Microalga-bacteria interaction mitigates adverse effects on microalga produced by ZnO nanoparticles

Abstract

Zinc oxide nanoparticles (ZnO NPs) used in myriad industries have become an emerging contaminant in various environments. Interactions of microalgae and bacteria, in suspension systems and immobilized, have been proposed for several biotechnology applications, such as wastewater treatment. However, the presence of NPs can affect the performance of these microorganisms during nutrient removal. This study assessed the effect of metal NPs on microalgae˗bacteria interaction, immobilized or in suspension. The effect of two concentrations of ZnO NPs (0.1 and 0.5 ppm) on the growth and photosynthetic pigments of the microalga Chlorella sorokiniana cultured in suspension or immobilized in alginate beads, alone or in the presence of the microalgae growth-promoting bacterium Azospirillum brasilense, was evaluated, and the effect of these NPs on indole-3-acetic acid (IAA) production by A. brasilense was also tested. The presence of ZnO NPs affected the growth of both microorganisms when cultured alone in suspension, and the IAA production by A. brasilense was reduced by 98 %. Nevertheless, when the two microorganisms were co-cultured in suspension, the negative effect of 0.1 ppm of ZnO NPs in C. sorokiniana was mitigated by A. brasilense, indicating a positive interaction between the two microorganisms. Furthermore, adding synthetic IAA enhances microalgae growth and photosynthetic pigment production regardless of NP concentration. On the other hand, immobilization of the cells produces a dose-response effect in the microorganisms, regardless of the partnerʼs presence. The results showed the positive effect of A. brasilense on stress mitigation of C. sorokiniana when 0.1 ppm of ZnO NPs are present and similar to the effect produced by synthetic IAA when the microorganisms are present in a suspension system. On the other hand, the immobilization of cells in alginate beads represents a physical barrier that acts as protection against the stress produced by ZnO NPs.