The responding mechanisms of Nannochloropsis oceanica to high CO2 and the regulating functions of H+-PPase.

High CO2 tolerance microalgae screen/breeding shows the urgent research priority when applying microalgae for flue gas CO2 sequestration. In this study, we chose the important resource microalgae Nannochloropsis oceanica as the target organism, the regulatory mechanisms of N. oceanica were elucidated under 5% and 20% high CO2 conditions, and the function of key regulating gene avp1 encoding H+-PPase was further explored. The results showed N. oceanica was tolerant to the 5% CO2 that maintained intracellular pH homeostasis, while severe cytoplasmic acidification was occurred under the 20% CO2 condition. Integrated physiological, biochemical, and transcriptomic analysis revealed that P-ATPase and H+-PPase activities were enhanced at 4 h and 4 d under the 5% CO2 condition, respectively. Concurrently, the reprogramming of organic acid metabolism and maintenance cellular energy supply additionally mitigated intracellular acidification. Further functional validation showed that overexpression of avp1 enhanced H+-PPase activity, increased cytoplasmic pH values, promoted pigments accumulation and growth of N. oceanica under the high CO2 condition. Therefore, this study clarified the working mode of N. oceanica to tolerant high CO2 and cytoplasmic acidification, and firstly timely proved the function of avp1, provided important data basis and gene candidates for the research of applying microalgae to CO2 sequestration.