Malathion biodegradation and biotransformation in water and sediment systems: bacterial pathways, environmental drivers, ecological impacts, and bioremediation strategies.

Malathion is one of the most widely used organophosphate insecticides, and its environmental fate is strongly influenced by microbial transformation in the water column and sediments. This review synthesizes current knowledge of the enzymatic and metabolic pathways that govern the persistence of malathion in aquatic, sedimentary, and wastewater systems. Members of the genera Pseudomonas, Bacillus, Acinetobacter, Rhodococcus, Micrococcus, and several other commonly studied taxa produce enzymes such as carboxylesterases and hydrolases that cleave the carboxyl ester linkage. The major metabolites formed are malathion monocarboxylic acid (MMC) and malathion dicarboxylic acid (MDC), which are generally far less toxic than the parent compound and, under favorable conditions, can be further mineralized to carbon dioxide and inorganic phosphate. A parallel transformation route, oxidative desulfuration, can generate malaoxon, a metabolite substantially more toxic and persistent than malathion and therefore important to consider in environmental risk assessment. Degradation rates are typically highest at 25-35 °C and neutral to slightly alkaline pH conditions that favor hydrolytic activity. Redox conditions further influence pathway predominance: oxic environments generally favor hydrolysis and oxidative transformation, whereas anoxic environments may slow overall degradation and promote alternative reductive processes. Nutrient availability can stimulate microbial growth and enzyme expression, accelerating degradation in eutrophic waters and organic-rich sediments, whereas in oligotrophic systems, transformation is often slower. Consequently, microbial activity can reduce malathion's environmental half-life from weeks to days in some systems, although the transient formation of toxic intermediates necessitates monitoring beyond the parent compound. Improved understanding of the microbial taxa, enzymatic machinery, and environmental drivers involved provides a stronger mechanistic framework for predicting malathion fate, refining ecological risk assessments, and informing targeted bioremediation strategies in contaminated ecosystems.