Carbon Dioxide Assimilation in Chemoautotrophic Bacteria, with Special Reference to Tricarboxylic Acid Cycle and Carbon Dioxide Assimilation Pathway of Nitrifying Bacteria

Abstract

CO₂ accumulation as a factor of global warming is requiring attention. Obligate chemoautotrophic bacteria grow independently of organic conditions with energy from the oxidation of reduced inorganic compounds and CO₂ from the atmosphere as sole carbon sources. Chemoautotrophic bacteria consume (fix) CO₂, and assimilate CO₂ via the reductive pentose phosphate cycle (Calvin cycle). CO₂ assimilation via the reductive tricarboxylic acid cycle (TCA cycle) occurs particularly in thermophilic hydrogen-oxidizing bacteria. This paper reviews studies on the tricarboxylic acid cycle in relation to CO₂ fixation reactions of chemoautotrophic bacteria, especially nitrifying bacteria.
Nitrifying bacteria promote nitrification, a stage in the nitrogen cycle. Ammonia-oxidizing bacteria (Nitrosomonas) oxidize ammonia as the sole nitrogen source of nitrite, and nitrite-oxidizing bacteria (Nitrobacter) oxidize nitrite as the sole nitrogen source of nitrate. Both bacteria fix CO₂ mainly via the Calvin cycle in which ribulose 1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) is a key enzyme. Less CO₂ is assimilated by the phosphoenolpyruvate carboxylase pathway. The TCA cycle of nitrifying bacteria was studied and CO₂ assimilation was clarified in greater detail.