A fungal growth model with application for soil bioremediation

Abstract

Fungi can be used as bioremediation agents because they actively colonize a soil matrix and they produce spores which can aid survival in extreme conditions. Genetic modifications to microorganisms allow flexibility in selecting the growth characteristics independently of the biochemical characteristics. The soil fungus Gliocladium virens was transformed to produce the bacterial enzyme organophosphate hydrolase, OPH. This enzyme can detoxify not only organophosphorus insecticides but also neurotoxins such as soman. The goal of this research was to describe the spatial growth of the transformed strain, GvT6, in relation to that of the parent strain, Gv29-8; and also to determine the temporal aspect of growth in response to added substrate and changes in temperature. Fractal geometry was used to compare spatial growth of GvT6 and Gv29-8. A system was devised to obtain images of subsurface colonies of the fungus without disturbing its structure. The fractal dimensions of the two colonies were not different indicating substantially the same branching and colonization behavior. Radial growth of GvT6 was slower than that of Gv29-8 implying higher inoculum densities would be required in a bioremediation system. Ergosterol can be used to quantify fungal biomass in soil but the production of this sterol varies among species and also depends on growth conditions. Experiments in liquid cultures and agar plates using a rich media (GYEC) and a soil extract media supplemented with maltose showed the ergosterol content of GvT6 was greatest when grown on GYEC agar plates (14.02 mg/g dry biomass). For a given media, plate cultures produced higher ergosterol content ratios than liquid cultures. Changes in ergosterol content over time were generally not significant. Data from experiments in soil bioreactors treated with different levels of substrate (0.5-8 mg maltose/g dry biomass) cultured at three different temperatures (22, 27 320C) showed subsurface growth of GvT6 can be described by the logistic equation. Culture conditions of 32'C and 8 mg/g substrate produced the highest levels of biomass, but growth at 32'C and 4 mg/g substrate was somewhat faster than at the higher substrate level. The combination of 22'C and 8 mg/g produced unexpectedly small levels of biomass.