In the absence of a rational method for an a priori predic- tion of a biosorption potential of a microorganism, the only method for identifying and developing newer and efficient biosorbents is sustained screening of microbes.24 Variations in metal tolerance among different species of a genus or within the same species might be due to the presence of one or more resistance mechanisms exhibited by different fungi.25 Sarkar et al.26 reported Trichoderma harzianum to be moderately toler- ant to up to 60 ppm of Ni, at that concentration the level of inhibition of mycelial growth was 33.3%. A further increase in the Ni concentration reduced the growth, and total inhibition was observed at 200 mg/L. Lima et al.27 also observed influ- ence of cadmium on radial growth of T. harzianum. The results of the present investigation are in line with these earlier obser- vations.
When the effects of different nickel concentrations on biomass production of the three promising Trichoderma iso- lates (in order of ranking) were studied, the maximum biomass weight was recorded for UBT-18 (359 mg), which was signifi- cantly higher than the values obtained for MT-4 (256.67 mg) and IBT-I (225mg), at a Ni concentration of 40ppm. With a further increase of the nickel concentration in the medium to 60ppm, biomass production by all the isolates screened was insignificant compared with their respective controls. At higher concentrations of nickel, from 100 to 200ppm, there was a significant reduction in biomass of all three isolates. Although isolate UBT-18 showed significantly higher biomass production at Ni concentrations of up to 150 ppm, IBT-I showed somewhat higher biomass production compared to the other two isolates at 200 ppm (Table 4).
Effects of different cadmium concentrations on biomass production were studied using the three most tolerant Tricho- derma isolates. The maximum biomass value was recorded for IBT-II (523.70 mg), and it was significantly higher than the val- ues obtained for UBT-18 (147.03 mg) and MT-4 (147.01 mg) at a Cd concentration of 40 ppm (Table 5). With a further increase