Trends and novel strategies for enhancing lipid accumulation and quality in microalgae Poonam Singh 1 , Sheena Kumari, Abhishek Guldhe, Rohit Misra, Ismail Rawat, Faizal Bux n Institute for Water and Wastewater Technology, Durban University of Technology, P.O. Box 1334, Durban 4000, South Africa article info Article history: Received 4 August 2015 Received in revised form 29 October 2015 Accepted 2 November 2015 Keywords: Microalgae Biodiesel Lipid Genetic engineering Phytohormones Gene expression Metabolic engineering abstract In order to realize the potential of microalgal biodiesel there is a need for substantial impetus involving interventions to radically improve lipid yields upstream. Nutrient stress and alteration to cultivation conditions are commonly used lipid enhancement strategies in microalgae. The main bottleneck of applying conventional strategies is their scalability as some of these strategies incur additional cost and energy. Novel lipid enhancement strategies have emerged to research forefront to overcome these challenges. In this review, the latest trends in microalgal lipid enhancement strategies, possible solutions and future directions are critically discussed. Advanced strategies such as combined nutrient and culti- vation condition stress, microalgae–bacteria interactions, use of phytohormones EDTA and chemical additives, improving light conditions using LED, dyes and paints, and gene expression analysis are described. Molecular approaches such as metabolic and genetic engineering are emerging as the potential lipid enhancing strategies. Recent advancements in gene expression studies, genetic and metabolic engineering have shown promising results in enhancing lipid productivity in microalgae; however environmental risk and long term viability are still major challenges. & 2015 Elsevier Ltd. All rights reserved. Contents 1. Introduction .......................................................................................................... 2 2. Nutrient regime and cultivation conditions to enhance lipid accumulation ........................................................ 2 2.1. Effect of nutrient regimes on lipid accumulation....................................................................... 2 2.2. Effect of cultivation conditions on lipid accumulation................................................................... 4 3. Novel approaches for enhancing lipid accumulation in microalgae by altering culture conditions (nutritional and cultivation) .............. 4 3.1. Two-stage cultivation ............................................................................................ 5 3.2. Combined nutrients and abiotic stress ............................................................................... 5 3.3. Phytohormones ................................................................................................. 6 3.4. Supplementation of Ethylene-diamine-tetra-acetic acid (EDTA) ........................................................... 6 3.5. Co-cultivation of microalgae–bacteria................................................................................ 6 3.6. Co-cultivation of microalgae–yeast .................................................................................. 6 3.7. Improving light conditions using LED, dyes and paints .................................................................. 6 3.8. Chemical additives............................................................................................... 7 3.8.1. Azide .................................................................................................. 7 3.8.2. Brefeldin A (BFA) ......................................................................................... 7 3.8.3. Surfactants .............................................................................................. 7 4. Molecular approaches for enhancing lipid accumulation and quality............................................................. 7 4.1. Microalgal genome and tools for genetic transformations in microalgae .................................................... 7 4.2. Genetic engineering.............................................................................................. 8 4.2.1. Chloroplast engineering.................................................................................... 8 4.2.2. Nuclear engineering....................................................................................... 8 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/rser Renewable and Sustainable Energy Reviews http://dx.doi.org/10.1016/j.rser.2015.11.001 1364-0321/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: faizalb@dut.ac.za (F. Bux). 1 Tel.: þ27 31 373 2346; fax: þ27 31 373 2777. Renewable and Sustainable Energy Reviews 55 (2016) 1–16