Nitrogen Sources

Similar conclusions are reached when attention turns to the 'best' nitrogen source, which usually proves to be one amino acid or a mixture of amino acids. In most cases inorganic nitrogen and ammonium salts fail to support fruit body development although they may support production of primordia, but amino acids are required to produce the mature fruit bodies (reviewed in Moore, 1998a). This suggests that the formation of fruit body initials may be an activity of the vegetative mycelium and it is their further development which constitutes the fundamental 'mode switch' into the fruit body morphogenetic pathway. At least some of the deleterious effects of ammonium salts may be due to their influence on the pH of the medium, though metabolite repression caused by ammonium ions in many Ascomycota may be another cause. Fruit body formation in some fungi is favoured by provision of protein as source of nitrogen. Several basidiomycetes (A. bisporus, Coprinus cinereus (= Coprinopsis cinerea) and Volvari-ella volvacea) are able to use protein as a carbon source as efficiently as they use glucose (Kalisz et al., 1986), so an advantage of protein is that it serves as a source of carbon, nitrogen and sulphur. In more natural conditions, A. bisporus and a wide range of other filamentous fungi can utilise dead bacteria as sole source of carbon, nitrogen, sulphur and phosphorus (Fermor and Wood, 1981; Grant et al., 1986).

Higher carbon than nitrogen concentrations are usually required for fruit body production but the optimum C:N ratio varies from ^30:1 to ~5:1 (references in Moore-Landecker, 1993). High concentrations of amino acids tend to delay and/or depress maturation of fruit bodies even in organisms in which fruit body formation is optimal on media containing lower concentrations of amino acids, an effect that may result from the production of large quantities of ammonium as a nitrogen-excretion product on such substrates. When grown on protein as sole carbon source, nitrogen needs to be excreted from the mycelium; when this happens in vitro the ammonium concentration of the medium increases drastically during mycelial growth. One-third to one-half of the supplied protein-nitrogen was metabolised to ammonia by batch cultures of three saprotrophic bas-idiomycetes when protein was the sole source of carbon (Kalisz et al., 1986).

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