Use of rare earth element (REE)-contaminated acidic water as Euglena gracilis growth stimulator: A strategy for bioremediation and simultaneous increase in biodiesel productivity

Rare earth elements (REEs) have emerged as contaminants in water systems because their use and disposal are on the rise in many industries. To achieve environmentally benign treatment of REE-contaminated acidic water, we examined a strategy that uses Euglena gracilis for the bioremediation of REEs and simultaneous biodiesel production from used biomass. The presence of lanthanum (La³⁺) in the medium (0–100 µM) stimulated the growth of E. gracilis. The removal efficiency of La³⁺ was 99.9%, indicating that bioremediation using E. gracilis is feasible. The La³⁺ treatment accelerated the endocytic activity of E. gracilis and enabled cells to take up more nutrients (glucose and macro/micro elements). Also, La³⁺ increased the paramylon yield of E. gracilis, with the fatty acid methyl ester (FAME) yield below 3 wt% in both the control and La³⁺-treated cells. To increase the FAME yield and productivity, wax ester fermentation was conducted under anaerobic conditions. After wax ester fermentation, the FAME yield and productivity of the La³⁺-treated sample were 9.49 wt% and 78.76 g/L/day, 1.4- and 1.6-fold higher than the yield and productivity of the control sample. Moreover, the series of La treatment and wax ester fermentation enhanced the quality of biodiesel by increasing its saturated fatty acid (SFA) content. We confirmed that other REE species (cerium, neodymium, and a mixture) also stimulated the growth and biodiesel production of E. gracilis. Taken together, our E. gracilis-based bioremediation of REE-contaminated water provides high removal efficiency and a chance to simultaneously improve biodiesel productivity and quality.