Uncracking the surface: Microbial biomineralization of calcium carbonate
Summary: Microbial metabolic process leads to production of inorganic substances as biominerals and precipitation of calcium carbonate as biominerals is called microbially induced calcium carbonate precipitation (MICCP). Ureolytic bacterial strains from various habitats are the most promising choice for the process with several applications starting from healing of surface cracks to study of lithology of outer neighbor planets and other bioengineering aspects.
What are biominerals?
Biominerals are beautiful and we are surrounded by them. Starting from earth crust to ocean/sea floor they are widespread and play a role in ecosystem functioning. Biominerals are nothing but induced minerals produced by living organisms starting from algae/bacteria like prokaryotes to eukaryotic vertebrates. Under specific condition, they are produced in specific location of the cell and excreted outside (extracellularly) due to cellular metabolic processes. Production of these biominerals has gained attention in scientific field to harness this technology in bioengineering and other related field. The minerals that have been associated with biomineralization, carbonates are the most prominent and promising. Microbially induced calcium carbonate precipitation (MICCP) is the most widely used biomineralization process for variety of fields ranging from Biotechnology, Geomicrobiology, Paleobiology to Civil Engineering. Most of their uses are in atmospheric CO2 fixation through carbonate sediment formation and remediation of surface cracks. The precipitation of CaCO3 is governed by four parameters;
(1) the calcium concentration,
(2) the carbonate concentration (dissolved inorganic carbon),
(3) the pH of the environment (which affects carbonate speciation and calcium carbonate solubility) and
(4) the presence of nucleation sites (Hammes and Verstraete, 2002).
The primary role of bacteria in the precipitation process has been ascribed to their ability to create an alkaline environment through various physiological activities (Al-Thawadi, 2011), where specific functional groups of microbial cell wall favors the binding of divalent cations (Ca2+ and Mg2+), serves as ideal crystal nucleation site (Chahal et al., 2011). MICCP can be induced by various metabolic processes of bacteria like nitrogen cycle, ammonification of amino acids, nitrate reduction and the hydrolysis of urea (Hammes et al., 2003).
Mainly four groups of microorganisms are seen to be involved in the process
(i) Photosynthetic organismsâ€"such as cyanobacteria and algae
(ii) Sulphate reducing bacteriaâ€"that are responsible for dissimilatory reduction of sulphates
(iii) Organisms utilizing organic acids
(iv) Organisms that are involved in the nitrogen cycle either ammonification of amino acids/ nitrate reduction/ hydrolysis of urea (Hammes and Verstraete, 2002).
Urea hydrolysis by heterotrophic bacteria is the simplest and most widely used method for precipitation of carbonates for several technical applications.
Ureolytic bacterial action through breaking of urea by urease results in the production of carbonate ions in the presence of ammonium (equation 1) leading to an increase in pH and carbonate concentration in the bacterial environment (Stocks-Fisher et al., 1999). Calcium carbonate is readily precipitated under these conditions, in the presence of sufficient calcium and carbonate.
CO(NH2)2 + 2H2O --> CO32- + 2NH42- (1)
Different bacterial groups have been assigned with MICCP activity with various polymorphs of carbonates on cell surface with successive stratification and continue to grow on carbonate crystals and can be assigned with ecological benefits for the precipitating microbe. Many groups of bacteria like Bacillus sp., Pseudomonas sp., Micrococcus sp., Halomonas sp., Leuconostoc sp., Myxococcales members and gram negative bacterial groups are most promising candidates as ureolytic CaCO3 precipitating microbes (Chaturvedi et al., 2006, Dick et al., 2006, Sarda et al., 2009). Among various forms of CaCO3 minerals precipitated by microbes, calcite, vaterite and aragonite are the three common anhydrous polymorphs and are species specific (Rusznyak et al., 2012 and Dhami et al., 2013a). Use of FTIR, XRD, Raman spectroscopy/NMR, EDX, SEM-TEM etc can help to characterize the form of polymorphic species and bacterial nucleation site of formation of these biominerals in in-vitro analysis.
Use of MICCP biominerals
1. Removal of Heavy metal and radionuclides
2. Removal of calcium ions and polychlorinated-biphenyls
3. CO2 sequestration
4. Alternative as filler for rubber, ink etc
5. As a biodeposition material
6. Remediation of surface cracks
7. Restoration of limestone building
Challenges and issues
1. Microbial precipitation method is comparatively slower than chemical method and depends on number of environmental factors
2. Inappropriate microbial candidate and its strain selection
3. Economic limitation of use of laboratory grade nutrient sources in field applications also restricts the use
4. Production of urea and its degradation causes secondary products and pose severe environmental concern
The potential of these biominerals has brought a new revolution in various engineering applications but still there has been much to explore in order to bring this environmentally safe, cost effective and convenient technology from lab to field scales using appropriate microbial candidate. Surface treatment is playing an increasingly important role for the protection of construction materials like limestone from harmful effects of rainwater due to global warming and climate change and use of MICCP technology can help huge.
1. Al-Thawadi, S. M. (2011). Ureolytic bacteria and calcium carbonate formation as a mechanism of strength enhancement of sand.Journal of Advanced Science and Engineering Research, 1(1), 98-114.
2. Hammes, F.,Verstraete, W. (2002).Key roles of pH and calcium metabolism in microbial carbonate precipitation. Reviews in environmental science and biotechnology, 1(1), 3-7.
About Author / Additional Info:
I am a microbiologist by profession and doing Ph.D at IIT Kharagpur on environmental microbiology field aiming to study the microbial role in arsenic contamination in West Bengal and development of its remediation strategies.