact that the basidiomycete life cycle includes in many cases a type of hyphae without clamp connections. So, basidiomycete hyphae could be mistaken for non-basidiomycete hyphae. Keep in mind many basidiomycetes in the clamp connectionless state often sporulate by the production of arthrospores, often leaving behind only spores with no other evidence of their existence. It should be noted that when a laboratory reports arthrospore-former, Geotrichum-like and Geotrichum spp. it is possible that a basidiomycete is being observed. In many cases, when a lab reports Geotrichum spp., it isnt. There are many fungi that labs confuse with Geotrichum spp. . Usually it is some other type of arthrospore-former, perhaps a basidiomycete. If you are concerned about basidiomycete decay, ask the lab specifically to look for basidiomycete hyphae. The clamp connections can often be hard to see and treating with KOH solution can enhance their visibility.
Molds in genera such as Chaetomium spp., Cladosporium spp., Stachybotrys chartarum (atra), Alternaria spp, Ulocladium spp., Trichoderma spp., Aspergillus spp., Penicillium spp., Acremonium spp., Fusarium spp., Phoma spp., Epicoccum nigrum, and many others are often found on cellulose based materials under direct examination and grow fantastically on cellulose based media. If you are concerned about cellulitic fungi, always ask the lab to include a cellulose plate in its dilution regimen as well as a 7-day incubation period. Do tape lifts whenever possible. Swabs break the fungal structures and make it hard or impossible to identify. Swabs are great if you want a culture or get at hard to reach places, but for ordinary direct examination, they are inferior to tape samples. Even better, send in a sample of the material and let the lab make the preparations.
The blue stain fungi are particular types of molds that are associated with wood. Their hyphae impart a bluish black color to infected wood. These fungi are placed in the Ceratocystis/Ophiostoma group. This group is composed of two ascomycete genera that are very hard to tell apart. The consensus is that these fungi do not damage wood in a way that would include reduction of structural integrity. They are often encountered and easily identified from tape lift samples and are often associated with a parasitic fungus called Gonatobotryum sp. Culturing for the Ceratocystis/Ophiostoma group on general laboratory media including cellulose is not useful. Direct examination is indicated if you suspect this group of fungi.
Investigators dealing with fungal issues could benefit from learning more about the biology of fungi, specifically those capable of cellulitic degradation. They are a large part of the indoor flora. Use your laboratory wisely by giving them information like sample location, what substrate the sample was taken from, the moisture conditions past and present. If you suspect a certain group of fungi, not only wood associated fungi but also others, ask them what techniques would be advantageous in isolating and enumerating them. Do not hesitate to ask them questions. Talk to them about your investigation, they can often recommend sampling techniques and strategies that would make sampling more meaningful. Laboratory personnel generally have some experience with field sampling strategies. By working with them, you may gain valuable insight by helping with information on site-specific sampling protocols that will help you with your investigations.
David Spero is an analyst at Aerobiology Laboratory Associates. He can be reached by e-mail at firstname.lastname@example.org or by phone at (877) 648-9150.