The soil organisms responsible for the enhanced biodegradation of metham sodium

On a farm near Perth, Western Australia, where metham sodium (sodium N-meth yldithiocarbamate) soil fumigant has been extensively used for the past dec ade. a dramatic case of enhanced biodegradation has been identified. On thi s soil, the applied dose of metham sodium produced less than half the maxim um concentration of the toxin methyl isothiocyanate (MITC) reached in simil ar, previously untreated soil. Also, the MITC formed was present in the soi l for less than 5% of the time of that in the untreated soil. On sterilisat ion of the affected soil in an autoclave, followed by treatment with metham sodium, the MITC concentration/time relationship was similar to that of th e previously untreated soil, confirming that the enhanced degradation was t he result of a biological process. Dry heat treatment of the affected soil (100 degreesC, 1 h) did not destroy the microorganisms responsible for enha nced biodegradation, as the soil recovered its degrading ability upon rehyd ration. This indicated that microorganisms with resistant stages were invol ved, at least in part, in the biodegradation of MITC. Agar containing MITC as a carbon source was used to isolate bacteria from fresh soil exhibiting enhanced biodegradation. All 18 isolates selected were Gram positive. Morph ologically, 11 isolates resembled Rhodococcus spp., 4 isolates resembled Ba cillus spp. and 3 were unidentified. Inoculation with these isolates groupe d together into soil that had been sterilised by autoclaving resulted in bi odegradation of MITC in that soil.