Mitigation of zinc toxicity through differential strategies in two species of the cyanobacterium Anabaena isolated from zinc polluted paddy field

The present study describes the physiological and biochemical mechanisms of zinc tolerance in two heterocytous cyanobacteria i.e. Anabaena doliolum and Anabaena oryzae, treated with their respective LC₅₀ concentrations of zinc (3 and 4.5 mg L⁻¹) for eight days. The feedbacks were examined in terms of growth, metabolism, zinc exclusion, zinc accumulation, oxidative stress, antioxidants and metallothionein contents. Although the growth and metabolic activities were reduced in both the cyanobacterium, maximum adversity was noticed in A. doliolum. The higher order of abnormalities in A. doliolum was attributed to excessive accumulation of zinc and enhanced reactive oxygen species (ROS) production. However, the comparatively higher growth and metabolic activities of A. oryzae were ascribed to the lower accumulation of zinc as a result of released polysaccharides mediated zinc exclusion, synthesis of zinc chelating metallothioneins and subsequent less production of ROS. The oxidative stress and macromolecular damages were prominent in both the cyanobacterium but the condition was much harsher in A. doliolum which may be explained by its comparatively low antioxidative enzyme activities (SOD, APX and GR) and smaller amount of ascorbate-glutathione-tocopherol contents than that of A. oryzae. However, sustenance of 50% growth by A. doliolum under zinc stress despite severe cellular damages was attributed to the enhanced synthesis of phenolics, flavonoids, and proline. Thus, differential zinc tolerance in A. doliolum and A. oryzae is possibly the outcome of their distinct mitigation strategies. Although the two test organisms followed pseudo second order kinetics model during zinc biosorption yet they exhibited differential zinc biosorption capacity. The cyanobacterium A. oryzae was found to be more efficient in removing zinc as compared to A. doliolum and this efficiency makes A. oryzae a promising candidate for the phycoremediation of zinc polluted environments.