Exploring Sustainable Biofuel Production: Comparative Analysis of Microalgal Oil Yield and Fatty Acid Composition Through Mixotrophic Cultivation on Agro-Industrial Wastes

With the increasing energy demand and elevated depletion rate of fossil fuels, there is a dire need for a cheap and environmentally stable fuel. Furthermore, the environmental and human health issues due to the overdependence on fossil fuels have been well addressed. Currently, mixotrophic (MT) cultivation of microalgae is gaining interest to overcome the world’s energy thirst. In this study, a microalgal strain (Chlorella vulgaris) that had already been isolated and characterized was grown in both photoautotrophic (PT) and MT conditions for 20 days. PT-cultivated C. vulgaris was provided with atmospheric CO₂ and BG-11 medium. While under MT conditions, C. vulgaris was cultivated on four kinds of agro-industrial wastes, i.e., sewage sludge, poultry waste, pressmud and livestock manure. The oil contents of the biomass raised under both cultivations were extracted using the Soxhlet apparatus and statistically compared. It was found that sewage sludge raised biomass produced the highest oil content (25.5 ± 0.35%), followed by poultry waste (18.51 ± 0.16%), pressmud (18.51 ± 0.16%), PT (10.58 ± 0.35%) and livestock manure (10.32 ± 0.09%). Furthermore, the extracted oils were subjected to GC-MS for the analysis of fatty acid methyl esters (FAME). The FAME analysis showed that sewage sludge raised biomass had the highest polyunsaturated fatty acids (PUFA) content of 59.61% with unsaturated fatty acids (UFA) content of 22.87%, followed by poultry waste (18.1% and 25.29%), livestock manure (49% and 8.2%) and pressmud (13.3% and 22.3%) raised biomass. While the PT-raised biomass showed 19.03 and 30.7% of PUFA and UFA, respectively. The frequent availability of PUFA and UFA ensured the production of high-quality biofuel. Based on these results, the MT growth of C. vulgaris can be utilized efficiently. The MT mode of nutrition on different wastes can ensure cost-effective production of biofuels.