A Chlorella sp.-Cupriavidus necator (C. necator) microcosm was artificially established for phenol degradation. The cooperation relationship between Chlorella sp. and C. necator was initially demonstrated, and then the effects of Chlorella sp./C. necator inoculation ratio, light intensity, temperature and pH on the performance of this microcosm were systematically evaluated and optimized. The optimal conditions for phenol degradation were as follows: a Chlorella sp./C. necator inoculation ratio of 1:1, a light intensity of 110 μmol m^-2 s^-1, a temperature in the range of 25-32 °C and a pH in the range of 5.5-7.5. Under optimal conditions, this microcosm could degrade phenol with a maximum concentration of 1200 mg L^-1 within 60 h. It was found that only when the phenol concentration was reduced to the tolerance concentration of microalgae, that is, the last stage of phenol degradation, the cooperation effect could be generated, indicating that the tolerance of microalgae to phenol may be more important than its degradation performance. Comparative transcriptomic analysis was conducted to discuss the cooperation mechanism of this microcosm subject to high phenol concentrations. The up-regulation of genes involved in photosynthesis and carbon fixation of Chlorella sp. demonstrated the CO₂ and O₂ exchange between Chlorella sp. and C. necator and their cooperation relationship. This study suggests that this microcosm has great potential for the bioremediation of phenol contaminants.

The emission of carbon dioxide (CO2) to the atmosphere at an increasingly high rate is the primary cause of global warming. Previous studies have mostly focused on low CO2 concentration inputs and the resulting CO2 concentrating mechanisms (CCM). However, there are relatively few reports on high-CO2 tolerant species and their tolerance mechanisms. In this study, Scenedesmus sp. was grown under 1-70 % CO2 conditions, and a transcriptomic analysis was performed to analyze the global gene expression profiles induced by extremely-high CO2 (70 %) relative to relatively-low CO2 (1%). Results showed that Scenedesmus sp. produced a maximum biomass of 3.92 g L-1 at 10 % CO2 and a biomass of 2.75 g L-1 at 70 % CO2. High relative CO2 levels (30-70 %) were favorable for accumulation of lipids and yielded a maximum lipid productivity of about 82.9 mg L-1 d-1. The abundance of genes related to oxidoreductase activities and photorespiration were stimulated under 70 % CO2 condition, which in turn led to overproduction of ROS. In addition, qPCR was used to validate the results from the transcriptomic analysis and found that most genes involving in the antioxidant system, such as CAT and SOD2, were upregulated around 16.5 times under the high CO2 treatments. In addition, the down-regulation of genes involved in CCM might ensure Scenedesmus sp. rapid adaption to the environmental signal to maintain microalgal growth through energy saving.