Effect of the veterinary ionophore monensin on the structure and activity of a tropical soil bacterial community

Abstract Monensin (MON) is a coccidiostat used as a growth promoter that can reach the environment through fertilization with manure from farm animals. To verify whether field-relevant concentrations of this drug negatively influence the structure and activity of tropical soil bacteria, plate counts, CO2 efflux measurements, phospholipid fatty acids (PLFA) and community-level physiological profiling (CLPP) profiles were obtained for soil microcosms exposed to 1 or 10 mg kg−1 of MON across 11 days. Although 53% (1 mg kg−1) to 40% (10 mg kg−1) of the MON concentrations added to the microcosms dissipated within 5 days, a subtle concentration-dependent decrease in the number of culturable bacteria (<1 log CFU g−1), reduced (−20 to −30%) or exacerbated (+25%) soil CO2 effluxes, a marked shift of non-bacterial fatty acids, and altered respiration of amines (1.22-fold decrease) and polymers (1.70-fold increase) were noted in some of the treatments. These results suggest that MON quickly killed some microorganisms and that the surviving populations were selected and metabolically stimulated. Consequently, MON should be monitored in agronomic and environmental systems as part of One Health efforts.