Enhanced astaxanthin accumulation in the microalga <i>Haematococcus lacustris</i> in seawater through the combination of salinity, nitrogen starvation and high light

The freshwater microalga Haematococcus lacustris, the most prolific natural producer of astaxanthin, has potential as a live microalgal diet for aquaculture. However, its low salinity tolerance restricts its use in marine aquaculture. To apply H. lacustris as live microalgal diet in marine aquaculture, this study examined the synergistic effect of nitrogen starvation and high light through four-shift salinity acclimation (FS) on the survival and astaxanthin accumulation in seawater. The living cell densities of cultures subjected to the FS were comparable between sufficient nitrogen and low light (LL + N; the control) and nitrogen starvation and high light (HL−N), reaching 0.50 × 105 and 0.58 × 105 cells mL−1, respectively. Rapid formation of red cells with stable survival was observed when cells were cultured in HL−N. HL−N achieved an astaxanthin yield of 5.50 mg L−1, a 7.9-fold increase over the control (0.70 mg L−1). Salinity acclimation maintained the survival, while nitrogen starvation and high light enhanced astaxanthin accumulation. The synergistic effect of salinity acclimation, nitrogen starvation and high light promoted the rapid formation of red cells, which enhanced tolerance to salinity stress (such as seawater salinity) and boosted the astaxanthin accumulation in H. lacustris.