(Chlorophyceae, Order Volvocales), a freshwater green microalgal species, has commercial value owing to its ability to accumulate high concentrations of astaxanthin (up to 5% of dry weight). Astaxanthin (3,3'-dihydroxy--carotene-4,4'-dione) is a red ketocarotenoid which has many important biological functions, including antioxidant activity, regulation of immune responses, and disease resistance, and has the potential for application in the aquacultural, nutraceutical, pharmaceutical, and cosmetic industries. has a distinctive lifecycle as it exhibits a green motile stage and a red non-motile resting stage called an aplanospore. In general, astaxanthin accumulation in is restricted to the aplanospore stage. Astaxanthin is accumulated in extra-plastidic lipid vesicles as a secondary carotenoid, and it is believed to be synthesized in response to oxidative stress in the red aplanospore stage under unfavorable environmental conditions such as high light, temperature, and salinity, or low nutrient availability. Several enzymes such as lycopene -carotene ketolase (Bkt) are involved in the astaxanthin bios-y-n-thesis pathway in -carotene from lycopene, and CrtR-B and Bkt cat-alyze further steps leading to astaxanthin synthesis. Changes in expression of the genes encoding these three enzymes can critically affect the biosynthesis and accumulation of astaxanthin in . In addition, various ant-i-oxidant enzymes including superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) also have important protective effects for combating oxidative stress under unfavourable environmental conditions. We aimed to explore the effect of salt stress on astaxanthin accumulation in , by examining the mechanism of astaxanthin synthesis and the relationship between the different antioxidant mechanisms in at high salinity. We examined its growth rate; astaxanthin content; gene expression levels; SOD, CAT, and GSH-Px activities; and malondialdehyde (MDA) content at four salinity levels (0.04 mol/L, 0.08 mol/L, 0.12 mol/L, and 0.16 mol/L) over three timescales (3 days, 6 days, and 9 days). Our results showed that the density of decreased under increasing salinity over different periods of time, while its mortality rate and aplanospore proportion increased with increasing salt stress concentration by the ninth day of stress. Asta-xanthin content, gene expressions increased over time with increasing salinity. SOD, CAT, and GSH-Px activities and MDA content also increased in comparison to grown at the control level of 0 mol/L NaCl, and significantly increased at the 0.12 mol/L NaCl level ( gene expressions were lower during the early and mid-stages of salt stress (e.g., on the third and sixth days of observation), and increased by the ninth day. Meanwhile, SOD, CAT, and GSH-Px activities and MDA content were higher during early and mid-stage stress, but were lower by the ninth day. These results suggest that salt stress can improve astaxanthin accumulation over time in at the appropriate level of salt stress, despite its negative effects on growth. Astaxanthin synthesis in is promoted mainly through an increase in the transcription level of astaxanthin synthesis-related enzyme genes under salt stress, and the antioxid-ant activities of astaxanthin and antioxidant enzymes complement one other to protect from oxidative damage under salt stress. This study provides a new insight into the astaxanthin synthesis and antioxidant mechanisms in .