Volume 10 Part 1 Article 36: Nitrogen Fixation by Pleurotus

Volume 10 Part 1 Article 36
Year 1979
Title: Nitrogen Fixation by Pleurotus
Author: R.H. Kurtzman

Abstract:

A number of authors have asserted that some higher fungi, particularly members of the genus Pleurotus, fix nitrogen (Duggar and Davis, 1916; Ginterova and Maxianova, 1975; Rangaswami et al., 1975). Although it has been many years since the first such assertion was made, and papers discrediting the methods have been published (Millbank, 1969), there appears to have been no adequate proof, even for a single species, nor any general disproof. However, many higher fungi, particularly lignicolous species have been shown to have a higher nitrogen content than the substrata on which they grow (Ginterova and Maxianova, 1975; Kurtzman, 1976; Rangaswami et al.).

It has been suggested that there are three non-mutually exclusive mechanisms by which lignicolous higher fungi might grow on the low nitrogen substrata with which they are normally associated : “(1 ) physiological adaptations that result in preferential allocation of available N to metabolically active substances and pathways that are highly efficient in the utilization of wood constituents; (2) reuse of available N by a dynamic and continuous system of autolysis of less active cells and reuse of nitrogenous constituents by more active mycelia without significant loss; and (3) utilization of N from sources outside the wood itself so that the fungi are not dependent upon N in the wood as their sole source of supply, for example by fixation of atmospheric N”, (Cowling and Merrill, 1966). While we may prefer to substitute a more general term for “wood” and to change the punctuation, these mechanisms seem to cover the possible explanations. There is considerable evidence for the existence of mechanisms (1) and (2), however, they are inadequate to explain the results of nitrogen balance experiments, unless faulty sampling techniques or other faulty procedures were used. Soil and other extraneous solids have been suggested as outside sources of nitrogen in nature, but nitrogen balance experiments have been run under conditions where the atmosphere would seem to be the only possible outside source of nitrogen.

All organisms proven unequivocally to fix nitrogen are procaryotic – bacteria or blue-green algae. Those who devote their major energies to the study of nitrogen fixation, automatically question any assertion that a eucaryote fixes nitrogen. It would, therefore, be a major break-through, if anyone could prove unequivocally that any eucaryotic species does fix nitrogen.

The data which has been used as proof of fixation by higher fungi, has always been nitrogen balance data. If one can show that a sample of substrate or substrate plus the sporocarps harvested have more total nitrogen than the initial substrate, then it would appear that fixation had taken place. That reasoning seems sound, but it has some weaknesses.

While most of the nitrogen in the atmosphere is N2, some exists as ammonia and as the oxides of nitrogen. It is possible that cultures of higher fungi are particularly able to scavenge such prefixed nitrogen from the air, and that it is such gases which are responsible for the net increase in fixed nitrogen in the cultures.

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