Booth, Matthew; (2023) Development of a turbidostat photo-bioreactor to investigate the effects of process parameters on the physiology and growth of micro-algae. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

The deployment of micro-algae as a viable bioproduction platform for industrial scale manufacturing has been hampered by low process yields and complex downstream processing. Chlorella variabilis NC64A has recently gathered significant scientific attention due to its relationship with the Paramecium bursaria chloro-virus (PBCV1) which can simplify downstream processing for the bio-production of ethanol or hyaluronan. However, photoautotrophic process optimisation is far from trivial, in part due to the unique characteristics of light as a critical process parameter (CPP) and its complex interactions on the physiology and metabolism of photoautotrophic microalgae. In the present thesis, a formalised statistical Design of Experiments (DoE) approach was employed for the optimisation of biomass concentration during photoautotrophic growth in closed, controlled and artificially illuminated batch cultures. The process design space was explored in two sequential rounds; initially six CPPs were screened using a fractional factorial design before a higher resolution face-centred central composite design was used for further optimisation. In most batch systems, extracellular conditions are highly dynamic as cells continuously grow and proliferate. This constant state of change compounds any efforts to discern whether observed changes are due to an introduced perturbation or due to the cells’ response to the ever-changing culture environment. To this end, a turbidostat module was created for a batch photo-bioreactor (Algem Pro ®) to allow operation in a time invariant and highly controlled continuous fashion. The developed experimental set-up enabled the isolated study of the effect of two CPPs (light intensity and biomass concentration) on the specific growth rate. Furthermore, an optimisation problem was formulated to fit kinetic parameters from a nonlinear model to the experimentally observed specific growth rates of several turbidostatic cultures. Using the model, a dynamic light profile was designed for batch cultures for a given geometry; this profile increased the incident light intensity as a function of biomass concentration, ensuring that the maximum specific growth rate was maintained across the entire growth period. When compared against control light profiles (operated at a constant light intensity), the dynamic light profile lead to a 30% increase in biomass concentration, biomass productivity and photosynthetic efficiency. Thus highlighting the benefits of using dynamic light profiles over constant light profiles in artificially illuminated cultures.

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