Mutualism is a type of symbiosis in which both partners benefit. Sometimes only one partner benefits and the other is not affected either beneficially or adversely, in which case the term commensalism is sometimes used. One example of commensalism that is cited by Deacon16 has to do with composting. Two fungi, Chaetomium thermophile and Humicola insolens var. thermoidea, are both thermophilic fungi and in abundance during the composting phases in which heat is generated. Chaetomium thermophile has strong cellulase activity, breaking down cellulose into cellobiose and glucose. Humicola, on the other hand, does not have strong cellulase activity, and it is thought that it utilizes the glucose made available by the cellulase activity of Chaetomium for
its own growth needs. Thus, Humicola benefits from the presence of Chaetomium, which is not harmed in the process. There is some suggestion that this may be more a case of mutualism than commensalism because Chaetomium may also benefit by the removal of glucose. Glucose, an end product of the cellulase activity, slows the rate of action of the enzyme, so the removal of glucose would make for better cellulase activity by Chaetomium. This is, however, just another example of a situation in which systems in nature do not always fit perfectly into human-made categories. The symbiotic relationships existing between fungi and algae (i.e., lichens) and fungi and roots
(i.e., mycorrhiza) fit into the term mutualism with less difficulty, however.