Microbial Degradative Capacities of Rhodococcus sp. as a Strategy to Reduce Pesticide Metamitron Pollution in Agricultural Systems

Despite increasing efforts to reduce the use of synthetic pesticides, their global consumption remains high due to the limited availability of effective and economically viable alternatives. Synthetic pesticides and their biodegradation products (metabolites) are major contributors to environmental contamination, raising significant concerns for ecosystem functioning and human health. This study focuses on metamitron, a synthetic herbicide widely used in sugar beet cultivation and among the most sold active substances in Belgium in 2020, with a total of 114,700 kg marketed. The objective of this work is to assess the environmental risks associated with metamitron and to develop effective and scalable bioremediation strategies for its removal from contaminated environments.
Metamitron is applied as a pre- and post-emergence herbicide and is characterized by a reported half-life of 11.1 days under field conditions. However, it rapidly transforms into desamino-metamitron, a metabolite that is approximately three times more persistent than the parent compound and frequently detected in soils and water bodies. The accumulation of this metabolite represents a significant environmental concern, particularly in agricultural landscapes. Nevertheless, previous studies have demonstrated that desamino-metamitron can be completely mineralized in soil microcosms through microbial activity, highlighting the relevance of biological approaches for mitigation.
The present study aims to isolate culturable microorganisms capable of degrading metamitron and its major metabolite and to evaluate their potential application in bioremediation strategies ranging from laboratory-scale microcosms to field conditions. Microbial strains were isolated from environmental matrices, including biobed (biobac) substrates, using enrichment cultures in mineral media supplemented with metamitron as the sole carbon and/or energy source. Pesticide degradation was monitored daily using ultra-performance liquid chromatography (UPLC), and microbial isolates were recovered upon confirmation of degradation activity.
A total of fifteen microbial strains exhibiting metamitron-degrading activity were successfully isolated from biobed substrates. Among these, strain 8M, identified as a Rhodococcus sp., showed exceptional degradation efficiency, achieving complete degradation of metamitron at an initial concentration of 50 mg·L⁻¹ within 48 hours. To our knowledge, this represents the fastest microbial degradation of metamitron reported to date. In addition, strain 8M demonstrated measurable degradation activity toward desamino-metamitron, and investigations into the associated metabolic pathways are currently ongoing.
The selected strain is currently being evaluated in controlled soil microcosms, including agricultural and non-agricultural soils, under planted and unplanted conditions, both in controlled environments and outdoors. The ultimate goal is to develop field-adapted bioremediation solutions to reduce the environmental persistence of metamitron.