Catalytic Degradation of the Herbicide Glyphosate by the Paddy Field Isolates of Cyanobacteria

Abstract

N-(phosphonomethyl)glycine (glyphosate) is the active ingredient of well known broad spectrum herbicide formulations such as ‘Round-up’, that are widely used in agriculture. Two strains of paddy field isolates of free-living, diazotrophic filamentous forms of cyanobacteria (Nostoc sp. L. ACN 101 and Westiellopsis sp. L. ACW 101) were tested for their responses to glyphosate treatment under culture conditions. Both the strains of cyanobacteria tested, registered an EC 50 (effective concentration causing 50% inhibition in growth) of 65 µM measured in terms of their protein content. In Nostoc while a 140% enhancement in intracellular C-P lyase activity was recorded, the enzyme showed only a 30% increase in C-P lyase activity in the culture filtrates, compared to their respective controls under P minus and in the presence of glyphosate. However, the intracellular and the culture filtrate activities of alkaline phosphatase showed a 51% and 5% reduction compared to their respective control. On the contrary, in Westiellopsis while the intracellular C-P lyase activity exhibited a 14% reduction, the enzyme activity in the culture filtrate was enhanced by 348%. Moreover, while the intracellular alkaline phosphatase activity registered a 233% enhancement, the enzyme activity was reduced by 12% in the culture filtrate. The patterns of induction kinetics of C-P lyase in the phosphate starved cultures exposed to glyphosate indicate that the optimum enzyme activity in the culture filtrate is obtainable between the 10th and 16th day in Westiellopsis and it was between 14th and 18th day in Nostoc. The magnitude of induction was nearly four-fold in Westiellopsis than Nostoc. Albeit the alkaline phosphatase showed a very high intracellular activity, its activity in the culture filtrate was not appreciable. This idicates that all the syntheszed alkaline phosphatase is not excreted out into the enviroment. Between the two cyanobacteria, Westiellopsis has a more efficient machinery to degrade glyphosate compared to Nostoc. This has been substantiated by the rate of synthesis of the phycobiliprotein which paralleled with the periods of optimum C-P lyase activity in the culture filtrates of both the organisms experimented with.