Bacillus megaterium is a unique microorganism that possesses dual enzymatic capabilities for the precipitation of calcium carbonate (CaCO₃), through enzymes urease a carbonic anhydrase (CA)Which of these metabolic pathways B. megaterium prefers is significantly influenced by the concentration of CO₂ in its environment, which has key implications for the biosynthesis of materials and carbon sequestration.

For atmospheric CO₂ levels (and in the presence of urea and CaCl₂), B. megaterium activates ureolytic pathway. This process involves the hydrolysis of urea (CH₄N₂O) to carbonate (CO₃²-) and ammonium ion (NH₄+). Subsequently, Ca²+ combines with carbonate to form CaCO₃, with calcite being identified as the precipitated polymorph. However, it is important to note that ammonium ion is an undesirable by-product that requires additional wastewater treatment.

The situation changes when elevated CO₂ levels, specifically at levels more than 470 times higher than atmospheric levelsIn such conditions B. megaterium activates the enzyme carbonic anhydrase (CA). Carbonic anhydrase catalyzes the hydration of the CO₂ molecule to bicarbonate (HCO₃-). The bicarbonate ions are then converted to carbonate ions (CO₃²-), which combine with free Ca²- cations to form CaCO₃.

Key experiments using C¹³-labeled urea confirmed this change in preference: at elevated CO₂ concentrations, up to 94 % of precipitated CaCO₃ formed by CO₂ mineralization, while only approx. 6 % came from ureolysis. This means that B. megaterium at high CO₂ concentrations, it primarily uses its carbonic anhydrase pathway for CaCO₃ synthesis.

The advantage of the CA-mediated path is obvious: it allows direct binding of atmospheric CO₂ into CaCO₃ and at the same time minimizes the formation of potentially dangerous metabolic byproducts, such as ammonia, which are associated with the ureolytic pathway. This understanding of the selective activation of metabolic pathways opens up promising possibilities for future strategies for microbially induced carbon dioxide sequestration (MICO2S) and the development of sustainable building materials. Spring

The study was published in Scientific Reports magazine .