Effect of N:P ratio on growth and chemical composition of Nannochloropsis oculata and Tisochrysis lutea Nadiah W. Rasdi & Jian G. Qin Received: 26 August 2014 /Revised and accepted: 2 December 2014 # Springer Science+Business Media Dordrecht 2014 Abstract The understanding of how nitrogen (N) to phos- phorus (P) ratios regulate growth and chemical composition of algae is important to control the nutritional value of microalgae for industrial application. This study compared the impacts of N:P ratio manipulations on the growth, ele- ments, lipid, fatty acids and protein contents of Tisochrysis lutea and Nannochloropsis oculata. F/2 medium was used as the basal formula to obtained six N:P ratios of 5:1, 10:1, 20:1, 30:1, 60:1 and 120:1 and tested on the algae species in triplicate. Growth rate was similar in both algal species across all N:P ratios, and the carbon content in T. lutea was higher than in N. oculata. However, the carbon contents were high in the N:P ratios of 5:1 and 120:1 and low from 10:1 to 60:1 N:P ratios for both T. lutea and N. oculata. There were no signif- icant differences in cellular N and P, but the protein contents depended on algae species and were significantly affected by N:P ratios. The N:P ratio of 20:1 favoured algal growth and protein content, while the N:P ratio of 120:1 reduced algal growth and protein synthesis but increased lipid in both algae. The 20:1 N:P ratio favoured eicosapentaenoic acid (EPA) production in N. oculata and the 30:1 N:P ratio favours docosahexaenoic acid (DHA) production in T. lutea. This study indicates that N:P ratio manipulation is an effective strategy to change biochemical composition in algae and N or P limitation tends to lower polyunsaturated fatty acids (PUFA) contents in algae. Keywords Algae . Phosphorus . Nitrogen . Protein . Lipid . N:P ratio Introduction Microalgae are photosynthetic organisms that constitute the first primary link of an aquatic food chain (Marchetti et al. 2012). The products of microalgae have various applications, particularly in pharmaceuticals, cosmetics, biofuel production, and aquaculture (Pulz and Gross 2004; Rosenberg et al. 2008; Borowitzka 2013). In nature, microalgae are the primary food for mollusks, zooplankton and crustacean larvae (Borowitzka 1997; Brown and Hohmann 2002; Duerr et al. 1998). Among various forms of alternative diets, such as algal paste (McCausland et al. 1999; Robert et al. 2001), yeast (Nell 2002), bacteria (Douillet and Langdon 1994) or even lipid compounds such as oil emulsions (Coutteau et al. 1996; Knauer and Southgate 1997), live microalgae remain essential to the production of larval fish and juvenile bivalves in cap- tivity (Robert and Trintignac 1997). As live food, the nutritional compositions, particularly polyunsaturated fatty acids 20:5(n-3) (eicosapentaenoic acid, EPA) and 22:6(n-3) (docosahexaenoic acid, DHA) in algae are critically important because they can affect the growth and reproduction of aquatic animals (Jonasdottir et al. 2009; Mueller-Navarra et al. 2000). However, the amount of EPA and DHA in algae differs greatly among algae species and environmental conditions. For instance, the EPA content is 4.8 mg g -1 in Tetraselmis sp. but is 23.4 mg g -1 in Nannochlopsis sp., while the DHA content is 0.2 mg g -1 in Tetraselmis sp. but is 15.8 mg g -1 in Isochrysis sp. (Patil et al. 2007). Marine microalgae such as Isochrysis sp. and Nannochloropsis sp. have received increasing attention as live food for aquatic animals because of their high contents of DHA and EPA (C-Pa and Lin 2001; Patil et al. 2007) which are the essential fatty acids in the diet of marine animals for growth, health and reproduction. Nutrient availability in the environment can regulate the growth and biochemical composition of algae (Qin and Culver N. W. Rasdi : J. G. Qin (*) School of Biological Sciences, Flinders University, GPO Box 2100, Adelaide 5001, Australia e-mail: Jian.Qin@flinders.edu.au J Appl Phycol DOI 10.1007/s10811-014-0495-z