Abstract
Carbamate pesticides are widely used as insecticides, nematicides, acaricides, herbicides and fungicides in the agriculture, food and public health sector. However, only a minor fraction of the applied quantity reaches the target organisms. The majority of it persists in the environment, impacting the non-target biota, leading to ecological disturbance. The toxicity of these compounds to biota is mediated through cholinergic and non-cholinergic routes, thereby making their clean-up cardinal. Microbes, specifically bacteria, have adapted to the presence of these compounds by evolving degradation pathways and thus play a major role in their removal from the biosphere. Over the past few decades, various genetic, metabolic and biochemical analyses exploring carbamate degradation in bacteria have revealed certain conserved themes in metabolic pathways like the enzymatic hydrolysis of the carbamate ester or amide linkage, funnelling of aryl carbamates into respective dihydroxy aromatic intermediates, C1 metabolism and nitrogen assimilation. Further, genomic and functional analyses have provided insights on mechanisms like horizontal gene transfer and enzyme promiscuity, which drive the evolution of degradation phenotype. Compartmentalisation of metabolic pathway enzymes serves as an additional strategy that further aids in optimising the degradation efficiency. This review highlights and discusses the conclusions drawn from various analyses over the past few decades; and provides a comprehensive view of the environmental fate, toxicity, metabolic routes, related genes and enzymes as well as evolutionary mechanisms associated with the degradation of widely employed carbamate pesticides. Additionally, various strategies like application of consortia for efficient degradation, metabolic engineering and adaptive laboratory evolution, which aid in improvising remediation efficiency and overcoming the challenges associated with in situ bioremediation are discussed.
Highlights
The sustenance of the ever-growing global population necessitates adequate crop production, making the use of pesticides crucial to prevent loss in the agriculture sector
Far excess use of carbamate pesticides has resulted in their dispersal across various compartments of the ecosystem
The reported bacterial degradation pathways for various carbamates primarily involve hydrolases and oxidoreductases; and depict conserved metabolic themes such as the initial hydrolysis by either amidases or esterases, ringhydroxylation of the hydrolysis products, generation of corresponding dihydroxy compounds and subsequent ring-cleavage to aliphatic intermediates, which are funnelled into central carbon metabolism
Summary
The sustenance of the ever-growing global population necessitates adequate crop production, making the use of pesticides crucial to prevent loss in the agriculture sector. Examples of some of the commonly used carbamate pesticides include fenobucarb, carbofuran, phenmedipham, carbendazim, aldicarb, methomyl, oxamyl, propoxur and Carbaryl (Table 1). Carbamates like propoxur, Carbaryl, carbofuran and carbendazim, which harbour aromatic nuclei are degraded through the formation of the respective dihydroxy intermediates, like catechols (Trivedi et al, 2016; Long et al, 2020; Mishra et al, 2020).
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