(460e) A Dynamic Model Coupling Photoacclimation and Photoinhibition in Microalgae

Algal-derived biofuel has been considered a potential alternative source of renewable energy since the 1970s [1]. The claimed advantages of this approach are a high actual photosynthetic yield compared to field crops, an independence to agriculturally usable soil, and a possibility to avoid fresh water consumption [2]. These advantages could lead to large-scale production of algal biomass which is not in direct competition with food production. Moreover, microalgae can contribute to CO₂ mitigation due to their inherent consumption of CO₂ during photosynthesis, they can be coupled with wastewater treatment technologies [3], and they can produce high added-value products such as cosmetics, pharmaceuticals and nutraceuticals [4]. All these possibilities have put microalgae in a favorable position for their integration into biorefinery concepts. Nonetheless, many problems have yet to be overcome on the path to making large-scale production of microalgal biofuel feasible.

For over a decade, much research effort has been devoted to enhancing lipid production, from which biodiesel can be derived. Lipid productivity can be stimulated by a deprivation of nutrients (nitrogen or phosphorus) as well as by a modification of the light conditions. Two key processes are involved in the way light conditions and nutrient supply affect the photosynthetic yield, namely photoinhibition and photoacclimation. The former causes a loss of photosynthetic yield due to an excess of irradiance which damages some of the key proteins in the photosynthetic apparatus; the latter alters the rate of photosynthesis production via adaptation of the pigment composition to the light intensity. These two processes operate at different time scales and are tightly coupled. Photoinhibition occurs in a time scale of minutes, whereas photoacclimation acts over a time scale of days. In order to achieve an optimal microalgae productivity, understanding the processes of nutrient assimilation, photoinhibition and photoacclimation, as well as their interactions, is therefore paramount. A number of mathematical models are available that account for photoacclimation in the slow time scale [5,6], yet they neglect the photoinhibition processes. On the other hand, models have also been proposed which describe photoinhibition in the fast time scale [7], but they do not account for photoacclimation.

It is precisely the objective of this paper to develop a dynamic model of microalgae growth that couples photoinhibition and photoacclimation with carbon and nitrogen uptake. Nutrient assimilation is described by the well accepted and validated Droop model [8].  Photoinhibition is described by a model proposed by Han [7], originating in the work of Eilers and Peeters [9] who introduced the concept of photosynthetic factories (or photosynthetic units). Photoacclimation effects are incorporated in the expressions of photosynthesis rate and pigment synthesis rate using empirical relations [10]. This last part constitutes the main novelty of the proposed model, namely the expression of the effective cross section and the number of photosynthetic units (which are parameters in the Han model) as functions of the chlorophyll content. We investigate the properties of the model under quasi steady-state conditions, and we also validate the proposed model against several experimental data sets from the literature. Finally, we discuss how the model can provide new insights into the mechanisms underlying photoacclimation..

References:

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