Degradation of the phosphonate herbicide glyphosate in soil: evidence for a possible involvement of unculturable microorganisms

The properties of microbial strains responsible for the rapid mineralization of the herbicide glyphosate in soil were investigated in soil-water mixtures supplemented with 10 mmol l(-1) active ingredient. Over 2 weeks degradation kinetics were linear, as expected in the case of non-growth-linked metabolization, and the rate of utilization was not enhanced following repeated treatment of the soil with increasing herbicide doses. The availability of exceeding phosphorus, nitrogen and carbon sources did not affect the rate of glyphosate utilization, that was maximal under conditions of neutral pH, high oxygen and low osmolarity. The screening of 1200 bacterial strains isolated on a rich medium in the absence of the herbicide failed to identify any strain able to cleave the glyphosate molecule. When antibiotics with different mode of action were added to the mixtures, while some inhibitors of protein synthesis exerted considerable effects, those that are active only against actively-proliferating cells were scarcely effective. An MPN analysis was performed to enumerate degrading microorganisms, but in no dilution the same extent of utilization measured in the original mixture could be found. Results suggest that at least the first steps in herbicide degradation could be accomplished by some microbial species unable to grow in vitro and form visible colonies on plates.