How soil is engineered into a strong fortress by termites

There is only one nature — the division into science and engineering is a human imposition, not a natural one” said Bill Wulf, an engineer. Researchers from the Indian Institute of Science (IISc), Bengaluru, have recently explained how a group of fungus-growing termites ‘bioengineer’ the soil with their secretions and construct stable mounds. This process is termed ‘bio-cementation’, where termites alter soil properties to build strong mounds.

Mutualism in nature

Fungus-growing termites are those that cultivate and grow a selective species of fungus in their mounds. These termites feed on wood and excrete undigested material in special structures inside the mounds, where the fungi are farmed. The fungi feed on this excreta and break down undigested food particles using enzymes. This product can now be digested by the termites. This is a classic example of mutualism in nature, where both the termites and fungi are benefitted by the relationship. Thanks to this mutualism between the two organisms, termites are able to absorb every last bit of nutrition from wood.

Renee Borges, a professor at the Centre for Ecological Sciences, IISc, and her team are exploring the bioengineering aspects of mound construction in these fungus-growing termites. Researchers in her lab have studied the mechanical and biological properties of the mound soil used by the fungus growing termite Odontotermes obesus in the past. Their research shows that these termites are excellent engineers who strengthen their mounds using bio-cementation processes.

Renee’s lab has recently published an article with inputs from civil engineers on the effect that bio-cementation has on the strength and stability of mounds. They found many differences between soil used in mounds and soil present around mounds. For example, mound soil was 10 times stronger and soil particles were smaller. However, mineral properties did not vary greatly in the two soils. This increased strength is the result of termite secretions and excreta. “Even though it is now clear that these termites strengthen their mounds by their secretions and by reducing the particle size of soil, the mechanisms of how they bring about this particle size reduction remains unclear,” says Nikita Zachariah, a PhD student, who is a member of the team.

Another interesting discovery was that termites seemed to modulate water content of the soil such that it could be easily moulded. Nikita also added that “Water is an essential requirement for mound construction. But the source of water is again not that clear. There are various water sources for the termites inside the mound like the metabolic water condensing inside the mound, the moisture in the soil or even the underground water.”

It is now clear that these termites strengthen their mounds by altering soil properties during construction with the help of their secretions. This bioengineering and bio-cementation process contributes to mound stability without changing the mineral content of the soil. Renee and her team are now exploring the different aspects of bio-cementation, mutualism between the farmed fungus and these termites.