Towards a low energy, stirless photobioreactor using photosynthetic motile microalgae

Abstract

Microalgae have a high potential for application in different industries including renewable energy production from light and atmospheric CO2. Biotechnologies should provide high production rates while using the mass and energy resources in a restrained manner. Often, mixing and gas provision are combined to minimize the energetic requirements of photobioreactors. Recent studies suggest that bioconvective patterns naturally developed in suspensions of motile microorganisms may provide biomixing. In this paper we report a study of the phototactic response of Chlamydomonas reinhardtii to induce biomixing in a small bioreactor at macro-scale. By exploiting the phototactic mechanism, we stimulated the microalgae to swim in opposite directions repeatedly in cycles of 40 min (20′ left, 20′ right) in the photobioreactor. The resulting Phototactic Oscillatory Motion (POM) lasted 7 days and maintained an amplitude close to the width of the bioreactor. Phototaxis allowed algae to remain in suspension without the use of any mechanical stirring system. We have demonstrated that light can be controlled to effectively provide mixing and ensure access to nutrients in form of an oscillatory motion at no extra energetic costs. This strategy can replace mechanical agitation in the manner of a ‘phototactic stirring’ under certain circumstances in pursuit of energetic sobriety.