Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Sustainable saline microalgae co-cultivation for biofuel production: A critical review Tasneema Ishika a , Navid R. Moheimani a, ⁎ , Parisa A. Bahri b a Algae R & D Centre, School of Veterinary and Life Sciences, Murdoch University, Murdoch 6150, Western Australia, Australia b School of Engineering and Information Technology, Murdoch University, Murdoch 6150, Western Australia, Australia ARTICLE INFO Keywords: Bioenergy Nutrients Salinity Open ponds Productivity ABSTRACT Microalgae have gained increased attention as a viable, eco-friendly and alternative source of green bioenergy. To compete in the fuel market, saline microalgae cultivation for biofuel production would need to be economically sustainable and co-cultivation of saline microalgae using only saline water and recycled nutrient can potentially be the best solution to reduce the excessive use and prompt downsizing of natural resources like fresh water and fertilizers. This review provides a critical analysis on the selection of potential biofuel producing marine, halotolerant and halophilic microalgae. Here we proposed a microalgae co-cultivation strategy from seawater salinity (35ppt) to salt saturation (300ppt) with biofuel as the main output. We focused that adaptation of a co-cultivation strategy could reduce 95%, 74% and 51% of the overall nutrient waste compared to the monoculture of marine, halotolerant and halophilic microalgae. This paper also highlights a cultivation strategy using both mono and mixed culture over the period of increased saline condition and compares mass industrial- scale biofuel production from microalgae in three sites in Western Australia. 1. Introduction Fossil fuels (i.e. coal, petroleum and gas) are used to meet over 80% of our energy demand [1]. Further, it is estimated that the natural reserves of fossil fuel will be completely exhausted between 2069 and 2088 [2]. Given our dependence on these resources, there is a world- wide need for an environmentally and economically sustainable source of renewable energy. To meet the demands of the growing population, food and oil crops are being used as first generation renewable bioenergy sources [3]. However, due to rising issues like fresh water, arable land and low energy content, these crops became unsustainable as biofuel source [4–6]. Non-food feedstock (e.g. residues of crops and forest, wastes, dedicated feedstock and short rotation forests) has been used as second generations biofuel [7]; however, due to their low energy density and high consumption of water and nutrients, their economic sustainability and viability are questionable [8]. Phototrophic microalgae as third generation biofuel have much higher biomass and oil productivity (73 t of biomass ha -1 y -1 with 25−40% of oil content in open ponds) compared to first and second generations biofuel [9–11]. Furthermore, due to the possibility of nutrient recycling, microalgae mass cultivation require 10–70% less fertilizers, than sunflower, canola, jatropha and soybean [11–15]. Additionally, microalgae could potentially be grown in non-arable land using non-potable water (e.g., brackish, saline, hypersaline and waste water). Thus, the cultivation of microalgae for fuel production could end the issues related to ‘food versus fuel’ (i.e. dependency on fresh- water, agricultural land) [16]. Fertilizer is one of the most important limited non-renewable resources. Microalgae and conventional agricultural crops require large amounts of fertilizer to ensure higher productivity [11]. Nitrogen fertilizer represents up to 45% of the effective energy input of microalgae cultivation [17]. Both phosphorus and nitrogen fertilizers are finite and their reserves are currently depleting; for example, it is estimated that phosphate rock reserves will be fully depleted within 100 years [18,19]. Additionally, the on-going price of fertilizer reduces the sustainability of low cost biofuel production. Consequently, to produce cost effective biofuel and reduce the direct competition between microalgae and food crops for the fertilizers, the conservation and recycling of fertilizer is critical in microalgae cultivation [11]. In recent decades, fresh water scarcity has increased from 69% to 77% [20]. While over the last 50 years, water withdrawals have tripled [20]. Therefore, using fresh water for microalgal biofuel production is unrealistic in regions with limited fresh water resource such as Australia where 20–30% of waste water (approximately 300,000 ML year -1 ) is recycled every year (about 300,000 ML year -1 ) to meet fresh water demand [21]. So, the use of saline water to cultivate microalgae http://dx.doi.org/10.1016/j.rser.2017.04.110 Received 10 July 2016; Received in revised form 25 March 2017; Accepted 28 April 2017 ⁎ Corresponding author. E-mail address: n.moheimani@murdoch.edu.au (N.R. Moheimani). Renewable and Sustainable Energy Reviews 78 (2017) 356–368 1364-0321/ © 2017 Elsevier Ltd. All rights reserved. MARK