Improving light conversion efficiency in microalgae-cyanobacteria consortia through chromatic adaptation: experiments and modelling

Microalgae and cyanobacteria are promising photosynthetic microorganisms for the sustainable production of biomass and high-value compounds (Richmond and Hu, 2013). However, the economic feasibility of large-scale photobioreactor cultivation is still limited by the inefficient utilisation of the available light at industrial scale (Borella et al., 2023). One promising strategy to overcome this limitation is the cultivation of microalgae–cyanobacteria consortia exploiting the complementary light-harvesting capabilities of the two microorganisms. Building on the previous work of Borella et al. (2023), this thesis further investigates the behaviour of the cyanobacterium Tolypothrix tenuis, the green microalga Tetradesmus obliquus, and their consortium cultivated in continuous flat-panel photobioreactors under artificial white LED illumination. Continuous cultivation experiments were carried out by varying photon flux density and hydraulic residence time. Biomass concentration, biomass productivity and pigment composition were evaluated for both the monocultures and the consortium. In addition, the relative abundance of the two microorganisms within the consortium was estimated using a semi-quantitative approach based on allophycocyanin as a cyanobacterial marker. The results showed that the benefits of complementary chromatic adaptation were evident only under specific operating conditions and provided a more comprehensive understanding of the effects of the operating conditions on biomass growth, pigment composition and species distribution within the consortium. Alongside the experimental investigation, a mathematical model describing the growth of T. tenuis was developed by adapting a previously proposed growth model for T. obliquus based on unpublished data. The biomass-specific absorption coefficient experimentally determined was incorporated into the model together with absorption spectra corresponding to different light acclimation conditions. After calibration, the model showed good agreement with the experimental data over the investigated operating conditions. Overall, this work provides new experimental and modelling tools to support the optimisation of continuous photobioreactor cultivation and represents a further step towards the development of predictive models for microalgae–cyanobacteria consortia.