Key genes expression of reductive tricarboxylic acid cycle from deep-sea hydrothermal chemolithoautotrophic Caminibacter profundus in response to salinity, pH and O₂

CO₂ fixation pathway of Caminibacter profundus, a chemolithoautotrophic ε-Proteobacteria from deep-sea hydrothermal vent, was determined and characterized by genetic and enzymatic analyses. Gene expression of key enzymes for CO₂ fixation in response to salinity, pH and O₂ in Medium 829 were also investigated. The results demonstrate that C. profundus contained aclB, porA and oorA, the genes encoding key enzymes of reductive tricarboxylic acid (rTCA) cycle. However, genes fragments of cbbL and cbbM encoding key enzyme of Calvin cycle were not recovered. Key enzymatic activities of ATP citrate lyase (ACL), pyruvate:ferredoxin oxidoreductase (POR) and 2-oxoglutarate:ferredoxin oxidoreductase (OOR) were also present in C. profundus. The combination of genetic and enzymatic analyses confirm that C. profundus adopted rTCA cycle for carbon assimilation. The results of aclB and oorA relative expressions of C. profundus demonstrate that the ranges of environmental factors for high genes expression were sea salt 3.0%-5.0% (optimum 3.0%), pH 5.0-6.5(optimum pH 6.5), anaerobic to microaerobic conditions (optimum 1.0% O₂). Gene expression patterns under different conditions show similar patterns with bacterial growth, revealing that key rTCA cycle genes provided molecular basis for bacterial growth and propagation. Our results suggest that C. profundus could regulate key genes of rTCA cycle for carbon assimilation and energy metabolism in response to environmental fluctuations in hydrothermal vent.