Nannochloropsis micro-algae: from cultivation to high-quality biodiesel: integrated bioprocess, lipid profiling, SCM...

Nannochloropsis micro-algae exhibit rapid growth, high lipid productivity, and robust marine adaptability, enabling sustainable biofuel production. This study critically examined the viability of different Nannochloropsis micro-algae species, namely, N. oceanica (IMET1), N. limnetica and N. granulata, in sustainable biodiesel development using advanced experimental and analytical techniques. The microalgal strains were cultured in a 20-L bubble column photobioreactor with a modified BG-11 growth medium under high nitrogen-deficient and elevated light intensity conditions to promote fatty acid storage as lipids. Furthermore, extraction was carried out in a bio-based automatic Soxhlet extractor system (model BFA-1S) using hexane solvent. The lipid profiles were analyzed using a Thermo Fisher Scientific Orbitrap Exploris GC 240 HRAM/ESI/MS spectrometer. A supercritical methanol (SCM) transesterification process was employed for biodiesel synthesis, and an FTIR technique was used for the analysis of the algae esters. The critical fuel properties of the biodiesel were determined by the ASTM D6751 and EN14214 standard protocols. N. oceanica exhibited the highest lipid productivity of 142.9 ± 3.42 mg per L per day and 57.5% oil content, followed closely by N. granulata with 128.4 ± 3.15 mg per L per day and 54.6% oil content. However, N. limnetica showed slightly lower values of 104.5 ± 2.48 mg per L per day and 45.3% oil content on a dry-weight basis. The substantially high methyl ester yields of 98.74%, 98.35% and 96.12% for N. oceanica, N. granulata and N. limnetica, respectively, validated the viability of the micro-algae species and the efficiency of the SCM transesterification process in sustainable biodiesel production. The Nannochloropsis biofuels satisfactorily met the ASTM D6751 and EN 14214 fuel quality standards. Finally, the N. oceanica micro-algae species (particularly the IMET1 strain) had an exceptionally high cetane number and calorific value of 57.63 and 41.85 MJ kg^-1, respectively. This facilitated its high-quality ignition characteristics and reduced engine fuel consumption rates.