Nitrogen-dependent lipid accumulation and fatty acid profile of microalga Parachlorella kessleri MDC 6524 for enhanced biodiesel production

The biomass of the unicellular green microalga Parachlorella kessleri is considered as one of the valuable sources of easily digestible protein, polysaccharides, pigments, and high-molecular fatty acids in the composition of synthesized lipids. The quality and quantity of these compounds vary based on intraspecific differences in strains, as well as growing conditions. Nitrogen is one of the most critical nutrients affecting cell growth and biochemical composition of microalgae, ultimately determining the lipid productivity for biofuels. In order to investigate the effect of nitrogen sources on newly isolated microalga Parachlorella kessleri MDC 6524 cell growth, lipid synthesis, and fatty acid composition, the strain was simultaneously grown on wastewater-simulating nutrient medium with and without a nitrogen source in bubble column photobioreactors. After 14 days of cultivation in the medium devoid of nitrogen sources, the total fatty acid content was 31.2% of the dry weight of the biomass, while in the medium rich in nitrogen sources, the fatty acid content was only 12.5%. Two methods for producing biodiesel were investigated: (i) lipid extraction followed by transesterification and (ii) direct transesterification from algal biomass. As a result, lipid extraction followed by transesterification increased biodiesel yield (per unit of biomass) by 1.2 times compared to the direct transesterification method. However, the direct transesterification method is significantly less time-consuming. Despite this advantage, the method has some limitations due to the interaction of lipids with the acid catalyst used in the process.