Substrate functionalization by cold plasma treatments as an alternative process to the cultivation of microalgae in biofilm: Application to Botryococcus

Microalgae are renowned for their diverse production of molecules, including biofuels. However, biotechnological processes aiming at producing these biomolecules have yet to achieve economic sustainability due to the high costs associated with downstream processing, which can make up to 80 % of the total production costs. Since microalgae immobilized on a flat surface are characterized by a higher productivity and an easier harvesting than bulk culture systems, flat cultures may present better economic viability. Nevertheless, immobilizing filamentous or colonial microalgae on a flat surface is challenging due to their inherent 3D development. In this study, we explored the effectiveness of a plasma-modified polyethylene terephthalate flat surface for improving the immobilization of the green freshwater colonial microalga Botryococcus protuberans, a promising taxon for biofuel production. Plasma treatments were found to alter the wettability and surface energy of polyethylene terephthalate substrates. Botryococcus adhesion was enhanced significantly on O₂ plasma-modified substrates compared to untreated substrates. The adhesion was strong enough to prevent colony development in the water column while allowing the development of a biofilm over one month, with minimal impact on their physiology.