High Cell Density Cultivation of Microalgae in Novel Thin-Layer Algal Reactor

Abstract

Microalgae are unicellular or simple multicellular photosynthetic microorganisms of rapid growth rate and high photosynthesis efficiency. They have been established as efficient cellular photoreactors with diverse applications in biomanufacturing and environment engineering. However, high algal farming costs have so far impeded large-scale growth of algal biomass and cell density culture (HCDC) of microalgae has shown great potential in reducing algal farming costs. This thesis presents an in-depth study on the HCDC of microalgae using a novel horizontal thin-layer algal reactor with green alga Neochloris oleoabundans as the model species. Systematic analyses on the effects of culture medium composition including nitrogen (N) and dissolved inorganic carbon (DIC) concentration as well as cultivation conditions, e.g., culture pH, light intensity, culture thickness, and intermittent mixing, were conducted. Biomass concentration of up to 35 g/L and areal productivities of up to 16.32 g/m²/day were achieved. About 98% savings in stirring energy was achieved by adopting a 10-min on and 2-hour off intermittent mixing strategy while obtaining algal growth kinetics similar to that of continuous mixing. It was demonstrated that an incident light intensity of 750 μmol photons/m²/s and DIC of 80 mM or above were inhibitive to cell division while promoting enlargement of individual cells. A simple equation was proposed and validated to correlate the extinction coefficient with the biomass concentration, allowing more accurate modelling of the light attenuation of algal culture using the Beer-Lambert equation. It was also demonstrated that, when the experimental kinetic data were generated using the correctly, the Monod equation, which has been widely applied to modelling the growth kinetics of heterotrophic microorganism, could be used to model the photoautotrophic growth of microalgae with satisfactory accuracy, resolving a long-standing challenge in this field. It was also demonstrated that the conversion factor in the spectrometric determination of biomass concentration, which has been wide applied to suspended cultures of microbial, plant, and animal cells, was dependent on cell size and cell composition and should therefore be experimentally determined carefully.