Toxicity of the mixture of nanoparticles to algae–bacteria consortia in OECD media†

The present study investigated the effect of single as well as a mixture of nanoparticles (ZnO and CuO NPs) on algae–bacteria consortia using the OECD 96-h toxicity test, one of the first efforts as per the authors' knowledge. Microalgae (Scenedesmus obliquus) and bacteria (Escherichia coli) were used as test organisms in OECD media. Effects of the different concentrations of NPs (0, 0.1 mg L⁻¹, 1 mg L⁻¹, 10 mg L⁻¹, and 100 mg L⁻¹) on 3 algae–bacteria ratios (1 : 1, 1 : 100 and 100 : 1) were studied using parameters, such as chlorophyll, biomass, lipid, and protein contents, reactive oxygen species (ROS) generation, and extracellular polymeric substance (EPS) components. At environmentally relevant NP concentration (0.1 mg L⁻¹), the order of toxicity of NPs to algae–bacteria consortia was found to be: CuO NPs (highest toxicity) > ZnO + CuO NPs > ZnO NPs (least toxicity). At 100 mg L⁻¹ NP concentration, structural changes and cell leakage in the samples containing NPs with algae–bacteria consortia were observed during TEM analysis. FTIR (Fourier transform infra-red) analysis indicated the addition of bonds and a difference in the peak location and its intensity values. The corresponding metal ions (Zn and Cu ions) resulted in higher toxicity to algae–bacteria consortia than metal oxide NPs. When nanoparticles were interacting with the algae–bacteria consortia in the suspension, it was observed that by absorption and dissolution, nanoparticles would enter inside the algal and bacterial cells simultaneously, altering the surface charge of the cell membrane. Due to the formation of EPSs, some of the nanoparticles would not be able to enter the cell cytoplasm but would interact with the EPS. ROS generation would take place extracellularly as well as intracellularly due to the interaction of nanoparticles. Overall, the mixture of NPs at environmentally relevant concentrations (<1 mg L⁻¹) poses lower toxicity to algae–bacteria consortia than individual nanoparticles. More experimental studies need to be performed to understand the effect of NPs on the functioning of algae–bacteria consortia for effective wastewater treatment using algae–bacteria consortia.