Improving the cultivation efficiency of microalgae for biofuels: spanning biofilm and bioprocessing scales
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Date
2014-10-15
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Johns Hopkins University
Abstract
Microalgae are versatile photosynthetic production platforms for recombinant proteins, nutritional commodities, and biofuels. The demand for liquid transportation fuels has recently propelled these organisms to the forefront of the bioenergy stage. While algal biomass is poised to become a widely recognized feedstock, existing bioprocessing challenges continue to limit its full production potential at large scales. This dissertation offers an analysis of the engineering approaches that have previously guided algae farm development in order to inform future process design at the scales necessary for biofuel applications. Ultimately, the findings confront conventional microalgal cultivation by exploring novel methodologies to increase oil yield, integrate alternative sources of nutrients, and minimize water usage.
In order to quantify resource demands for algal biofuels using conventional raceway ponds, technoeconomic modeling identified carbon dioxide (CO2) sourcing logistics, lipid content, and water handling as the major factors affecting cost of production. To address CO2 feedstock availability, integrated algae production using biogenic CO2 from an ethanol biorefinery was assessed. Together, these models predicted the cost of unrefined algal oil to be $10–40/gal at baseline biomass productivities of 15–20 g/m2/d and 25% total extractable lipids. The technical and economic obstacles associated with both production scenarios motivated experimental work to increase cellular lipid content with strategic nutrient supplementation and explore the potential for adherent algal growth systems.
By investigating the differential impact of light and organic carbon on microalgal lipid composition, a species selection pipeline focused on strains with the potential to accumulate triacylglycerol (TAG) as a biofuel precursor. From a collection of over thirty phylogenetically distinct Chlorella strains, C. sorokiniana UTEX 1230 was chosen based on its robust autotrophic growth and enrichment with TAG during heterotrophy— as much as 90% of the total lipid fraction. However, its lipid productivity was suppressed under mixotrophic conditions with light and glucose. This constraint on carbon flux offered insights into the metabolic regulation of lipid biosynthesis in this organism and prompted further analysis of two-stage bioprocessing to maximize TAG yield.
Finally, to improve the volumetric efficiency of algal cultivation, photosynthetic biofilms were examined as an alternative to suspension culture. The formation of algal monolayers on membranes was improved 8-fold by employing protein-mediated cellular attachments. To encourage multi-layer outgrowth, natural microbial communities containing filamentous cyanobacteria were studied as potential scaffolds for biofilm architectures. The spatial and temporal dynamics of unicellular algal growth and migration within these biofilm microenvironments were characterized using molecular genetic tools and fluorescent microscopy to resolve cellular-level detail. Ultimately, immobilized biofilms inhabited by C. sorokiniana UTEX 1230 reached a maximal thickness of 150 µm and achieved areal productivities comparable to raceway pond production.
Collectively, the results in this thesis address bioprocessing bottlenecks associated with algal cultivation by increasing oil content and reducing water requirements. By investigating the influence of nutrient regimes on algal lipid composition and interrogating the fate of single algal cells within biofilms, this dissertation connects biochemical and physical factors to the economics and environmental impact of algal biofuels.
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Keywords
algae, biofuels, bioprocessing, technoeconomics, Chlorella, lipids, heterotrophy, biofilms, immobilization