Bioluminescent in fungi: prospection, biological function and toxicological assays

Abstract

Although fungal bioluminescence has been reported since ancient times, the chemical pathways involved in light emission, the identity of the substrate and enzymes involved and the biological function of bioluminescence remain unsolved. Likewise the bacterial bioluminescence, whose study harnessed the development of the toxicological bioassay Microtox®, utilized by environmental protection agencies, industries and universities, the comprehension of fungal bioluminescence has the potential to generate a similar bioassay and also furnish information about the ecological and biological role of bioluminescence. The toxicity of metal-contaminated soils to basidiomycete fungi isnot well understood, although fungi are one of most important organisms in the soil ecosystem. A possible reason forthe lack of information about the fungal toxicity is the absence of a standard methodology for the evaluation of metaltoxicity. The present work describes the development and use potential of an assay for the evaluation of mono anddivalent metals to the naturally bioluminescent basidiomycete fungus Gerronema viridilucens. Initially, the optimal conditions for bioluminescence (BL) were studied by ANOVA factorial analysis varying the nutrients (i.e., sugar canemolasses and yeast extract), pH and temperature (81 experiments). The results indicated the optimal culture conditionsfor the emission of BL were 1.0% (w:v) sugar cane molasses, 0.10% (w:v) yeast extract, non-buffered pH 6.0 at 25oCand the minimum number of experiments was 28. Moreover, the 24h-BL-based assay must be performed on the 10thday after the inoculation of 35 mm Petri dishes with the mycelium. The higher the concentration of metallic cations, thelower the intensity of BL. The median effective concentration (EC50) was obtained from the BL vs. metallic cation concentration plots yielding the following series of fungal toxicity: Cd2+ > Cu2+ > Mn2+ H Ni2+ H Co2+ > Zn2+ > Mg2+ > Li+> K+ H Na+ > Ca2+. Quantitative Ion-Character Activity Relationships (QICARs), which correlate the metallic cationphysical-chemical properties with the free ion EC50 values (logEC50), were used for modeling mathematical expressions for the prediction of metal toxicity. The best fitting was obtained for the covalent index (Xm2r), which reflects the tendency of metallic species to form covalent bonds with sulfur-containing low weight biomolecules and/or proteins. The bioassay herein described has the potential to be used as a standard method for the evaluation of toxicity of metal and organic compounds as well as industrial pesticides to basidiomycete fungi. (AU)