Metabolic regulation and environmental sustainability of Chlorella ellipsoidea for chromium bioremediation and biodiesel production

With the acceleration of global industrialization, chromium (Cr) contamination from industrial effluents and solid wastes presents a critical environmental challenge. Although phytoremediation, especially microalgae-based bioremediation, provides a promising eco-friendly solution, it is still difficult to link the basic mechanism of chromium detoxification with real solid waste recycling and scalable bioenergy production. In this study, we systematically investigated the metabolic regulatory mechanisms of Chlorella ellipsoidea under pure chromium conditions and under stress conditions using actual argon-oxygen decarbonization (AOD) slag, and integrated these physiological findings with a life cycle assessment (LCA) of biodiesel production at the laboratory scale. We identified a synergistic, three-layered compartmentalized defense mechanism in Chlorella ellipsoidea , driven by the stimulated secretion of extracellular polymeric substances (EPS) and intracellular vacuolization, which facilitated efficient Cr sequestration (up to 64.00%) and intracellular reduction (up to 98.81%). Crucially, under optimized multi-source chromium stress conditions (AOD slag), the microalgae strategically redirected carbon allocation toward lipid biosynthesis, increasing lipid content to a maximum of 43.42 wt% with a fatty acid profile dominated by the ideal C16-C18 composition, and achieving a biodiesel yield of 78.00%, demonstrating excellent fuel suitability. However, the cradle-to-gate LCA revealed that medium preparation remains the primary environmental bottleneck, contributing to 60% of the total carbon footprint (782.97 kg CO₂ eq) and dominating toxicity indicators. This study not only elucidates the subcellular detoxification mechanisms of microalgae under complex heavy metal stress, but also demonstrates the untapped potential of utilizing industrial solid waste to trigger high-value lipid accumulation. Furthermore, our LCA findings provide a crucial roadmap for identifying green solvent alternatives and energy reduction strategies, advancing the sustainable deployment of microalgal waste-to-energy technologies.