Effect of Glyphosate on Dehydrogenase Activity of Bacterial and Filamentous Fungal Isolates from a Freshwater Sediment

The effect of glyphosate on bacteria and filamentous fungi isolated from Yeghe River sediment in Bori LGA, Rivers state, Nigeria was investigated. The isolates were phenotypically identified using standard methods and assayed to determine the Minimum Inhibitory Concentration (MIC) of the glyphosate. The dehydrogenase activity (DHA) of the most sensitive isolates were determined using 2, 3, 5-triphenyltetrazolium chloride (TTC) as the artificial electron acceptor. The triphenylformazan (TPF) produced was extracted in 4 ml of amyl alcohol and determined spectrophotometrically at 500 nm. The DHA was expressed as the mg of TPF formed per dry weight of cell biomass per hour and was plotted against the glyphosate concentration. A total of thirteen (13) bacteria of the generaAmphibacillus, Brochothrix, Aeromicrobium, Staphylococcus, Sporosarcina, Kurthia, Enterococcus, Acinetobacter, Erysipelothrix, Vagococcus, Alcaligenes, Caryophanon and Escherichia, and nine (9) moulds of the genera Microsporum, Acrophialophora, Colletotrichum, Histoplasma, Trichophyton, Aspergillus, Cladosporium, Scedosporium and Chrysosporium were isolated. The result of MIC assay showed that 50% of the bacteria and 0% of the mould isolates were susceptible to concentrations of the glyphosate between 0.075 and 5.0 mg/ml while all isolates were susceptible-to concentration of 9.0 mg/ml glyphosate. However, all the microbial isolates used for DHA were found to be sensitive to concentration of 0.075 mg/ml glyphosate, showing that DHA is more sensitive than the MIC assay in toxicity testing. The results have also shown that low concentrations of glyphosate, usually associated with run-off and spray drifts, are sufficient to reduce the dehydrogenase activity of microorganisms and have the capacity to debilitate freshwater systems.