Tolerance of wetland rice field's cyanobacteria to agrochemicals in cultural condition

The wetland rice fields are favorable for cyanobacterial growth, which play an important role in building soil fertility. Since agrochemicals are intensively being applied in rice fields to boost crop productivity, this study was undertaken to observe their effects on dominant N2-fixing cyanobacteria i.e. filamentous, heterocystous Nostoc linckia and unicellular Aphanothece pallida. These cyanobacteria were isolated from flooded rice fields and used to find out the effects of urea N-fertilizer, herbicide benthiocarb and insecticide metacid (in isolation and combinations) on their growth and N2-fixation ability. It was observed that N. linckia and A. pallida grew up to 25 days after inoculation (DAI) whereas acetylene reduction activity (ARA) increased up to 15 DAI in N-free medium without supplementation of agrochemicals in both cyanobacteria where N. linckia exhibited higher growth and ARA than A. pallida. Urea-N (30 ppm) was inhibitory for the growth of N. linckia whereas it stimulated the growth of A. pallida. Addition of benthiocarb (0.5 ppm) was found to have no significant impact on the growth of N. linckia but showed initial enhancement in the growth of A. pallida up to 10 DAI. Metacid supplementation at 0.25 ppm supported growth and ARA of both the cyanobacteria. It was also revealed that the use of urea-N, benthiocarb and metacid in combination reduced the growth and N2-fixation of N. linckia, however, these chemicals together were promotary for A. pallida. Overall, it was observed that combinations of these agrochemicals caused additive toxicity to heterocystous cyanobacterium whereas these were promotary to unicellular ones.