Concerns over the reporting of inconsistent data on fatty acid composition for microalgae of the genus Nannochloropsis (Eustigmatophyceae)

Abstract

Letter to editor The current research focus on the development of nextgeneration alternative sources of biofuel from microalgal oil has led to a considerable increase in the number of studies on various oleaginous species. The studies have been examining species promising for high neutral lipid (mainly triacylglycerol; TAG) and biomass productivity under diverse cultivation conditions, with the aim to develop sustainable, commercially feasible, and economic processes for the production of biofuels (Rodolfi et al. 2009; Tredici 2010). Given the relative robustness and marine origin of microalgae of the genus Nannochloropsis and their high content of neutral lipids (Sukenik 1999; Sukenik et al. 2009; Rodolfi et al. 2009), a large number of projects have been devoted to these microalgae, including studies focusing on evaluating the effects of growth conditions on lipid productivity and fatty acid composition (Boussiba et al. 1987; Chini Zittelli et al. 1999; Chiu et al. 2009; Hodgson et al. 1991; Renaud et al. 1991; Renaud and Parry 1994; Richmond et al. 2003; Sukenik 1999; Sukenik et al. 2009). Nannochloropsis species are also a valuable source of the ω3 long-chain polyunsaturated fatty acid, eicosapentaenoic acid (20:5n-3, EPA) for aquaculture and even for human nutrition. These eustigmatophytes are characterized by a typical fatty acid composition that features four abundant fatty acids (Table 1): palmitic (16:0), palmitoleic (16:1n-7), arachidonic acid (20:4n-6), and eicosapentaenoic acid (20:5n-3) (Sukenik 1999; Volkman et al. 1993). C18 fatty acids are represented mainly by oleic acid (18:1); unsaturated 18:2 and n-3 and n-6 isomers of 18:3 account for only a small portion of the total fatty acids, with the actual amounts depending on the growth conditions. Although the proportions of the major fatty acids vary under different cultivation conditions, their abundances are fairly consistent across the different strains and thus constitute a useful chemotaxonomic feature. Table 1 presents the well-established characteristic features of several well-studied strains of N. salina (strain CS-190) and three strains of Nannochloropsis oculata (CS-170, CS179, and CS-216; data taken from Volkman et al. 1993; Renaud and Parry 1994). We have noticed that a recent publication reports fatty acid composition data for Nannochloropsis cultures (Su et al. 2010) that are inconsistent with the above data in that it describes high relative percentages of 18:2 and 18:3(n-3) (Table 1). It is likely that the high proportions of these fatty acids are due to severe contamination of the cultures with green microalgae, typically rich in C18 PUFA. Moreover, the marked increase in the proportion of oleic acid 18:1(n-9) and disappearance of 16:1(n-7) under nitrogen starvation conditions further indicates the enhanced formation of storage TAG in cells of the contaminating algae but not in N. oculata. Characteristically, TAG of Nannochloropsis species are rich in 16:0 and 16:1(n-7) fatty acids (Sukenik 1999); their combined proportions increase up to 70–80% of total fatty acids under stress conditions, such as nitrogen starvation or high light intensity (Pal, Khozin-Goldberg, Boussiba, unpublished), while 18:1 accounts for about 10% of total fatty acids. Results for biomass and lipid productivity that are obtained in the cultures featuring fatty acid profile I. Khozin-Goldberg (*) : S. Boussiba Microalgal Biotechnology Laboratory, French Associates Institute of Agriculture and Biotechnology, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion 84990, Israel e-mail: khozin@bgu.ac.il