The foundation of much of modern civilization? Sand, actually. It's a key ingredient of concrete, and to construct any building you need a lot of it. It has to be sand from oceans, beaches, riverbeds, deltas, lakes. All that desert sand made slippery-smooth by wind erosion won't do. Concrete remains the top used resource after water and that clamor isn't going anywhere, since annual global demand for concrete is predicted to rise to 4.7 billion tons by 2025. Civilization is one very big sandcastle, and here's the problem with that: the world is running out of sand.
Ever heard of the "sand mafia"? Sand is in such high demand that it has spawned a black market that runs amuck with murders and hush-hush deals and rival gangs in a power struggle over the highly lucrative dealings. The sand mafia gangs have been particularly infamous in India, which has one of the world's largest construction sectors. Aside from the humanitarian issues, illegally digging up sand with reckless abandon impacts the infrastructure of ecosystems. Sucking up sand en masse from a riverbed alters the depth, width, and the water flow rate, which in turn affects the wildlife and increases the risk of cave-ins. Not to mention all the carbon emissions that result from shipping sand to far-away countries.
It's microbes that present alternatives to the problem – as so often seems to be the case.
Conventional concrete is made by combining a coarse material like sand and limestone with a binding agent. This latter ingredient is most often cement. Eliminating cement – which accounts for around 8% of CO2 emissions worldwide – from the narrative is one eco-friendly step. This nixes the step in which limestone is crushed and heated to 1450°C, a process that demands serious investments of thermal energy.
Bacteria offer one alternative via a process called microbially induced calcite precipitation (MICP). Harnessing MICP to build "bio-bricks" is a clever co-opting of processes that occur in nature. Here, limestone is the main ingredient. First adding lactic- and acetic-acid-producing Bacillus pumilus lowers the pH and dissolves the limestone. Next, this slurry is mixed with sand inoculated with Sporosarcina pasteurii, whose urease splits urea into ammonia and carbon dioxide, which become ammonium and carbonic acid. This amps up the pH again and kick-starts MICP, which winds up with the production of calcium carbonate crystals. Calcium carbonate, aka chalk, is the binding agent. Voila. No need for extreme temperatures.
The urea is a key bio-brick ingredient, as it splits into ammonium and carbonic acid and thereby raises the pH to trigger chalk formation. In the above example, it was added exogenously. But what about a more familiar source of urea? Urine is in fact an ingredient in a bio-material invented by a design student. This material has 70% of the compressive strength of concrete, its recipe consisting of Sporosarcina pasteurii, sand, calcium, and of course – chef's kiss – urea derived from urine.
The other key bio-brick ingredient is the living bacteria. The bacteria work a little chemistry that does one of three things to kick-start MICP: convert starter material into carbonate ions, encourage the precipitation of calcium carbonate, or help spur along the formation of calcium carbonate crystals. Bacteria like Sporosarcina pasteurii produce the urease that does the pH-upping trick. Different non-ureolytic bacteria that are also MICP-friendly include nitrate-reducing bacteria, sulfate-reducing bacteria, and cyanobacteria.
One example of such handy cyanobacteria is the ubiquitous marine dweller Synechococcus. Scientists at the University of Colorado Boulder and at the construction firm Katerra have developed a concrete-like material by sticking the photosynthetic bacteria into scaffolds made of gelatin, media, and sand. Instead of increasing pH through urease-produced carbonic acid, Synechococcus achieves the same endpoint via the hydroxide ions it exports during photosynthesis. In this way, it too triggers MICP and produces regenerating "bio-bricks."
These methods, while eliminating the over-the-top energy consumption of conventional cement production, still use sand. Going completely sand-free enlists the aid of some fungi – in the vein of STC's Fungomania. Mushroom mycelia have proven useful in the bio-brick arena. Bio-bricks produced by companies such as the New York biotech Ecovative and the UK start-up Biohm are an amalgam of mushroom mycelia and agricultural waste like corn husks and leaves. No sand, no cement, no sky-high temperatures – but it's undeniable that the current mushroom bricks' compressive strength (~30 psi) is a far cry from the compressive strength of concrete (2,500-10,000 psi).
And so, we continue to search for a way out of our sandcastle-existence…