Polyamines capture carbon dioxide and accelerate calcium carbonate formation

Abstract

Since the beginning of the last century, it has been known that bacteria, including cyanobacteria, are involved in the extracellular formation and precipitation of calcium carbonate (CaCO₃). Such carbonate precipitation by bacteria has been observed both in vitro and in natural habitats, suggesting that this precipitation could partially contribute to the formation of marine calcareous skeletons, carbonate sediments, and deposits of carbonate in soils. Bacterial CaCO₃ formation has been shown to include several pathways such as the reduction of CaSO₄ to CaS by sulfate-reducing bacteria, the hydrolysis of urea leading to the formation of ammonium carbonate by urea-hydrolyzing bacteria, and the removal of CO₂from a bicarbonate-containing solution by cyanobacteria and by extracellular polymeric materials such as exopolysaccharides and capsular polysaccharides isolated from Bacillus firmus and Nocardia calcarea. Despite such explanations, the mechanisms involved in the extracellular production of CaCO₃by bacteria have remained largely unclear.

It is known that CO₂ at high concentrations in exhaust gases can be captured by solutions containing alkanolamine. This is called the “amine method,” which has been used since the 1930s in various industries to fix CO₂gas. Recently, another synthetic amine, polyethylenimine, has been investigated as an agent for the capture of CO₂ from the air. Biogenic polyamines are ubiquitous cellular components that perform multiple functions and are essential for normal growth and development. Levels of cellular polyamines are elaborately maintained at an optimum by biosynthesis, degradation, and transport. It is known that bacteria such as Escherichia. coli contain high concentrations of polyamines. For example, intracellular concentration of putrescine is approximately 20 mM while that of spermidine is about 6 mM. It is also known that biogenic long-chain polyamines do play roles in silica formation in diatoms. To our knowledge, the functional relationship between biogenic polyamines and calcification by organisms has never been reported. In this report, we examine the interaction between biogenic polyamines with CO₂ and suggest roles of the polyamines in extracellular formation of CaCO₃ crystals by bacteria_. This new mechanism of CO₂ fixation adds a novel pathway to the global carbon cycle.