Ecotoxicological response of Spirulina platensis to coexisted copper and zinc in anaerobic digestion effluent.

Copper ion (Cu) and zinc ion (Zn) are widely co-existent in anaerobic digestion effluent as typical contaminants. This work aims to explore how Cu-Zn association affects physiological properties of S. platensis using Schlösser medium (SM) and sterilized anaerobic digestion effluent (SADE). Microalgae cells viability, biochemical properties, uptake of Cu and Zn, and risk assessment associated with the biomass reuse as additives to pigs were comprehensively assessed. Biomass production ranged from 0.03 to 0.28 g/L in SM and 0.63 to 0.79 g/L in SADE due to the presence of Cu and Zn. Peak value of chlorophyll-a and carotenoid content during the experiment decreased by 70-100% and 40-100% in SM, and by 70-77% and 30-55% in SADE. Crude protein level reduced by 4-41% in SM and by 65-75% in SADE. The reduction ratio of these compounds was positively related to the Cu and Zn concentrations. Maximum value of saturated and unsaturated fatty acids was both obtained at 0.3 Cu + 2.0 Zn (50.8% and 22.8%, respectively) and 25% SADE reactors (33.8% and 27.7%, respectively). Uptake of Cu in biomass was facilitated by Zn concentration (> 4.0 mg/L). Risk of S. platensis biomass associated with Cu was higher than Zn. S. platensis from SM (Cu ≤ 0.3 mg/L and Zn ≤ 4.0 mg/L) and diluted SADE (25% and 50% SADE) reactors could be used as feed additives without any risk (hazard index <1), which provides sufficient protein and fatty acids for pig consumption. These results revealed the promising application of using S. platensis for bioremediation of Cu and Zn in anaerobic digestion effluent and harvesting biomass for animal feed additives.