Project Description

Climate change is the most pressing problem of our generation, causing extreme weather events, ecosystem collapse and mass migration [IPCC,2022]. Tackling the climate crisis will ultimately require every sector of human life to adapt. The construction industry, which accounts for 40% of all global emissions [Wieser et al., 2021], has escaped significant public scrutiny, as its immense carbon emissions are hidden from consumers as indirect emissions locked into materials and practices used.

Cement, the key component of concrete, ranks among the most environmentally harmful materials, contributing up to 8% of global carbon dioxide emissions . Production of cement releases the most carbon emissions as fossil fuels are used to heat up the mixture of limestone (calcium carbonate) and clay to 1400 ºC. When the mixture is heated, it releases 600 kg of carbon dioxide for every tonne of cement prduced[nature, 2021]. Concrete is then created by mixing sand, gravel and water with cement.

Why is it a useful application of synthetic biology

At Biocrafter, we are developing a biological alternative to cement by engineering Bacillus subtilis to enhance its biomineralisation capabilities. The engineered bacteria can precipitate calcium carbonate from waste streams and fuse together pieces of aggregate, creating a strong biomaterial.

Biocrafter and Biomineralization

Wide variety of organisms have biomineralising properties, from bacteria, shelfish and even humans. Certain bacteria biomineralise through expression of enzymes which facilitate capture of carbon dioxide into insoluble minerals, most commonly calcium carbonate. We can highjack this natural mechanisms and untilise it in construction.

We engineered Bacillus subtilis to overexpress two enzymes: urease and carbonic anhydrase. Urease breaks down urea (commonly found in waste water and fertalisers) into amonia and carbon dioxide. In turn, carbonic anhydrase acts as a hydrator, catalysing the reaction of carbon dioxide (CO2) with water into carbonic acid, which spontaneously decays into carbonate ions.

Our Process

Calcium ions get stuck on the cell’s outer surface due to its slight electronegativity. As carbonate ions difuse out of the cell, they meet calcium and calcium carbonate is formed. The cell outer surface then acts as a nuceation site for stratified formation of calcite crystals and hence crystaline calcium carbonate is precipitated (Burbank et al., 2012). The crystal act as a glue, fusing together cells with the sand and aggregate particles, creating a solid material.

Design

We work with Sodium alginate (SA) and Methyl cellulose (MC) hydrogels, because of their inherent binding capacity. These hydrogels provide provide moisture for the biological components of the material and a structural matrix the aggregate particles become embedded in. The matrix binds aggregate particles in place so the biomineralisation process can take place.

Mycelium is a root-like structure of a fungus consisting of mass of branching, thread-like hyphae. The mycelial cell walls contain chitin, making it strong and elastic. Mycelium became a staple in biomaterial development with many companies and researchers using it to create novel materials. In our biomaterial, mycelium has three functions:

Mycelium fibres aid tensile stength that conventional concrete lack.
Hyphae act as additional nucleation sites for calcite formation.
Mycelium will create a hydrophobic coating of the material preventing weathering

Sand is made of fine rock particles and is necessary to form a strong material. The compression strength is given by the strength of individual aggregate particles, while tensile strength is dictated by the aggregate binding. In our prototypes we use fine sand with mean aggregate size of < 2 mm, hence our bacteria can fill spaces between aggregates and fill the gaps with calcium carbonate crystals.

We chose to engineer B. subtilis for four reasons:

It is naturally occuring, hardy and abundant soil bacteria Although less developed than E. coli, it has a substanial genetic engineering toolkit. It has natively expresses our chosen enzymes.

Proof of Concept

We successfully combined all the components to form a completely new biomaterial enabled by synthetic biology. The transformed bacteria precipitate significant amounts of calcium carbonate crystals compared with the negative controls.

The prototypes are currently being tested for compression and tensile strengths. The results will be revealed on the Jamboree.

Project Goals

Overexpressed two reinforcing calcium carbonate precipitation pathways.
Develop a novel biomaterial with structural properties comparable to cement.
Create novel research within the biomaterial field by combining mycelium and engineered bacteria

References

Burbank, M.B., Weaver, T.J., Williams, B.C. and Crawford, R.L. (2012). Urease Activity of Ureolytic Bacteria Isolated from Six Soils in Which Calcite Was Precipitated by Indigenous Bacteria. Geomicrobiology Journal, 29(4), pp.389–395. doi:10.1080/01490451.2011.575913.
IPCC, 2022: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R.
Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926
nature (2021). Concrete Needs to Lose Its Colossal Carbon Footprint. Nature, [online] 597(7878), pp.593–594. doi:10.1038/d41586-021-02612-5.
Wieser, A.A., Scherz, M., Passer, A. and Kreiner, H. (2021). Challenges of a Healthy Built Environment: Air Pollution in Construction Industry. Sustainability, 13(18), p.10469. doi:10.3390/su131810469.