Improvement of the potential of Dunaliella tertiolecta as a source of biodiesel by auxin treatment coupled to salt stress Hicham El Arroussi, Redouane Benhima, Iman Bennis, Najib El Mernissi, Imane Wahby * Moroccan Foundation for Advanced Science, Innovation and Research, Green Biotechnology Center, Rabat Design Center, Rue Mohamed Al Jazouli, Madinat Al Irfane, Rabat 10 100, Morocco article info Article history: Received 5 March 2014 Accepted 2 December 2014 Available online Keywords: Microalgae Dunaliella tertiolecta Lipid Biodiesel Renewable energy abstract Increased world demand for energy and decrease of petroleum reserves have focused worldwide efforts to develop new and renewable energy sources. Liquid microalgal biofuels such as biodiesel represent a sustainable alternative to fossil energy besides being environmentally friendly. However, optimization of culture conditions to maximize productivity (biomass and lipid) is still a great challenge for a commercial biodiesel production. This is due to the difference between optimal culture conditions required for biomass and lipid accumulation. The current study proposes a two-phase method allowing both biomass and lipid increase in Dunaliella tertiolecta. The used this method consists of a first phase when microalgae were cultivated in presence of 2,4-Dichlorophenoxyacetic acid (2,4-D) to enhance algal division and growth. Later, at the end of log-phase, culture conditions were modified by adding NaCl to suddenly increase the salinity from 0.5 M to 2 M. Results showed that combination of auxin with salt stress resulted in a considerable improvement both in growth and lipid production in D. tertiolecta. During the first phase auxin addition improved biomass accumulation by 40% and salt stress in the second phase led to lipid increase from 24% to 70% with the optimal concentrations combination of auxin and NaCl. © 2014 Published by Elsevier Ltd. 1. Introduction Use of microalgae as feed stock for biofuel production is increasingly being considered as renewable alternative to fossil energy [1e3]. This interest toward microalgae is principally due to its: 1) fast growth comparing with oleaginous plants used for first generation biofuel's production, 2) use of saline waters and non-arable lands for culture, 3) lack of competition with food security concerns, in comparison with first generation biofuels and 4) potential to modify metabolism toward a higher lipid production through physical or chemical stress. Under optimal growth condi- tions, intracellular lipid contents can rise to 80% of cell dry weight making microalgae an attractive source for biodiesel production. Therefore, microalgae seem to be the only renewable biofuel source capable to meet the global demand for transport fuels [1]. Large scale culture of microalgae can be carried out in closed systems (bioreactors) or in open ponds like raceways. The latter are mostly used because of their lower construction and operational cost. Nevertheless, the major problem confronted in open pond systems is culture contamination that can cause serious loss in production [4,5]. For this reason, microalgae producers usually prefer work under extreme conditions (especially high pH and salinity) to limit contamination in these systems. This fact implies use of microalgae tolerating such conditions for example strains belonging to Duna- liella, Haematococcus and Spirulina. Besides limiting contamination, high salinity or nutrient deficient culture media is known to in- crease the lipid content mainly triacylglycerols (TAG) in various microalgae species [6e8]. Triacylglycerols are preferred over other lipids like phospholipids for biodiesel production since phosphate hinders the transesterification process and consequently decreases the biodiesel yield [9]. Dunaliella tertiolecta is euryhaline unicellular green algae considered as potential candidate for biodiesel production [1,10e12]. This is mainly due to its capacity to respond to external stimuli (salt stress, nitrogen depletion, etc.) by increasing its lipid content [7,13,14]. Nevertheless, use of D. tertiolecta for biodiesel production is hindered by their slow growth rates. Thereby, for an economic exploitation of D. tertiolecta for biodiesel production two issues have to be optimized: growth rate and lipid content. Un- fortunately, only few works reported improvement of lipid content * Corresponding author. Tel.: þ212 657 12 40 87; fax: þ212 530 27 58 28. E-mail address: i.wahby@mascir.com (I. Wahby). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene http://dx.doi.org/10.1016/j.renene.2014.12.010 0960-1481/© 2014 Published by Elsevier Ltd. Renewable Energy 77 (2015) 15e19