Metabolism and degradation of glyphosate in aquatic cyanobacteria: A review

Abstract

Use of glyphosate (N-phosphonomethylglycine), a broad-spectrum, non-selective, post-emergence herbicide has been increased steadily with the introduction of genetically modified glyphosate-resistant crops. Increased reliance on herbicides for suppressing weeds and aggressive marketing have also contributed substantially to rising demand for glyphosate. Degradation of glyphosate was basically done by soil microorganisms; however, once the herbicide reached to the aquatic systems, cyanobacterial strains were reported to be involved in the process of biodegradation. Upon glyphosate exposure, a remarkable tolerance was reported in many strains, where cell proliferation was found to be completely unaffected by the herbicide at the concentration of micromolar to millimolar range. However, the mechanism through which cyanobacteria exhibit the tolerance seemed to be widely varied and species-dependent. Carrier-independent uptake of glyphosate has been suggested as the resistance mechanism at micromolar level concentrations. Presence of resistant form of the target enzyme EPSP (5-enolpyruvylshikimate-3-phosphate) and the ability of some strains to metabolize glyphosate have also been reported to be responsible for the tolerance. A remarkable ability to degrade glyphosate has been identified from some cyanobacterial strains such as Spirulina spp. where degradative pathway was however reported to be different from those exhibited in other bacteria. Exploitation of cyanobacteria in biological treatments of waste water contaminated with glyphosate has not yet been reported, mainly due to lack of research evidence on as to how cyanobacteria deal with biodegradation of glyphosate under field conditions. Key words: Glyphosate, cyanobacteria, biodegradation, biological treatments.