Nutritional influence on the ability of fungal mycelia to penetrate toxic metal-containing domains

Metal-contaminated soils often contain a spatially heterogeneous distribution of metal concentrations, and the ability of fungi to colonize such metal-contaminated domains will be influenced by the nutritional resources available. An experimental system based upon tessellated agar tiles was used to study the influence of nutrients upon the ability of soil fungi Trichoderma virens and Clonostachys rosea to colonize spatially discrete toxic metal (copper and cadmium) containing domains. The growth parameters recorded demonstrated a decrease in apparent metal toxicity with increasing concentration of available carbon source. It was shown that maximum hyphal extension rates and the efficacy of carbon substrate utilization of both species decreased with increasing concentration of toxic metals. It was also observed that in the gap between metal-free and metal-containing tiles, the presence of toxic metals led to negative chemotropic reactions and cessation of growth, swelling and lysis of some hyphal tips. Penetration of the hyphae into the metal-containing domain was often followed by the formation of very dense mycelia or mycelial 'bushes' representing an associative (constraining, exploitative or 'phalanx') growth strategy of the mycelial system. After the fungi entered the toxic metal-containing domains, they often produced long sparsely-branched or branchless explorative hyphae representing a dissociative (expansive, explorative or 'guerrilla') growth strategy. Our data therefore demonstrate that fungi efficiently use both 'phalanx' and 'guerrilla' states of the mycelial system as well as shifts in these growth strategies as a response to toxic metal stress combined with nutritionally-poor conditions.