Glyphosate and fungi in agricultural environments

Abstract

Chemical herbicides are widely used throughout the world in food production. At the present time, they represent approximately 40% of chemical products sales, followed by insecticides, fungicides, and other types of pesticides. Among these products, commercial formulations based on N-phosphonomethyl-glycine (PMG), commonly known as glyphosate, are the most used worldwide. Such herbicidal formulations play a key role in promoting crop yields. PMG is a broad-spectrum compound that kills weeds that compete with crops in agricultural soils (such as soils for soybean and maize growth) and non-agricultural environments (such as green spaces and gardens). In the last decade, some annual broadleaf weeds have developed resistance to PMG. For this reason, it has become necessary to apply higher and repeated doses of PMG, which may lead to contamination of the environment. Some works have reported a relatively fast biodegradation of PMG in contaminated soils, with a half-life period of nearly 20 days. However, after 30-years of application of glyphosate-based herbicides (GBH), this herbicide and its primary degradation product, aminomethylphosphonic acid (AMPA), still are the most frequently detected compounds in water bodies, mainly in agricultural areas. In addition to environmental pollution and human health risks, these residual compounds can have a phytotoxic effect on the next crop and become a threat to food production. Several processes act in the dissipation of organophosphorus compounds on soil, being the microbial activity one of the most relevant. Since the 1980s, applied researchers have become increasingly interested in the study of the effects of GBH on microbial communities. Although considerable research has been done on biodegradation of PMG by bacteria, much less is known regarding this capacity in fungi. Fungal species belonging to genera such as Phanerochaete, Trametes, Aspergillus, Fusarium, Penicillium and Trichoderma are able to metabolize important environmental pollutants such as herbicides. They possess a number of advantages that can be exploited in bioremediation systems. They can degrade a diverse range of persistent toxic environmental pollutants and carry out a relevant role in co-metabolic degradation processes. The persistence of PMG and AMPA in soils and aquatic sediments depends on the type of soil, tillage, and climatic conditions, among other factors. Both are polar compounds that strongly bind to Fe and Al oxides, clays and organic matter (humic acids) of the soil, decreasing the availability of free herbicide. The purpose of this chapter is to review information on the presence of GBH in the environment, their potentially harmful effects, their influence on soil microbial communities and their capacity to adsorb to clay particles (which affects their environmental availability).