Toxicity of three herbicides to some nitrogen-fixing cyanobacteria.

The effects of some common rice field herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), Machete, and Saturn, on the paddy field nitrogen-fixing cyanobacteria Nostoc linckia, Nostoc calcicola, Nostoc sp., and Anabaena doliolum were studied. These cyanobacteria were found to be more tolerant to 2,4-D (lethal doses 1500-2000 micrograms/ml) than to Machete and Saturn (lethal doses 6-8 micrograms/ml). The effects of these three herbicides on some physiological processes of N. linckia were studied. The 2,4-D stimulated the growth and nitrogen fixation up to 100 micrograms/ml concentration (a dose higher than the field dose, i.e., about 40 micrograms/ml), recommended for field application. However, with Machete and Saturn this type of stimulation was not observed even at lower concentrations. Similarly, the uptake of nutrients, such as NO3- and NH+4, was also inhibited by Machete and Saturn. However, 100 micrograms/ml 2,4-D stimulated the uptake of NO3- but not of NH+4; higher doses of 2,4-D inhibited the uptake of both nutrients. Factors such as pH, organic carbon sources (glucose and acetate), and amino acids were found to regulate the toxicity of all three herbicides to N. linckia. Lower pH enhanced the toxicity of all three herbicides, whereas higher pH (up to 9.0) lowered it. Glucose and acetate (each 500 micrograms/ml) protected against the toxicity of 2,4-D and Saturn, but not against Machete. Whereas glutamine, arginine, serine, and tryptophan conferred upon N. linckia a greater protection against the toxicity of all three herbicides, methionine did not do so, and the presence of methionine with herbicide in the culture medium resulted in greater toxicity to N. linckia than that in the presence of the herbicide alone.