Simultaneous production assessment of triacylglycerols for biodiesel and exopolysaccharides as valuable co-products in Navicula cincta

The marine benthic diatom Navicula cincta was cultured in order to evaluate its capacity to produce both neutral lipid-triacylglycerols (TAG) for biodiesel, and exopolysaccharides (EPS), as valuable co-products, under the same environmental conditions. The species was cultured without applying stress conditions, except for the ones naturally created by the culture age. Peaks of neutral lipid accumulation were estimated by fluorimetry by Nile Red. Consequently, lipids were extracted and fractionated into neutral and polar fractions and the fatty acid profile of each fraction analysed by GC. The stationary phase began on day 6, when phosphate and silicate reached limiting values for diatom growth. Total lipids and lipid fractions did not show differences between harvesting time points, reaching total lipid up to 41% of ash-free dry weight (AFDW) and TAG the dominant fraction (ca. 90% of total lipids). Particularly noticeable was the storage of palmitoleic acid (ca. 54% of total fatty acid methyl esters) and a lower level of polyunsaturated fatty acids, which may impart overall favourable properties to a biodiesel fuel, especially cold flow and oxidative stability. The maximum concentrations of EPS corresponded to soluble fraction, which was most significant when the cultures reached the stationary phase and when the medium was almost phosphate and silicate depleted. The characterization of soluble EPS indicated the presence of N-glycopeptides. According to the present results, a hypothetical scheme of demonstrative cultures under a biorefinery approach is proposed for N. cincta.