Mixotrophic production of microalgae in pilot-scale photobioreactors: Practicability and process considerations Jean-Sébastien Deschênes ⁎, Alexandre Boudreau, Réjean Tremblay Université du Québec à Rimouski, 300 Allée des Ursulines, Rimouski, Québec G5L 3A1, Canada abstract article info Article history: Received 23 January 2015 Received in revised form 31 March 2015 Accepted 16 April 2015 Available online xxxx Keywords: Mixotrophic growth Microalgae Bacterial contamination Feeding strategy Pilot-scale photobioreactor The practicability of culturing microalgae in mixotrophic mode is evaluated for pilot-scale photobioreactor systems, as a general consideration for new process development: higher biomass production rates, production of (potentially new) high-value products from algae and the transformation (recovery) of other carbon sources are the typical advantages of this production mode. This study is conducted with Scenedesmus obliquus, a species known for its ability to assimilate a wide variety of carbon sources. Autotrophic experiments are first conducted to obtain the typical behavior (growth curve and nutrient consumption rates) of the alga in standard medium composition. Mixotrophic experiments then follow: aside from productivity aspects, results show that bacterial contaminations can remain an issue at this scale, even for closed systems such as photobioreactors. It is however shown possible to exploit some intrinsic dynamic properties of the algae to limit bacterial growth, with no sig- nificant impact on algal growth and biomass productivity. Anionic analyses on the mixotrophic cultures show that particular attention should be drawn on the phosphate and sulfate ions for further process optimization. © 2015 Elsevier B.V. All rights reserved. 1. Introduction An important bottleneck for the industrial advent of new bioprocesses using microalgae is tied to productivity [1,2]. Economically viable industrial productions of microalgae have yet only been realized for a limited number of (high-value) products and microalgae species [3,4] in open systems and photoautotrophic mode [2,3]. As a way to increase productivity, addition of an organic carbon source has often been shown effective [5,6]. However, this introduces a high risk of contamination by competing organisms (especially bacteria), thus re- quire such cultivation to take place in a more controlled environment (closed systems) such as a photobioreactor or fermenter. These systems allow a broader range of microalgal species to be cultured [4,7,8], thus enhancing the possibilities for new bioprocesses using microalgae: note that this is considered in a very general (non-limitative), for any product potentially obtained from algae (which may or may not include biofuels). Mixotrophic mode of cultivation for microalgae has received increased attention recently [9–12]. In this mode, a dissolved organic carbon (DOC) source is supplemented in addition to inorganic carbon (photosynthesis) for growth. Compared to strict heterotrophic growth (on a DOC source in the absence of light), mixotrophic growth still allows CO 2 capture, which is normally the primary interest for the use of algae in a bioprocess [12]. In comparison to the autotrophic case, the presence of the DOC source reduces the culture's dependency on the lighting conditions [3,9,10], thus allowing for larger photobioreactor vessels to be used, further supporting scale-up. In addition, for many algae, the mixotrophic biomass productivities are at least equal to those obtained in heterotrophy [13] or greater [3,12,14], as the metabol- ic pathways involved may operate in some kind of symbiotic effect [12]. Compared to strict autotrophy, productivities are often increased by a factor of 5 to 10 [9,13], making it a highly efficient means of producing microalgal biomass. Since photobioreactors allow a further 10-fold in- crease in productivity when compared to open (raceway) pond systems [3], comparable productivities can already be obtained using 100 times smaller volume photobioreactor. Thus, a 1.2 ML raceway system could be replaced with a 12,000 L photobioreactor, which is an attainable size for photobioreactors. Such reduction in size with similar productiv- ities could also make photobioreactors cost-competitive with those field applications, with a better control over the process. This paper addresses a first practical study for the mixotrophic pro- duction of microalgae in pilot-scale photobioreactors with particular focus on bacterial contaminations and anionic concentrations in the ex- tracellular medium. Note that no particular products are discussed in this paper (not even biofuels), such that the scope of this study is not limitative. Since the paper is strictly about these two process consider- ations, an artificial light source is used (to minimize fluctuations on the available light intensity through time and thus eliminate this as a factor of variation) and the selected (model) carbon source is glucose: this DOC source is easily assimilated by both the algae and bacteria, and is already known to significantly increase the biomass yield and productivity of microalgae cultures. The objectives of this paper are to evaluate the practicability of implementing mixotrophic growth in Algal Research 10 (2015) 80–86 ⁎ Corresponding author. E-mail address: jean-sebastien_deschenes@uqar.ca (J.-S. Deschênes). http://dx.doi.org/10.1016/j.algal.2015.04.015 2211-9264/© 2015 Elsevier B.V. All rights reserved. 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