Herbicide monuron mediated alterations in carbon and nitrogen fixation in the cyanobacterium Nostoc muscorum Meg 1

This study aimed to investigate the impact of the herbicide monuron on various aspects of photosynthesis and diazotrophy in the cyanobacterium Nostoc muscorum Meg 1 isolated from a rice field in Cherrapunji, Meghalaya, India. The consequences of monuron exposure on various photosynthetic pigments, functioning of PSII, RuBisCO, nitrogenase and glutamine synthetase enzyme activities, heterocyst frequency, and on production of proteins and carbohydrates in the cyanobacterium were studied in a range of monuron doses (20–100 ppm) to gauge the herbicide’s effect on CO2 and N2 fixation and on net biomass production. The total amount of D1 protein (the host for PSII complex), RuBisCO (the key enzyme for CO2 fixation), nitrogenase (the enzyme responsible for the reduction of atmospheric nitrogen to ammonia), and ammonia assimilating enzyme glutamine synthetase (GS) contents under western blot analysis indicated interruption of new protein synthesis and breakdown of their existing enzyme molecules when exposed to higher monuron concentrations. All parameters studied showed enhanced expression under low dose monuron treatment (20 ppm) indicating a hormetic effect in the exposed organism. The expression of monuron toxicity on various parameters of CO2 and N2 fixation in a dose-dependent manner was immediately visible when cultures were treated with higher doses (40–100 ppm). Scanning and transmission electron microscopic studies further uncovered several undesirable changes in the morphology and ultrastructure of the organism due to herbicide treatment that could be correlated to compromised CO2 and N2 fixation.