Use of bicarbonate salts in algal growth for enhancement of lipid content

Abstract

Production of biofuel from microalgae has thus far been cost prohibitive due in part to expenses associated with providing the necessary nutritional requirements for growth of the algal culture. In particular, inorganic carbon must be supplied in higher concentrations than available atmospherically to achieve high density cultures necessary for biofuel production strategies. Cost of algal biomass, as a feedstock, will be the limiting factor to the realization of algal biofuels moving forward. Prior research has demonstrated bicarbonate to enhance lipid content in select algal cultures grown under stress conditions, such as nitrogen depletion. This phenomenon has come to be known as bicarbonate-induced lipid accumulation, colloquially known as 'bicarbonate triggering', and has unrealized potential in executing economical and productive algal biofuel. Still, this method has only been demonstrated in select microalgal species and relatively little metabolic information is available regarding its use. Here, two species were investigated with the use of bicarbonate salts for algal growth and lipid accumulation. Nannochloropsis gaditana is a marine microalga which produces relatively high lipid content under nutrient stressed conditions, and has not been thoroughly studied under use of bicarbonate. This organism was studied under bicarbonate supplementation in batch photobioreactor systems. Chlorella vulgaris is a fresh water green alga which has received attention as a biofuel candidate, due to high growth rates and lipid content. This organism was investigated under bicarbonate supplementation during nitrogen depletion with the use of high resolution-magic angle spinning (HR-MAS) nuclear magnetic resonance (NMR) spectroscopy over 38 hours with a 14:10 diel cycle. N. gaditana showed best growth rates under pH controlled growth strategies during nitrogen replete conditions, and bicarbonate was seen to increase lipid content following nitrogen depletion when paired with this growth strategy. However, N. gaditana may not be an ideal candidate for biofuel production as it has relatively low growth rates compared to other industrially relevant organisms, and demonstrated low productivity in batch systems. Metabolite investigation in C. vulgaris revealed large incorporation of inorganic carbon from bicarbonate amendments into biomass, specifically monitored as increases to the biological sucrose pool and subsequent synthesis of fatty acids as carbon storage compounds.