Graduate Thesis Or Dissertation
 

Stimulation of mycelial growth of Endothia parasitica by specific heavy metals that detoxify accumulated oxalate

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  • Mycelial growth of Endothia parasitica was stimulated by relatively high concentrations of CuSO₄ in potato dextrose agar medium (PDA). Stimulation of E. parasitica by the high levels of CuSO₄ and the relatively poor growth of this fungus on standard culture media suggested that stimulation might be due to interaction of the copper toxicant with a self-inhibiting fungal metabolite secreted into the medium such that both were inactivated. The present study was designed to test this hypothesis and determine the influence of other metals on the growth of E. parasitica. Mycelial growth stimulation by CuSO₄ occurred over the concentration range of 10⁻⁵ to 10⁻² M, and over a wide pH range of 4.0 to 8.0. Sixteen other metal cations were assayed to determine if they were capable of enhancing growth but only Cu⁺, Fe⁺⁺, and Fe⁺⁺⁺ were stimulatory at relatively high concentrations (10⁻⁵ to 10⁻²M), and Zn⁺⁺ stimulated growth to a much lesser extent. Growth enhancement of E. parasitica, similar to that initiated by CuSO₄, was produced by high concentrations of dextrose (160 to 320 g/l) in PDA and occurred when the fungus was grown on cellophane over PDA. Stimulation did not occur in liquid culture. When cultured on PDA and other solid media, E. parasitica secreted large amounts of oxalate that precipitated as calcium oxalate trihydrate at the periphery of the fungal colony causing a halo to form in the medium. Mycelial growth of E. parasitica was greatly retarded when calcium oxalate accumulated, but retardation was reversed by copper and iron salts that prevented accumulation of the calcium oxalate crystals. E. parasitica grew well on potato infusion medium containing copper oxalate and copper-calcium oxalate as the main carbon source, poorly on calcium oxalate, and was inhibited by sodium oxalate in the medium. The specificity by which only Cu⁺⁺, Cu⁺, Fe⁺⁺⁺, and Fe⁺⁺ stimulate mycelial growth of E. parasitica suggests that interaction of metal and oxalate ions to form specific oxalate complexes that reverse the inhibition of simple oxalate salts accounts for enhanced growth in the presence of toxic levels of metals and oxalate.
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