Beyond Suspension: Light and Growth Dynamics in Immobilised Algal Cultures
With growing commercial interest in algal biotechnology, immobilised cultures, such as biofilms and matrix encapsulation, have emerged as viable techniques with potential advantages for the commercial extraction of valuable metabolites and applications in functional photosynthetic materials. Nonetheless, little attention has been paid to the natural formation of algal aggregates within encapsulating matrices and the loss of cell motility in a biofilm setting. This thesis explores the physiological effect of mechanical confinement on unicellular flagellate Chlamydomonas reinhardtii, a model organism studied extensively in the laboratory. Serving as a study model of algal biofilm dynamics, the use of timelapse microscopy and single-cell tracking within microcolonies uncovered how external matrices and exogenous nutrient supply influence cell cycle and morphology. The findings revealed spatial heterogeneity among cells within colonies, providing insights into the effects of contact inhibition and micro-gradients of mass transfer. The radial propagation of ring-like oscillations, characterised by variations in parent cell size and chlorophyll autofluorescence emanating from the colony centre, indicated a complex spatio-temporal dynamic in the regulation of the cell cycle within a constrained cellular environment. Beyond the impact of confinement on a single-cell level within individual clusters, the collective environment and spatial distribution of algal aggregates within a gel matrix were also investigated. By microscopic investigation of the algal aggregates confined in the gels, a spatial gradient of their sizes depending on the distance from the gel-air interface was revealed. To better understand this distribution, we studied the interaction of light with such immobilised cultures. As the algal aggregates strongly affect the light distribution within the colony, a comparative study between the heterogeneous distribution of cell aggregates in a gel matrix and a flat, uniform biofilm was performed. The obtained findings not only allowed to better understand the properties of immobilised cultures in the context of biomass yield, but also provided a potential route for the enhancement of light harvesting efficiency, by simple incorporation of scattering particles within the hydrogel matrix. Finally, the last part of the thesis focuses on biomass characterisation, specifically the bulk analysis of macronutrients within algal biomass, and mainly on the lipid productivity. Macromolecular composition was assessed for cultures encapsulated within agar beads and for biofilms where cells were confined between membrane layers. These studies highlight the structural complexity and dynamic nature of cellular communities under confinement, where specific growth characteristics and population heterogeneity contribute to emergent cell functions.
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