Fungal biodegradation of chlorinated herbicides: an overview with an emphasis on 2,4-D in Argentina

Abstract

Chlorinated herbicides are one of the main types of pesticide used in agriculture. In Argentina, 2,4-dichlorophenoxyacetic acid (2,4-D) is the most applied herbicide for the control of broadleaf weeds, but the risks it poses for the environment and human health are cause for great concern. A promising technology to remove this kind of pollutants, or neutralize them in such a way that they become less or non-toxic, is the use of degrading or detoxifying microorganisms from contaminated sites. Filamentous fungi can bioremediate xenobiotics thanks to their efficient enzymatic machinery. However, most studies on the degradation of 2,4-D have been carried out with bacteria, and little is known about whether it can be efficiently biodegraded by fungi. In the environment, fungal strains and native microbiota may detoxify contaminants through mechanisms like biosorption, bioabsortion, biotransformation, and/or degradation. Whether these processes occur separately or simultaneously depends on the metabolic ability of the strains that conform the microbial community. Another important concern when attempting to introduce detoxifying microorganisms into a contaminated environment is the GRAS ("Generally Recognized As Safe") assessment or status. These are studies that help predict a biodegrading microorganism's pathogenicity, toxicity, and infectivity before in situ application. This application, moreover, is regulated by different legal frameworks. The present review aims to outline the main aspects of 2,4-D degradation by fungi, and to summarize the current state of research on the topic in Argentina.

Keywords: Bioremediation; Characterization of 2,4-dichlorophenoxyacetic acid-removing strains; Chlorinated herbicides; Fungal species; Regulatory aspects.

Fig. 1

Mechanisms involved in fungal bioremediation of herbicides

Fig. 2

Performance of different fungal enzymes in biodegradation

Fig. 3

Brief history of 2,4-D herbicide

Fig. 4

Different pathways for 2,4-D degradation proposed in the literature: a. p-chlorophenoxyacetic pathway proposed by Clifford and Woodcock (1964); b. pathway proposed by Faulkner and Woodcock (1965) and c. 2-DCP pathway proposed by Shailubhai et al. (1983)