In recent years, the interest in obtaining microbial cement has gained popularity. This is in part because of the potential of microbial cement to overcome problems such as fractures and fissures in concrete structures which are created by weathering, land subsidence, faults, earthquakes and human activities. Synthetic biology has proposed a novel way to repair and remediate these problems. One of the possible solutions is biomineralization of calcium carbonate using microbes such as Bacillus species.The application of microbial concrete in construction may simplify some of the existing construction processes and revolutionise them.
Back to basics
Biomineralization is process by which living organisms are naturally able to produce minerals.
Production of microbial calcium carbonate (CaCO3) is a widely studied and a promising technology with various engineering applications. The use of CaCo3 include: treatment of concrete, manufacturing of construction materials (such as building bricks and fillers for rubber), synthesis of plastics and inks.
There are three distinct pathways of bacterial calcium carbonate precipitation:
1) biologically controlled - cellular specific control of formation of the mineral (exoskeleton, bone or teeth) ,
2) biologically - influenced - passive mineral precipitation caused through the presence on the surface of the cell of organic matter and
3) biologically- induced - which is the chemical alteration of an environment by biological activity.
The most commonly found mechanism in bacteria for CaCO3 precipitation has been to generate an alkaline environment through different physiological actions. Precipitation of CaCO3 by ureolytic bacteria is the most straightforward and most easily controlled mechanism of microbially induced calcium carbonate precipitation. It also has the potential to produce high amounts of carbonates in short period of time.
Besides the CaCO3 precipitation induced naturally by microbes, many other organisms also have the power to produce calcium carbonate, such as corals. In the stony coral, Stylophora pistillata, 4 acid-rich proteins (CARPs 1–4; GenBank accession numbers KC148537–KC148539 and KC493647) were identified to be responsible for calcium carbonate precipitation. These proteins were found in the study of changes in the growth of corals with increasing of acidity in the ocean.
As such, bioreaction of calcite formation is far from the thermodynamic equilibrium. It may even compromise with acidification and very low mineral saturation state (E. Tambutté & A. A. Venn et al. 2015).
In our project, coral acid-rich proteins (CARPs) was cloned and expressed in E.coli BL21 strain. They were characterized for their ability to induce calcium carbonate precipitation.
According to the putative mechanism of calcium carbonate nucleation by CARP, a highly acidic pocket brings together a calcium ion and a carboxylate molecule thus favouring their reaction (Figure 5). Evidence based on high-resolution magnetic resonance spectroscopy has shown that the calcification in stony coral is mainly controlled by CARPS embedded in skeleton organic matrix.
The key advantage of CARPs is their power to bypass the acidification of the growth medium and the urea synthesis associated with the classical urease pathway. Furthermore, CaCO3 precipitation with CARPs occurs in one enzymatic step, greatly reducing the metabolic cost for the cell.