Effect of Bacillus cereus on the ecotoxicity of metal-based fungicide spiked soils: Earthworm bioassay.

Soil microorganisms exhibit varying levels of metal tolerance across a diverse range of environmental conditions. The use of metal-based fungicides such as mancozeb and copper oxychloride could potentially result in increased levels of manganese, zinc and copper which may adversely affect soil mesofauna. Under standardized earthworm bioassay conditions (temperature, pH, soil type and water content), we investigated the effect of Bacillus cereus on mancozeb and copper oxychloride ecotoxicity towards Eisenia andrei. A metal-tolerant Bacillus cereus strain previously isolated from a gold mining site was introduced into fungicide spiked soils. Earthworms were exposed to bacterial inoculated and non-inoculated substrates of mancozeb (8, 44, 800 and 1250 mg kg-1) and copper oxychloride (200, 450, 675 and 1000 mg kg-1). Experimental trials assessed avoidance-behavior, growth and reproduction utilizing standardized protocols (ISO and OECD). In the avoidance-behavior, E. andrei showed significant (p < 0.05) preference for inoculated substrates. Further, significant (p < 0.05) increases in biomass, survival, cocoons, juveniles and lower soil and tissue Mn, Cu and Zn contents were recorded at 8 and 44 mg kg-1 mancozeb and copper oxychloride 200 and 450 mg kg-1 inoculated soils compared to non-inoculated. However, at 800 and 1250 mg kg-1 mancozeb and 675 and 1000 mg kg-1 copper oxychloride concentrations, reproductive success in both inoculated and non-inoculated treatments was negatively (p < 0.05) affected. In conclusion, Bacillus cereus decreased the ecotoxicity of metal-based fungicides towards Eisenia andrei at 8 and 44 mg kg-1 mancozeb and 200 and 450 mg kg-1 copper oxychloride concentrations. The outcome observed with the inoculated substrates at elevated fungicides concentrations maybe as a result of the environmental conditions (pH and temperature).