Adaptive Laboratory Evolution of Scenedesmus obliquus for Increased Carbohydrate Content and Biomass Productivity

The economics of microalgal bioproduct commercialization would benefit from increased accumulation of energy storage compounds, such as carbohydrates and lipids, and increased biomass productivity. This thesis explores two adaptive laboratory evolution strategies for improving Scenedesmus obliquus: single UV-mutagenesis and low light cultivation at a high dilution rate to produce cultigens with greater carbohydrate content and iterative UV-mutagenesis and selection under outdoor conditions in a raceway pond at a high dilution rate to increase biomass productivity. Two cultigens were generated with the goal of increased carbohydrate content: K5 and K7. Both were mutagenized for 5 seconds and cultivated in 50-mL Klavins reactors. K5 was maintained at 95 μmol/m2-sec for 16 hours/day, and K7 was maintained at 250 μmol/m2-sec for 6 hours/day. When evaluating the two cultigens in triplicate 800-mL bubble columns against wild-type Scenedesmus obliquus, neither K5 nor K7 demonstrated an increase in carbohydrate content. To increase biomass productivity, Scenedesmus obliquus was iteratively UV-mutagenized (aliquots exposed for 5-60 seconds were combined) and cultivated in a 100-L, 0.5-m2 outdoor raceway pond fed with filtered reclaimed wastewater at a high dilution rate. Three rounds of mutagenesis and selection took place during the spring, summer, and winter months, with the MBE 509 and MBE 510 cultigens being produced after the summer and winter selection rounds, respectively. When evaluated in triplicate 800-mL bubble column reactors: MBE 509 (491 ± 42 mg/L-day, expressed as the mean of the time series ± SD) was 24% more productive than wild-type (397 ± 39 mg/L-day), and MBE 510 (443 ± 26 mg/L-day) was 13% more productive than wild-type (391 ± 9 mg/L-day). When evaluated in replicate 1,350-L, 4.5-m2 raceway ponds, MBE 509 (15.2 ± 2.2 g/m2-day) and MBE 510 (16.1 ± 1.8 g/m2-day) were 5% and 12% more productive than wild-type (14.5 ± 2.4 g/m2-day), respectively. Selection under low light and high dilution did not result in cultigens with higher carbohydrate content relative to wild-type Scenedesmus obliquus, but iterative mutagenesis and selection in an outdoor raceway pond did generate cultigens with higher biomass productivity in both indoor and outdoor environments