Optimisation of lipid production, harvesting processes and the mass culture of isochrysis galbana U4 for biodiesel production
Due to the numerous disadvantages associated with the use of fossil fuels, focus has been drawn on the environmentally friendly, renewable and carbon-neutral alternative, algalbased biofuels. Many microalgal species have been studied due to their ability to produce significant lipid yields which may be converted to biodiesel. In the present study three microalgal species were screened and a model organism that produced maximal lipid yields, had the greatest lipid productivity and showed potential to be used on a large scale basis, was selected. The selected species was identified, using both morphological and molecular methods, as Isochrysis galbana U4. Nitrogen (N) limitation and depletion studies showed that an internal N reservoir determines cell growth and eventual lipid accumulation in I. galbana when the external N reserves are deplete. Intracellular N depletion was associated with a decrease in the pyrenoid size and chlorophyll content, a breakdown of the chloroplast and the production of large lipid bodies which is advantageous in terms of lipid sequestration for biodiesel production. Cost reduction approaches for the mass culture of I. galbana were investigated. Factors that were proven to reduce costs, without altering the final lipid yield, included the use of urea as a N source and the supply of lower phosphorus (P) levels since this species is capable of growing optimally with as little as 0.25 ppm P. Furthermore, I. galbana cells demonstrated spontaneous flocculating abilities when cultured for prolonged periods. This is advantageous in the cost reductions of downstream harvesting processes. Both, 7 L and 16 L photobioreactors (PBR) were tested. Culture upscale resulted in the concomitant decrease in algal growth rate which was attributed to the limitations on the carbon dioxide and light supply in scaled up systems. Hence, it is suggested that multiple smaller units be used in an industrial setup. Overall, I. galbana is a promising candidate for biodiesel production, due to its ability to produce large amounts of lipid, its elevated growth rates and low P demand. The use of a two-phase PBR (The first phase being nutrient replete, promoting cell growth and division, and the second phase nutrient deplete, promoting lipid production) for the mass culture of this species in industry is recommended.