Bioenergetics of growth and lipid production in Chlamydomonas reinhardtii Kübra Küçük a , Rahul Tevatia b , Esra Sorgüven c , Yas ¸ ar Demirel b , Mustafa € Ozilgen a, * a Department of Food Engineering, Yeditepe University, Kayisdagi, 34755 Istanbul, Turkey b Department of Chemical and Biomolecular Engineering, University of Nebraska, Lincoln, NE 68588, United States c Department of Mechanical Engineering, Yeditepe University, Kayisdagi, 34755 Istanbul, Turkey article info Article history: Received 6 September 2014 Received in revised form 10 February 2015 Accepted 16 February 2015 Available online xxx Keywords: Chlamydomonas reinhardtii Cumulative degree of perfection Exergetic efficiency Flagella work Lipid production Photosynthesis abstract The study of thermodynamic aspects of the lipid, e.g., raw material for biodiesel, production in micro- algae is important, as the non-lipid producing biological activities of the algal cultivation consume part of the solar energy captured during photosynthesis in expense of the exergetic efficiency of the lipid production process. The cultivation of Chlamydomonas reinhardtii (a unicellular biflagellate fresh-water microalga) is modeled as a three-step chemical mechanism representing growth, respiration, and lipid production. Further, the comprehensive thermodynamic analysis of these mechanisms is presented. The cumulative degree of perfection of the cellular proliferation, after excluding the lipid synthesis, fluctuates with no trend around 0.52 ± 0.19. The exergy analysis has indicated that C. reinhardtii prefers to maxi- mize the lipid production when it is difficult to generate new cells. Under batch production of algal biomass, the highest heat and exergy loss per unit biomass production are accountable under the most favorable biological growth conditions, whereas the highest exergetic efficiency of the lipid production accounted under the least favorable growth conditions, which is in line with the previous studies. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The energy management in a living cell is called bioenergetics [1]. Bioenergetics, when combined with the fundamental principles of thermodynamics proved to be a highly useful tool for the analysis of the biological systems [2]. Lipids are perfect chemicals to store internal energy in their interatomic high-energy bonds for future use. These high energy bonds serve as the energy reserve for or- ganisms [3,4], as well as for engines in form of biodiesel [5]. Photosynthetic microalgae proliferate under the light, while consuming carbon dioxide and other nutrients necessary for maintenance [6e9]. Their large-scale cultivation may play an important role in reducing the atmospheric CO 2 concentration [5]. Further, the use of oleaginous algae may also stop the use of the food oils for biofuel production [5]. Microalgae promises for meeting the global renewable biodiesel demand for transport fuels [5]. Since, this industry is developing rapidly; there is continuous research on identification of thousands of different oleagineous algal strains, cultivation and harvest methods, wide range of algal products, and related conversion technologies. The non-lipid pro- ducing biological activities of the algal cultivation consume part of the solar energy captured with photosynthesis, in expense of the exergetic efficiency of the lipid production process. Hence, bio- energetics appears to be highly important concept. In the last decade, numerous studies are published regarding the exergy analysis of the biological systems [5,10e19], especially for the assessment of the efficiency of the exergy utilization via metabolic processes, and to relate kinetic parameters with ther- modynamic functions [20]. The living cells are far-from- equilibrium systems; they generate entropy, but also self- organize themselves to minimize it [21]. Eco-exergy is referred to as the work capacity possessed in the ecological network of or- ganisms due to the information embodied in their genome. The exergy of the eco-system is calculated relative to a ground system at the thermodynamic equilibrium where there are no gradients and all components are inorganic at their highest possible oxidation state (Fig. 1). With the exergy available the ecosystem moves as far away from thermodynamic equilibrium as possible [22]. * Corresponding author. E-mail address: mozilgen@yeditepe.edu.tr (M. € Ozilgen). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy http://dx.doi.org/10.1016/j.energy.2015.02.054 0360-5442/© 2015 Elsevier Ltd. All rights reserved. Energy xxx (2015) 1e8 Please cite this article in press as: Küçük K, et al., Bioenergetics of growth and lipid production in Chlamydomonas reinhardtii, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.02.054