Harnessing bioenergy potential of Chlamydomonas reinhardtii: A comprehensive characterization and valorization of biomass towards energy conversion under circular bioeconomy

Abstract

Despite the immense potential of microalgal bioenergy, limitations in biofuel precursors and ineffective zero-waste management hinder economic scalability. This study explores the response of Chlamydomonas reinhardtii to bicarbonate supplementation under nitrogen-sufficient (Stage I) and deficient (Stage II) conditions within a circular bioeconomy framework aimed at maximizing bioenergy potential. Optimal bicarbonate supplementation resulted enhancement in 3.35-fold biomass, 5.38-fold GS activity, NH₄⁺ removal (90.45 %), and a reduced zeta potential (−33.27 mV). Elevated ACCase and GPAT activities under bicarbonate supplementation boosted lipid accumulation (49.61 %) while enhancing antioxidant enzyme activities (SOD, CAT, APX) at Stage II. The increase in lipids was further validated by distinct FTIR peaks (2968 cm⁻1, 2855 cm⁻1, and 1744 cm⁻1) and 1H NMR signals (1.0–1.4 ppm, 4.1–4.25 ppm), indicating the metabolic rewiring of C. reinhardtii towards neutral lipid accumulation. Enhanced biofuel precursors contributed to improved thermochemical conversion efficiency, as revealed by TGA/DTG analyses. P-XRD confirmed the increased amorphous nature of the biomass. CHNS and SEM/EDX analyses revealed a marked increase in carbon content with a reduction in oxygen levels reflected in the HHV (21.52 MJ kg⁻1) and an elevated H/C ratio under nitrogen-starved conditions, further highlighting the biomass's enhanced suitability for efficient energy conversion. In addition, a 1.76-fold theoretical biomethane yield (TMP) was recorded against the control. Interestingly, lipid-extracted algal biomass (LEAB) yielded 0.158 g ethanol/g LEA. Thus, the study presents the potential of bicarbonate supplementation in C. reinhardtii for bioenergy precursors, advancing sustainable biofuel production and biomass valorization within a circular bioeconomy framework under nitrogen starvation. © 2024 Elsevier Ltd