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

Copper ion (Cu 2+) and zinc ion (Zn 2+) are widely co-existent in anaerobic digestion effluent as typical contaminants. This work aims to explore how Cu 2+-Zn 2+ 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 2+ and Zn 2+, 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 2+ and Zn 2+. 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 2+ and Zn 2+ 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 2+ concentration (> 4.0 mg/L). Risk of S. platensis biomass associated with Cu 2+ was higher than Zn 2+. S. platensis from SM (Cu 2+ ≤ 0.3 mg/L and Zn 2+ ≤ 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 2+ and Zn 2+ in anaerobic digestion effluent and harvesting biomass for animal feed additives.