Urea-N influences biomass yield, neutral lipids accumulation, and unsaturated fatty acid production in photoautotrophically grown microalga Chlorella sp

Abstract

Microalgal cultivation using cost-effective nutrients is found to decrease the biodiesel production cost significantly. Therefore, in this study, various nitrogen sources namely sodium nitrate, ammonium nitrate, potassium nitrate, and fertilizer-grade urea, were tested to evaluate their impact on the biomass, lipids, and fatty acid content of microalga Chlorella sp. For biodiesel production. Onset of the study, the highest biomass yield, about 1.35g/L, was obtained in urea-N, potassium nitrate, and sodium nitrate. For lipid extraction, different methods such as sonication, stirring, Soxhlet, homogenization, and sonication + Soxhlet were compared, and the highest efficiency (19.86% lipid content) was noticed in the combined sonication + Soxhlet method with chloroform: methanol (2:1). Regarding the lipid content, 19.86, 19.91, 18.44, and 19.13% were extracted from sodium nitrate, ammonium nitrate, potassium nitrate, and fertilizer grade urea-grown cultures, respectively. Further optimization of urea-N revealed that, of the 0.0, 0.75, 1.5, 2.25, and 3.0 g/L urea-N concentrations, total lipid content and neutral lipid was decreased when increasing the urea-N level in the medium and maximum content of 23.6%, and 14.7 %, were observed, respectively in 0.0 g/L urea-N. Biodiesel yield with respect to urea-N concentration showed a maximum of 64.7% biodiesel from 0 g/L grown microalgal cells. The fatty acid fingerprinting of the biodiesel by gas chromatography showed higher levels of UFAs (57.18% w/w lipids) in nil urea-N concentration (0 g/L) and higher levels of SFAs (39.84% w/w lipids) in higher urea-N (3.0 g/L) grown culture.