EFFECT OF PESTICIDES ON THE GROWTH, PHOTOSYNTHETIC OXYGEN EVOLUTION AND NITROGEN FIXATION OF WESTIELLOPSIS PROLIFICA

Abstract

We examined the effect of the pesticides Furadan (Carbofuran, 3%), Sevin (Carbaryl, 50%), Rogor (Dimetnoate, 30% EC) and Endotaf (Endosulfan, 35% EC) on the survival, growth, photosynthetic oxygen evolution and nitrogen fixation of the cyanobacterium Westiellopsis prolifica. Lower concentrations of the carbamate and organophosphate pesticides (10ppm) increased the growth and nitrogen fixation while higher concentrations (>20ppm) had an inhibitory effect. The organochlorine pesticide Endotaf was toxic even at 10ppm. Survivability and nitrogen fixation of the cyanobacterium was reduced to 72 and 93% respectively in the presence of 100ppm of Endotaf. An exposure of the filaments to over 100ppm of Furadan, Sevin or Rogor caused about a 40% inhibition of the photosynthetic oxygen evolution. Endotaf was more toxic than the other pesticides. Oxygen evolution was totally suppressed after incubation of the organism with over 250ppm of Endotaf; there was similar suppression of O₂ evolution on treatment with more than 500, 1, 000, and 1, 500ppm of Rogor, Sevin and Furadan respectively. Various cyanobacterial species occur abundantly in rice-fields of tropical and sub-tropical countries, whose agricultural technologies involve extensive use of pesticides for selective elimination of pests of rice crops (1, 3, 8, 14). Therefore, the indiscriminate use of pesticides may be causing adverse effects on these rice-field nitrogen-fixing cyanobacteria which have a direct influence on the total productivity.