Chlorella vulgaris bioaccumulates excess manganese up to 55× under photomixotrophic conditions

Abstract

Manganese is a transition metal that can accumulate in waterways in concentrations above natural abundance due to the mining, metallurgy, and agricultural industries. While chemical technologies for manganese removal are abundantly available, the high solubility of manganese in aqueous solutions causes steep decreases in removal efficiency and generates hazard waste that can be problematic to store. Chlorella vulgaris, a robust, unicellular green alga, offers an environmentally friendly alternative to chemical methods with the added economic potential of generating value-added products. In this study, C. vulgaris was exposed to increasing concentrations of manganese and monitored for bioaccumulation and adsorption. Additionally, cells were monitored for biochemical changes by assaying for terminal biochemical composition and determining photosynthetic activity throughout the life cycle of the culture. Evidence suggests that C. vulgaris can bioaccumulate manganese to an intracellular concentration of 733.3 mM and remove up to 56.74% of the manganese from highly concentrated media through both intracellular bioaccumulation and membrane-bound adsorption. Interestingly, the cultures exposed to high concentrations were able to accumulate manganese up to 55× the external concentration without experiencing inhibitory effects resulting from metal toxicity. Furthermore, cultures exposed to increased manganese displayed higher amounts of protein biosynthesis and an increase in photosynthetic capacities, potentially related to structural changes in the light harvesting antenna complexes of photosystem II.