(682a) Relationships Between CO2 Consumption and Co-Product Formation during Bubble-Column Photobioreactor Cultivation of the Diatom Cyclotella Under Light-Limited and Light-Saturated Growth Conditions

The photosynthetic biorefineries of the future must rely on microorganisms that can flexibly produce a diverse array of valued co-products in addition to liquid transportation fuels. Towards this end, the unique biosynthetic capacities of the marine diatom Cyclotella were harnessed to flexibly make N-acetyl glucosamine biopolymer (chitin) microfibers as well as C16-18 lipids for advanced biofuels. The light intensity delivered to the diatom cell is a critical variable for controlling the net biosynthesis rate and selectivity of these products. This study characterized the influence of mean light intensity on carbon dioxide (CO₂) consumption, and then correlated CO₂ consumption to the production of biomass, lipids, and chitin during the cultivation of the diatom Cyclotella under four different mean light intensities ranging from light-limited conditions (50 µmol/m²-sec) to light-saturated conditions  (300 µmol/m²-sec) in a bubble-column photobioreactor. The photobioreactor was equipped to deliver photosynthetically active radiation (PAR) to the diatom cell suspension at a stepped incident light intensity needed to maintain a given final mean light intensity, and was also instrumented to provide controlled CO₂ delivery in the inlet aeration gas as well as real-time measurement of CO₂ concentration in the outlet gas. During photobioreactor cultivation, the cell suspension was analyzed for cell number density, ash-free biomass, total lipid, chitin, and elemental (C,H,N) content. Integral analysis of real time CO₂ measurement combined with respiration analysis successfully predicted biomass carbon production with cultivation time. The final cell number density was controlled by silicon limitation, and was similar at all light intensities.  However, the CO₂ consumed for biomass and lipid production increased with increasing mean light intensity. Chitin production started after silicon depletion, and cells diverted up to about 20% of biomass carbon to the production of this metabolite during stationary phase under nitrogen-replete conditions. This study demonstrates the unique capability of the diatom Cyclotella to simultaneous produce two value-added metabolites, and also shows how real-time measurement of CO₂ consumption can be used to track carbon flow into these products.