Direct versus indirect methods for growth measurements in batch cultures of Nitzschia palea

Direct measurements such as cell density, are considered the most reliable methods for growth monitoring of microalgae. In comparison, indirect methods are undoubtedly less time-consuming, but their relevance needs to be evaluated case by case for the species and the experimental conditions used. Our study focused on the relevance of optical density (OD) and basal fluorescence (F_O) measurements for batch cultures of the benthic diatom Nitzschia palea. Three Canarian strains of N. palea were cultured in triplicate 1 L Erlenmeyer flasks with 270 mL FDMed medium for 26 days with 1.3 x 10⁵ cells mL^-1 ± 27% (RSD). The three strains showed their maximum growth rate between days 4 and 7 (BEA0255B: 0.21 day^-1 ± 0.023 (SD), BEA0256B: 0.23 day^-1 ± 0.058 (SD), BEA0392B: 0.42 day^-1 ± 0.065 (SD)), 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 F_O was performed using the data of each replicate of the three strains in the same analysis. The relevance of OD and F_O measurements was demonstrated during the exponential phase, as they both positively correlated with cell density in a linear regression model (R² > 0.89, n = 45). No linear correlation was found during the stationary phase (R² < 0.46, n = 45). Further studies will address the study of photosynthetic efficiency, biomass weight and composition throughout the growth phases of N. palea, with the aim of determining the optimal harvesting time for biotechnology.