Light, salinity, and nitrogen modulation of growth and carotenoid synthesis in six Dunaliella strains

Dunaliella species are a unique group of microalgae known for their remarkable ability to thrive in extremely saline environments, making them a subject of significant interest in biotechnology. In this study, we evaluate six representatives belonging in five species of Dunaliella, isolated from Greek solar saltworks (D. asymmetrica TAU-MAC 0620, D. viridis TAU-MAC 0920, D. granulata TAU-MAC 1120, D. minutissima TAU-MAC 1220, D. granulata TAU-MAC 1420 and D. polymorpha TAU-MAC 1520) for their growth performance and their capacity to accumulate carotenoids in response to various growth parameters. We employed an L9 Taguchi fractional experimental design to assess the impact of illumination, salinity, and nitrogen supply, each tested at three different levels. Analysis of variance revealed that the growth performance of all Dunaliella strains was primarily dictated by the applied level of light intensity, while for some strains the salinity and nitrogen supply also contribute as a significant co-factor for their growth. Overall, and despite some strain-specific behaviors in response to the different levels of the applied factors, Dunalliella TAU-MAC strains seem to follow a closely similar pattern for their growth. Thus, they maximize their growth performance under the influence of the moderate level of illumination (100 μmol m^-2 s^-2), the moderate or even the high level of salinity (60 or 120 g L^-1 NaCl, respectively), and the low level of nitrogen availability (0.2 g L^-1 KNO₃). Our results showed that all the examined strains were non-carotenogenic. Among the studied Dunaliella strains, D. granulata TAU-MAC 1420 showed an elevated carotenoids accumulation capacity especially under the influence of the moderate level of light intensity and the high level of salinity, achieving almost 10 mg of carotenoids production L^-1, while the highest carotenoid concentration for the other Dunaliella strains was significantly lower, typically below 5 mg L^-1. Through this research, we anticipate uncovering critical insights into the growth behavior and carotenoid accumulation patterns of several lesser-studied Dunaliella species.