Cypermethrin toxicity to rice field cyanobacterium Calothrix sp.

Abstract

Cyanobacteria are one of the potent group of microbes in wet land soils, especially in rice fields. Most of them have the capacity to fix atmospheric nitrogen and thus play a crucial role in nitrogen budget of soil where they grow. With the advent of green revolution these microbes have been gradually exposed to pesticides in large scale that poses a great threat to them. The present study was undertaken to investigate the influence of cypermethrin, a pyrethroid insecticide, on the growth and physiological activities of a rice field nitrogen fixing cyanobacterium Calothrix sp. (strain GUEco 1003) under controlled laboratory conditions. The test cyanobacterium showed varying degree of sensitivity against cypermethtin. To evaluate the toxicity, the organism was exposed to varying concentrations of the insecticide (8.75–70 ppm) based on LC50 for a period of 20 days. Results revealed that cypermethrin negatively affected its growth (65%), biomass (67%), chlorophyll-a (68%), protein (53%) and nitrogen content (65%) in a time-dose dependent manner. However the organism showed increase carbohydrate content with the increasing concentration of the insecticide over the control. Reduction in growth, biomass, chlorophyll-a, protein and nitrogen content with the increasing concentration of cypermethrin was an indication of its toxicity to the Calothrix sp., which is one of the natural biofertilizer in any agricultural field.