Physiological Adaptation of Unicellular Microalgae To Environmental Stress and Their Potential For Biofuel Production

The crisis of producing enough liquid fuels in an environmentally friendly way is expected to increase in the future. At the moment most liquid fuels are produced from crude oil, which is a fossil fuel contributing to global warming. Microalgae offer a high potential for the production of biodiesel which is a reliable, non-toxic, and biocompatible liquid fuel that can replace the existing unsustainable fossil fuel sources of liquid fuels. In this study, Nannochloropsis salina was examined for its ability to produce neutral lipid, the precursor of biodiesel. Nannochloropsis salina was obtained from the Culture Centre of Algae and Protozoa, Oban, Scotland, UK and its identity was confirmed using molecular techniques (18S rDNA sequencing) and light microscopy. Two growth media (defined f/2 medium and AMCONA medium) were used to determine which medium is the most appropriate based on the highest growth potential. Effects of stress conditions, which included high salinity up to 1 M NaCl, pH values from pH 5 to pH10 and nitrogen depletion conditions (75%, 50%, 25% NaNO3 as nitrogen source and nitrogen free) on neutral lipid accumulation in N. salina cells were investigated using Nile red technique to confirm their suitability for biofuel (biodiesel) production. It was found that 0.6 M NaCl, high pH values (pH10) and nitrogen free mediumtended to induce higher levels of neutral lipid in the algal cells (74.65%, 76.89% and 72.12% of total dry weight respectively). Total lipid content in N. salina cells grown under different treatments was also investigated using the gravimetric technique. Nitrogen free medium induces more total lipid content in cells. Fatty acid methyl ester (FAME) profiles of N. salina cells were analyzed by gas chromatography-mass spectrometry (GC-MS) after direct transesterification. Cells of N. salina grown in 0.6 M NaCl showed the highest levels of fatty acids which were principally C16 and C18 - ideal for biodiesel production. Lipid content in N. salina
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cells was also examined using 1H-NMR and the results confirmed that the fatty acid content was much higher in the 0.6 M NaCl grown cells. Further analysis of neutral lipid in N. salina cells grown under different conditions was established using a flow cytometry technique.