Growth and survival potentials of immobilized diazotrophic cyanobacterial isolates exposed to common ricefield herbicides

Abstract

The effect of graded concentrations of four common ricefield herbicides (Arozin, Butachlor, Alachlor, 2,4-D) on diazotrophic growth, macromolecular contents, heterocyst frequency and tolerance potentials of Ca-alginate immobilized diazotrophic cyanobacterial isolates Nostoc punctiforme, N. calcicola, Anabaena variabilis, Gloeocapsasp., Aphanocapsa sp. and laboratory strain N. muscorum ISU (Anabaena ATCC 27893) was studied and compared with free-living cultures. Cyanobacterial isolates showed progressive inhibition of growth with increasing dosage of herbicides in both free and immobilized states. There were significant differences in the relative toxicity of the four herbicides. Arozin proved to be more growth toxic in comparison to Alachlor, Butachlor and 2,4-D. Growth performance of the immobilized cyanobacterial isolates under herbicide stress showed a similar diazotrophic growth pattern to free cells with no difference in lethal and sub-lethal dosages. However, at lethal concentrations of herbicides, the immobilized cells exhibited prolonged survivability of 14–16 days as compared to their free-living counterparts (8–12 days). The decline in growth, macromolecular contents and heterocyst frequency was found to be similar in both the states in graded dosages of herbicides. Of the test organisms, A. variabilis showed maximum natural tolerance towards all the four herbicides tested. Evidently immobilization by Ca-alginate seems to provide protection to the diazotrophic cyanobacterial inoculants to a certain extent against the growth-toxic action of herbicides.