Combining multiple nutrient stresses and bicarbonate addition to promote lipid accumulation in the diatom RGd-1

Algal biofuels represent a renewable, potentially viable, solution to mitigate transportation fuel demands. A novel diatom strain, RGd-1, isolated from Yellowstone National Park, produces high concentrations of lipids that can be converted to biodiesel. To increase the cell concentration and determine optimal conditions for growth, RGd-1 was grown without added Si, in the presence of four Si concentrations within the soluble range (0.5-2mM), and one above the soluble range (2.5mM). Medium Si concentrations and intracellular triacylglycerol (TAG) content were monitored daily by inductively coupled plasma mass spectrometry and Nile Red fluorescence, respectively (end-point TAG values were measured using gas chromatography). Si depletion with or without combined nitrate (NO₃^-) limitation was shown to induce TAG accumulation. Additionally, the effects of sodium bicarbonate (NaHCO₃) supplementation were examined on cultures grown using two NO₃^- concentrations (2.94 and 1mM NO₃^-), which also resulted in increased TAG accumulation. It was determined that utilizing a combination of two independent physiological stresses and HCO₃^- supplementation resulted in the highest total and per cell TAG accumulation.