Carotenoid biosynthesis and productivity in diatoms

Due to their versatility and modest cultivation requirements, microalgae are a promising potential source of sustainable fuel, chemicals, and food. At present, microalgal production at scale is not economically viable. A major hurdle to productivity is the inefficient use of light energy by dense microalgal cultures. Due to extensive photopigmentation, microalgae closest to the light source absorb more light than they can use, and wastefully dissipate the rest. As a result, light penetrance into the culture is steeply attenuated. Reducing light-harvesting or dissipation capacity of microalgal cells is a promising solution to uneven light distribution in mass cultures. Most efforts to do so have focused on chlorophytes, with some successes. Diatoms are a class of microalgae that is very promising in terms of productivity and has evolved light harvesting and photoprotective strategies that differ substantially from those utilized by chlorophytes. This dissertation explores the notion of improving diatom productivity through manipulating their light-harvesting and dissipation capabilities. Because microalgal performance in production conditions can differ substantially from what is observed in the laboratory, the responses of a wild-type production candidate diatom to simulated outdoor conditions are examined in Chapter 1. Substantial diel changes in hypothetical product yields were observed and discussed in terms of what variables need to be optimized to maximize productivity. Main light-harvesting and photoprotective carotenoid-derived photopigments were found to respond differently to chloroplast division and changes in irradiance, suggesting differential regulation. Chapter 2 examined carotenoid biosynthesis in diatoms, because diatom carotenoids play major light-harvesting and photoprotective roles. Targets for genetic manipulation were identified, transgenic lines with two distinct altered photopigmentation phenotypes were generated, and a model for how diatom carotenoid biosynthesis may be differentially regulated in response to chloroplast division and irradiance increase was developed. Chapter 3 focused on examining photosynthetic performance, growth, and productivity of two transgenic strains created in Chapter 2 and identified a strategy that may substantially improve diatom productivity. Overall, the dissertation substantially advances the understanding of diatom carotenoid biosynthesis, identifies strategies for improving light utilization efficiency in diatom cultures, and contributes to the understanding of practices to maximize the productivity of commercial microalgal cultivation