Investigation of the colony size effect on light access and growth of the colonial microalga Botryococcus braunii in photobioreactor
Résumé
The colonial microalga Botryococcus braunii is recognized for its original hydrocarbon content, making it a promising candidate for the production of biofuels and other bio-based products. However, challenges related to its slow photosynthetic growth rate hinder economically viable processes and large-scale applications. This study investigates the effect of colony size on light absorption and growth efficiency in B. braunii BOT-22. To examine this, large colonies were disrupted into smaller ones using a high-pressure homogenizer, resulting in a tenfold increase in the mass absorption cross-section (from 8.0 to 79 m2·kg−1), highlighting how colony size significantly affects light absorption. Next, cultivation in a photobioreactor allowed the calculation of the Mean Rate of Photon Absorption (MRPA), supporting a relationship between the colonial behavior of B. braunii and a photoprotective strategy. Higher incident light flux was associated with larger colonies, which is likely a mechanism to manage the light stress on embedded cells. Building on these results, medium-sized and larger colonies were successfully generated by applying different light conditions during pre-cultivation. Photobioreactor cultures of these medium-sized colonies (145 μm average diameter) showed 34 % higher biomass productivity (0.123 g·L−1·d−1 or 4.92 g·m2·d−1) and 74 % higher hydrocarbon productivity (0.033 g·L−1·d−1 or 1.30 g·m2·d−1) compared to the larger colonies (434 μm). These findings highlight the important role of optimizing colony size for better light absorption and productivity, providing valuable insights for improving photobioreactor performance in B. braunii cultivation.
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