Batch cultures of native strains of Nitzschia palea from the Canary Islands: growth monitoring using direct and indirect methods

Several methods for monitoring the growth of microalgae in the laboratory have been described in the literature. Direct measurements, such as cell density and biomass yield, are considered as the most reliable methods. Indirect methods generally rely on specific parameters that can be measured as a consequence of the phototrophic activity. While indirect methods are undoubtedly less time-consuming, their relevance needs to be evaluated case by case for the species and the experimental conditions used. In this study, the main objective was to test the relevance of optical density (OD) and basal fluorescence (F0) measurements for monitoring the growth of batch cultures of the benthic diatom Nitzschia palea. Further experiments addressing photosynthetic efficiency, biomass weight and composition, lipid content and fatty acid profile were conducted with the aim of determining the optimal harvesting time for biotechnological applications. Three Canarian strains of Nitzschia palea (BEA 0255B, BEA 0256B and BEA 0392B) were cultured in triplicate 1 L Erlenmeyer flasks with 270 mL FDMed medium for 26 days at an initial cell density of 1.3 x 105 cells mL-1 ± 27% (RSD). Biomass was harvested and further analyzed at two points of the growth curve, one corresponding to the exponential phase (day 9) and the other corresponding to the stationary phase (day 26). The three strains showed their maximum growth rates (μmax) between days 4 and 7 (0.21 – 0.29 day-1), and entered the stationary phase at day 17, maintaining an average concentration of 1.5 x 106 cells mL-1 ± 11 % (RSD). Comparison between cell density, OD and F0 was performed using the data of each replicate of the three strains in the same analysis. The relevance of OD and F0 measurements was demonstrated during the exponential phase, as they both positively correlated with cell density in a linear regression model (days 4 – 17, R2 > 0.87, n = 45). Very poor linear correlation was found during the stationary phase (days 17 – 26, R2 < 0.46, n = 45). The biomass yields of the three strains were comprised in a range of 3.1 – 3.8 mg mL-1 dry weight (DW), without significant differences between strains or growth phases. At day 9, lipid content represented between 9 and 11 % DW, whereas at day 26, lipid content was between 6 and 8 % DW. The lipid content in all three N. palea strains was slightly higher at day 9 compared to day 26, but not significant, and overall low compared to other studies. The preliminary studies on fatty acid composition opens up potential applications for these strains in different biotechnological areas, including nutraceutical and aquaculture sectors