Good­bye Sand­cas­tle, Hello Mi­cro­bial Bio-Brick

by Janie

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.

Fig­ure 1. Sand min­ing in Goa. Source

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.

Fig­ure 2. A de­pic­tion of mi­cro­bially in­duced cal­cite pre­cip­i­ta­tion (MICP) at a sin­gle-cell level, fea­tur­ing a ure­olytic bac­ter­ium. Source

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."

Fig­ure 3. An ex­am­ple of mycelium-based build­ing ma­te­r­ial. Source (Fron­tispiece)

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…