Utilisation of complex sugars for biomass production, and low-energy lipid extraction from Chlorella vulgaris and Nannochloropsis oculata for sustainable biodiesel production

In this study, growth media BG 11 supplemented with complex sugars such as β-glucan and β-mannan used as carbon sources were utilised to cultivate two selected microalgae species Chlorella vulgaris (C. vulgaris) and Nannochloropsis oculata (N. oculata) adopting mixotrophic mode in two scenarios (with culture aeration and without culture aeration). The BG 11 supplemented with these complex sugars and their corresponding monomers such as glucose and mannose served as positive control. The synergistic effects of these complex sugars and nitrogen were studied and optimised using response surface methodology (RSM). Also, various low-energy lipid extraction methods such as sonication, osmotic shock, bead shaking using TissueLyser, and the combination of osmotic shock with bead shaking using TissueLyser were studied to identify the most effective and energy efficient method of lipid extraction. The extracted lipids were trans esterified to biodiesel. Empirical method was employed to estimate the physicochemical properties, pinpointing the effects of these complex sugars on the properties the yielded biodiesel. Results showed that complex sugars retard biomass biosynthesis but boost lipids yield and biodiesel quality. Comparatively, biomass productivities of 0.032, 0.023, 0.030 and 0.026 gl⁻¹d⁻¹ were obtained from C. vulgaris, and 0.025, 0.018, 0.027 and 0.021 gl⁻¹d⁻¹ were obtained from N. oculata when cultured in BG 11 supplemented with glucose, β-glucan, mannose and β-mannan respectively. Lipids yield by weight of dry biomass of 26, 18, 23.5, 17.5 and 11.5 % were obtained from C. vulgaris, and 23.5, 15, 22.5, 17 and 8.5 % from N. oculata when cultured in BG 11 supplemented with β-glucan, glucose, β-mannan, mannose and media without carbon source respectively. More so, the major physicochemical properties of the produced biodiesel such as viscosity, cetane number and oxidative stability quantified in this research fall within the set range of international standards, indicating that complex sugars are potential carbon sources for sustainable biodiesel production.