FACTORS AFFECTING THE TOXICITY OF SEVERAL LICHEN ACIDS: EFFECT OF PH AND LICHEN ACID CONCENTRATION

The effect of pH 5-8 and lichen acid concentration gradients (2.7 x 10-3 2.7 x 1o-6 M) on the toxicity of the following lichen acids: usnic, lecanoric, evernic, vulpinic, stictic, fumarprotocetraric, psoromic, and atranorin, on spores of Funaria hygrometrica was tested. Percent germination and sporeling growth were used as indicators of toxicity. None of the lichen acids were significantly toxic, for either percent germination or sporeling growth at concentrations equal to or below 2.7 x 10-5 M at pH 7.0, but many of the lichen acids which increased in toxicity at values different from pH 7 may have been toxic at lower concentrations if a different pH was used for the assay. Lichen acid toxicity showed a good correlation with pH for the parameter of spore germination, or sporeling growth, or both. Some lichen acids did not inhibit germination but were effective in retarding sporeling growth, or vice versa. This observation is discussed in relation to changing fatty acids and other lipid composition as germination occurs. Two of the three O-methylated lichen acids (evernic and psoromic) were among the most effective in inhibiting growth over all, but at lower pH values these were less effective than non-O-methylated lichen acids. Stictic, which is also an O-methylated lichen acid, was the least effective inhibitor over all the pH values for both parameters, while vulpinic was the most toxic over all the pH values. The order of relative toxicity for the lichen acids is different, depending on the pH and concentration at which they are tested and depending on the parameter measured. Thus, in an ecological sense, it is difficult to evaluate the adaptive significance of a particular compound or group of compounds without knowing what factors influence the toxicity of those compounds and how these factors vary in the organism's habitat. LICHENS produce a number of chemically diverse secondary products known as lichen acids, of which many are unique to lichenized fungi. These can be found in concentrations as high as 25% of the dry weight of the lichen thallus but more typically 5-10% of the dry weight. A single lichen thallus may produce several different acids. The majority of the compounds are depsides and depsidones but other classes of compounds also exist (Culberson, 1969). Lichen acids are found crystallized on the outer surface of the medullary hyphae of the mycobiont in the lichen thallus. In this position they are readily available to the environment, as well as to systematists who wish to extract them in a simple quick manner. Aside from their usefulness in systematics, lichen acids have been postulated to have several natural roles. These roles include biochemical weathering of rock surfaces, acting as sun-screens to protect the phycobiont from 1 Received for publication 20 December 1979; revision accepted 8 April 1980. We would like to thank Drs. Page W. Morgan and Robert S. Egan for helpful suggestions during the conduction of the experiments and for critical reading of the manuscript. Reprint requests should be sent to the second author. intense radiation, and acting as allelopathic agents against both plants and animals (cf. review by Rundel, 1978). Many studies have dealt with the toxic potential of lichen acids. Some have been shown to retard the growth of gram-positive bacteria (Vartia, 1973), several pathogenic and decay fungi (Henningsson and Lundstrom, 1970; Vartia, 1973), and mycorrhizal-forming fungi (Brown and Mikola, 1974). Symbiont and non-symbiont algae, as well as growth of vascular plants, also are retarded by lichen acids (Follmann, 1965; Follmann and Villagran, 1965; Kinraide and Ahmadjian, 1970; Huneck and Schreiber, 1972; Lawrey, 1977). While most lichen acids are effective in most assays conducted, differing reports of relative toxicity have emerged from the data. Where comparisons were attempted, the relative toxicity of many of the lichen acids was different from assay to assay and changed as concentration was changed. Recently, Lawrey (1977) suggested that certain lichen acids (0-methylated), which are considered evolutionarily advanced, are of greater ecological significance than the hydroxylated counterparts. The data on toxicity of lichen acids to bacteria would bear out such a hypothesis (with notable ex-