Freshwater microalga Haematococcus lacustris rich in astaxanthin, as a supplemental live diet can directly supply natural astaxanthin to the aquaculture organisms, except marine aquaculture organisms, since H. lacustris cannot tolerate seawater salinity. The objective of the present study is to provide a salinity acclimation method that allows H. lacustris to survive and accumulate astaxanthin with the aim of developing a novel supplemental live diet for marine aquaculture organisms. H. lacustris cultured in freshwater was subjected to different stepwise salinity acclimation processes (two-, three-, and four-shift). As the controls, H. lacustris was exposed to five constant salinities conditions (0, 0.05, 0.075, 0.3, and 0.6 M NaCl, respectively). Among the controls, almost all cells in the 0.3 M and 0.6 M NaCl conditions died immediately. In contrast, H. lacustris in the stepwise salinity acclimation processes survived in 0.6 M NaCl (equivalent to seawater salinity of 35 psu), showing the highest living-cell proportion (50.0%) and astaxanthin yield (0.72 mg·L^-1) in the four-shift. The present study first demonstrated that H. lacustris tolerated seawater salinity through a stepwise acclimation process, proving a new strategy to supply live microalgal diets rich in natural astaxanthin for marine aquaculture.

Declarations. Conflicts of interest: Qianyi Gu, Yoshiki Takayama, Noriaki Natori and Tatsuki Toda are inventors on the Japanese Patent Application No. 2022 − 004929. The authors have no other relevant financial or non-financial interests to disclose.

• JP19H03035/Japan Society for the Promotion of Science
• JPMJSA1509/Japan Science and Technology Agency
• JPMJSA1509/Japan International Cooperation Agency
• JPMJSA1509/Science and Technology Research Partnership for Sustainable Development