Microalgae has been regarded as a promising method for reducing CO₂ emission. High CO₂ concentration generally inhibits algal growth, and previous studies have mostly focused on breeding freshwater algae with high CO₂ tolerance. In this study, one marine algal strain Dunaliella salina (D. salina) was grown under 0.03%-30 % CO₂ and 3% NaCl conditions, and was evaluated to determine its potential for CO₂ assimilation. The results showed that D. salina could tolerate 30% CO₂, and its maximum biomass concentration could reach 1.13 g·L⁻¹ after 8 days incubation, which was 1.85 times higher than that of incubation in air (0.03%). The phenomenon of high-CO₂ tolerance in D. salina culture was discussed basing on transcriptome analysis. The results showed that D. salina was subjected to oxidative stress under 30% CO₂ conditions, and the majority genes involving in antioxidant system, such as SOD, CAT, and APX genes were up-regulated to scavenge ROS. In addition, most of the key enzyme genes related to photosynthesis, carbon fixation and metabolism were up-regulated, which are consistent with the higher physiological and biochemical values for D. salina incubation under 30% CO₂.

The discharge of industrial phenol wastewater has caused great harm to the environment. This study aims to construct microalgae and bacteria co-culture system to remove phenol from simulated high-salt phenol wastewater and accumulate microalgae biomass. The degradation of phenol by marine microalgae Dunaliella salina (D. salina) and phenol-degrading bacteria Halomonas mongoliensis (H. mongoliensis) was investigated preliminarily, and then the effects of co-culture H. mongoliensis and D. salina on the degradation of phenol and the growth of D. salina were studied. The effects of D. salina/H. mongoliensis inoculation ratio, light intensity, temperature and pH on the performance of the co-culture system were systematically evaluated and optimized. The optimal conditions for phenol degradation were as follows: a D. salina/H. mongoliensis inoculation ratio of 2:1, a light intensity of 120 μmol m⁻² s⁻¹, a temperature of 25°C and a pH around 7.5. Under optimal conditions, this co-culture system could completely degrade 400 mg L⁻¹ of phenol within 5 days. Correspondingly, the phenol degradation rate of D. salina monoculture was only 30.3% ± 1.3% within 5 days. Meanwhile, the maximum biomass concentration of D. salina in coculture was 1.7 times compared to the monoculture. This study suggested that this coculture system had great potential for the bioremediation of phenol contaminants and accumulate microalgae biomass.