Enhanced Fatty Acid Productivity by Parachlorella sp., a Freshwater Microalga, via Adaptive Laboratory Evolution Under Salt Stress

Salt stress can be used to increase intracellular lipid content of microalgae. However, the growth of microalgae is decreased under salt stress. Thus, a two-stage cultivation strategy with a growth phase followed by a stress stage is required to improve lipid content. In this study, adaptive laboratory evolution (ALE) with salt stress was conducted to improve biomass and fatty acid (FA) productivity using a locally isolated freshwater microalga Parachlorella sp. in a single-stage cultivation. Algal cell growth and FA production under conditions of salt stress (0–40 g of NaCl/L) were compared to those in a 0.5 L bubble column photobioreactor and appropriate levels of salt stress (10 and 20 g/L) were determined for ALE. During the ALE process, the average cell size increased from 3.0 to 3.9 µm. After eight consecutive ALE cycles, microalgal growth rate was remarkably increased to close to that of the culture without salt stress. Furthermore, FA content in microalga was improved from 7.5% to 25%. This result was confirmed by observations with various types of microscopes. Eventually, overall FA productivity was increased up to 219.0 ± 10.7 mg/L/day with the addition of 20 g of NaCl/L, which was 233% greater than that of the culture without salt stress (93.8 ± 5.3 mg/L/day) due to increased FA content. Recovered biomass productivity was 80% of that in the culture without salt stress. These results suggest that microalgal FA production can be significantly improved by a simple ALE process without an additional stress culture step.