Salinity-Driven Modulation of Growth and FAME Composition in Auxenochlorella protothecoides for Industrial Applications

Microalgal lipid production requires cultivation strategies that reduce freshwater demand while maintaining biomass productivity and improving the quality of fatty acid methyl ester (FAME) profiles for downstream applications. In this context, salinity is a relevant but still insufficiently characterized factor, particularly for Auxenochlorella protothecoides, whose response to salt stress in terms of growth dynamics, lipid productivity, and FAME composition remains poorly understood. This study investigated the effect of NaCl concentrations ranging from 0 to 50 g L−1 on the growth, lipid accumulation, FAME profile, and predicted biodiesel-related properties of A. protothecoides. Growth kinetics were described using logistic and Gaussian models. The highest carrying capacity was observed at 10 g L−1 NaCl, whereas the modeled optimum for specific growth rate occurred under moderate salinity, close to 20 g L−1 NaCl. Lipid analyses showed that C16–C18 fatty acids (FAs) dominated across treatments, accounting for more than 90% of total FAMEs, and that increasing salinity shifted the profile from a more saturated C16:0-rich composition toward higher proportions of unsaturated C18 FAs, particularly oleic and linoleic acids. Biodiesel property estimations indicated that mild salinity (5 g L−1) improved some fuel-relevant parameters relative to the other salinity treatments, including cetane number, iodine value, cold-flow-related properties, and oxidative stability. Nevertheless, the predicted viscosity values were below the EN 14214 specification range for all treatments, and oxidative stability at 5 g L−1 only marginally met the European minimum requirement, indicating partial rather than full compliance with biodiesel standards. These findings indicate that salinity can be used as a practical tool to tune biomass production and lipid quality in A. protothecoides, supporting its potential use in microalgal biorefineries and oleochemical applications. Furthermore, the ability of the strain to tolerate saline conditions may support the future use of brackish water, seawater, or saline waste streams, potentially reducing freshwater demand; however, this environmental benefit was not quantified in the present study and should be validated through dedicated water-footprint or life-cycle assessment. Direct biodiesel application also remains limited by incomplete compliance with fuel standards, and further optimization through cultivation in real saline or wastewater-based media, process scale-up, blending, or downstream upgrading will be required to improve industrial feasibility.