(682f) Control of Chitin and Lipid Production in the Photosynthetic Diatom Cyclotella Sp By Co-Limitation of Silicon and Nitrate in Batch and Fed-Batch Cultivation

Controlling the metabolic pathways that microalgae have in a photobioreactor system to produce high value metabolites and biofuels is achieved by specific feeding strategies. Diatoms use silicic acid to create their silica cell wall, which is made during cell division. The deliberate addition of silicic acid controls cell division, henceforth the production of stress-induced metabolites, such is lipids. Lipid production in diatoms is a way of energy storage. Nitrate is used by diatoms to produce cellular material and nitrogen bearing metabolites, such as chitin. Cell biomass, lipid and chitin productivity by the marine diatom Cyclotella sp was studied in a batch and fed-batch cultivation under sequential co-limitation of two macronutrients, silicon and nitrogen in a bubble column photobioreactor. During the cultivation in the photobioreactors the cell suspension was assayed for cell number density, dissolved silicon and nitrate concentrations in the media. In specific points of the experiment, biomass samples were analyzed for lipid and chitin content. In batch experiments, the experiments were carried out by co-limitation of nutrients, with initial nitrate concentration varying from 0.5 to 5.0 mM at a constant dissolved silicon concentration of 0.8 mM. In fed-batch, experiments are carried in two stages, stage one for cell synchronization, and stage two for the controlled addition of nutrients. The initial silicon to nitrate concentration ratio (Si/N) was 0.7, ratio which is maintained along the fed-batch cultivation.  The rate of addition of nitrogen is the controlled sequentially, which is performed in a time range from 1 to 21 days.  In batch cultures, chitin is produced at silicon depletion within the first 48 hours of stationary phase. The range of chitin production is 2 to 10 mg/100 mL of culture. Lipid production is induced at silicon depletion, and ranged from 10-15 wt% lipid in dry biomass. In fed-batch experiments, results show that at low medium addition rates, the nitrogen that is incorporated into chitin can reach above 30% of the total nitrogen delivered, and at high medium addition rates, the nitrogen incorporated into chitin can be as low as 5% of the total nitrogen delivered to the system. Low nutrient medium addition rate shuts down lipid production, and is selective for chitin production, and cell production rate is limited by the addition of silicon and nitrogen. In high nutrient medium addition rate, lipid production is maintained during cell production, chitin production is maintained, but reduced, and the cell production rate is not limited by the addition of silicon and nitrogen. This concludes that the rate of addition of silicon and nitrogen (co-limitation) divert the production of chitin and lipids in Cyclotella sp, chitin production is linear during the stage two cultivation of the fed-batch experiments, and the nutrient flow rate changed the pattern of lipid and chitin product formation.