Diatom biofuels : optimizing nutrient requirements for growth and lipid accumulation in YNP isolate RGd-1
Moll, Karen Margaret
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The world's crude oil supply is decreasing at an alarming rate and no longer represents a long-term solution to meet energy needs. Development of renewable energy sources is required to meet transport fuel demands. Algal biofuels represent a potentially viable option. Diatom strain, RGd-1, isolated from Yellowstone National Park, produces high concentrations of lipids that can be used for biodiesel production. To increase cell numbers, RGd-1 was grown in six silica concentrations: without added silica, four silica concentrations within the soluble range (0.5-2mM), and one just above the soluble range (2.5 mM). Increasing the silica concentration resulted in an increase in total cell numbers and dry cell weight (DCW) with R ²=0.965. Silica depletion was verified by inductively coupled plasma mass spectrometry (ICP-MS). When grown in higher silica concentrations the medium reached a higher pH, which remained elevated. Nile Red fluorescence can be used as measurement of triacylglycerol (TAG). Once silica was depleted, Nile Red fluorescence increased. Unlike green algae and other diatoms, nitrate was never depleted when using the standard Bolds Basal Medium concentration (2.94 mM). RGd-1 never depleted nitrate from the growth medium and utilized only 1/3 of the original nitrate concentration (1 mM) by the time cells reached stationary phase. Therefore, the nitrate concentration was decreased to 1mM to induce a dual nitrate and silica stress. To increase the lipid content further, sodium bicarbonate was added to cells grown with each nitrate concentration (2.94 and 1 mM NO ₃-). Coupling nitrate limitation with sodium bicarbonate addition resulted in higher Nile Red fluorescence. RGd-1 fatty acids were primarily observed as C16:0, C16:1, C18:1-3 and C20:5, averaging at approximately 35, 30, 16 and 10%, respectively of the total lipid content. With exception of cells grown without added silica, the percent lipid content was approximately the same (30-40% (w/w) TAG (Triacylglycerol) and 70-80% (w/w) fatty acid methyl ester (FAME) grown under all conditions within the soluble range. However, when factoring in the dry cell weight from each system, it was observed that the TAG and FAME yields increased with silica concentration when normalized to DCW.