Sustainable biodiesel production from the green microalgae Nannochloropsis: Novel integrated processes from cultivation to enzyme-assisted extraction and ethanolysis of lipids

[Display omitted] •An integrated process was developed for biodiesel using microalgae Nannochloropsis.•This process consisted of cultivation, lipid extraction and enzymatic ethanolysis.•Three phase partitioning was an efficient approach for extracting microalgal lipids.•Ethanolysis by Thermomyces lanuginosus lipase was a cost-effective way for biodiesel.•The integrated process was the potential strategy for biodiesel from Nannochloropsis. A novel integrated process from cultivation to enzyme-assisted extraction and ethanolysis of lipids was established and implemented to produce sustainable and clean microalgal biodiesel by three marine Nannochloropsis strains (N. oculata, Nannochloropsis sp. and N. oceanica). Among the selected strains, N. oculata was selected as a model for the evaluation of lipid extraction performance and biodiesel conversion, due to its highest biomass (4.38 g/L) and total fatty acids (TFAs) yield (1.55 g/L). In the lipid extraction, three phase partitioning (TPP) exhibited superior lipid extraction performance when N. oculata cells were treated by four mixed enzymes (cellulase, hemicellulase, papain and pectinase). After optimization, the highest FAMEs recovery (92.59%) was obtained by TPP at 1.5% microalgal biomass, 6% K2HPO4, 65% ethanol content, 80 °C extraction temperature, 1 h incubation time and two extraction cycles. In the enzymatic ethanolysis, the extracted lipids from the ethanol phase of TPP were directly used for biodiesel with liquid lipase from Thermomyces lanuginosus (TL) and results showed that lipase TL performed the greatest biodiesel conversion (90.24%) under the optimized conditions. Lastly, this integrated process was re-evaluated with two Nannochloropsis strains. Results showed that the integrated process attained 85.12% biodiesel conversion for Nannochloropsis sp. and 76.33% for N. oceanica, demonstrating that the lipidic class and fatty acid composition influenced the biodiesel conversion by enzymatic ethanolysis. Taken together, the integrated process mediated with enzymes and ethanol as green media was a promising strategy for cost-effective and low-value bulky biodiesel by oleaginous Nannochloropsis strains.