Eukaryotic marine microalgae like Dunaliella spp. have great potential as a feedstock for liquid transportation fuels because they grow fast and can accumulate high levels of triacylgycerides with little need for fresh water or land. Their growth rates vary between species and are dependent on environmental conditions. The cell cycle, starch and triacylglycerol accumulation are controlled by the diurnal light:dark cycle. Storage compounds like starch and triacylglycerol accumulate in the light when CO2 fixation rates exceed the need of assimilated carbon and energy for cell maintenance and division during the dark phase. To delineate environmental effects, we analyzed cell division rates, metabolism and transcriptional regulation in Dunaliella viridis in response to changes in light duration and growth temperatures. Its rate of cell division was increased under continuous light conditions, while a shift in temperature from 25°C to 35°C did not significantly affect the cell division rate, but increased the triacylglycerol content per cell several-fold under continuous light. The amount of saturated fatty acids in triacylglycerol fraction was more responsive to an increase in temperature than to a change in the light regime. Detailed fatty acid profiles showed that Dunaliella viridis incorporated lauric acid (C12:0) into triacylglycerol after 24 hours under continuous light. Transcriptome analysis identified potential regulators involved in the light and temperature-induced lipid accumulation in Dunaliella viridis.
Microalgae are a promising biofuels feedstock, theoretically yielding concentrations of triacylglycerides (TAGs) per unit area that are far higher than traditional feedstocks due to their rapid growth. Dunaliella is particularly advantageous as a feedstock because it is currently commercially mass cultured, thrives in salt water, and has no cell wall. Fourteen strains of Dunaliella have been investigated for growth rates and lipid production in mass culture and tested for enhanced lipid production under a range of environmental stressors including salinity, pH, nitrogen and phosphorus limitation, and light regime. The nuclear genome has been sequenced for four of these strains, with the objective of increasing carbon flux through genetic engineering. Electroflocculation followed by osmotic membrane rupturing may be a very energy and cost efficient means of harvesting the lipid bodies from Dunaliella. A technically feasible and scalable thermo‐catalytic process to convert the lipids into replacements for liquid transportation fuels has been developed. The lipids were converted into long‐chain alkanes through continuous thermal hydrolysis followed by fed‐batch thermo‐catalytic decarboxylation. These alkanes can be reformed into renewable diesel via conventional catalytic hydrocarbon isomerization reactions to improve cold flow properties, if desired. © 2013 American Institute of Chemical Engineers Environ Prog, 32: 916–925, 2013
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