Abstract
This work presents a kinetic model that quantifies the effects of the algae’s circadian rhythms and natural light- dark cycle on algal growth in photobioreactors. Such a model is necessitated by the fact that the classical Monod growth model applies to algal growth obtained under continuous exposure to light. However, any large scale production of algae under natural light in photobioreactors or raceway ponds subjects the algae to a day-night cycle, switching on the algae’s natural circadian rhythm through upregulation of certain genes. This circadian rhythm results in a biphasic algal growth pattern during alternating light-dark cycle – a phenomenon that cannot be quantified by the classical Monod growth model. Our new growth model, capable of quantifying the algae’s circadian rhythm in the presence of day-night photo-cycles, lends itself to both analytical and numerical solutions, both of which are obtained and compared in this study. Our growth kinetic model has been validated by comparing with experimental data on the effect of light-dark photoperiods on the growth of the microalgae Chlorella sorokiniana in phototrophic and mixotrophic culture media in air-sparged bubble- column photobioreactors. Our kinetic model shows that the growth during the light dependent phase can be studied as a biomass concentration dependent negative sigmoid function, whereas the dark phases shows a constant decrease in the biomass concentration which has been termed as 'night biomass loss'.
