Microbial synthesis of nanoparticles and their effect on the growth of nitrogen fixing cyanobacteria

Abstract

Zinc oxide nanoparticles are of great interest due to their versatile applications and noble properties such as high refractive index, high thermal conductivity, binding energy, antibacterial UV-protection etc. They can be synthesized by various physical and chemical methods which are generally toxic, need very high temperatures and release hazardous chemicals. Therefore, we report green approach for the synthesis of Zinc oxide (ZnO) nanoparticles (NPs) using zinc acetate as metal precursor from Aspergillus niger, Anabaena variabilis and Trichoderma reesei which were further characterised by XRD, FTIR and SEM. Sharper and stronger diffraction peaks of XRD analysis confirmed synthesized ZnO NPs. SEM images confirmed the size of nZnO synthesized from various sources. The size of nanoparticles was 64-126 nm from A. niger, 72-74 nm from A. variabilis and the nano-structures in T. reesei remained so compactly arranged that their average size could not be estimated. FTIR analysis confirmed the polysaccharides present in the aqueous extract of A. variabilis that are involved in the formation of nZnO. Whereas, functional groups present in the ZnO nanoparticles synthesized from A.niger and T. reesei was confirmed through the peaks in the range of 3075 to 859 cm-1 and 1615.05 to 1099.99 cm -1 . In order to assess the role of these synthesized nanoparticles in the environment, effect of ZnO NPs and other sources of Zn (0 to 100 ppm) was studied on the growth of A. variabilis and N. muscorum. Biomass and chlorophyll content were studied as growth parameters, whereas NR activity and IAA production were studied as physiological parameters. It was found that 2 ppm of ZnO NPs are stimulatory and above 2 ppm were inhibitory for the growth of A. variabilis, whereas in N. muscorum even 2 ppm of ZnO NPs were inhibitory for growth. Effect of different zinc sources such as ZnCl2, ZnNO3, ZnO, Zn(CH3COO- )2 and ZnSO4 was studied on the growth of A. variabilis and Calothrix sp. The biosynthesized zinc oxide nanoparticles are expected to be clean, non-toxic and environmentally acceptable and can have applications in diverse fields.