Scientists unravel mystery of earthworms' digestive system
They are known as a "gardener's best friend" and without them the countryside would not look the same.
But just why the humble earthworm is so good at enriching the soil has remained a mystery until now.
The secret is a unique digestive system that relies on metabolite molecules dubbed drilodefensins to counteract plant toxins, scientists have discovered.
Thanks to drilodefensins, earthworms are able to digest organic material such as rotten roots and fallen leaves that would defeat other herbivores. At the same time, their castings return concentrated nutrients and carbon to the soil.
A world without earthworms and their drilodefensins would be very different, according to the researchers .
Lead scientist Dr Jake Bundy, from Imperial College London, said: "Without drilodefensins, fallen leaves would remain on the surface of the ground for a very long time, building up to a thick layer. Our countryside would be unrecognisable, and the whole system of carbon cycling would be disrupted."
Plants produce antioxidant polyphenol compounds that can have health benefits for humans but are toxic to many herbivores, inhibiting enzyme activity that aids digestion.
Earthworms generate large amounts of drilodefensins - a name coined by the scientists - in their guts to counteract polyphenols. For every person on Earth, the researchers estimate that at least a kilogram of drilodefensins exists in all the worms that populate the world's soil.
Their importance was illustrated by the fact that earthworms recycled them over and over again to harness their effects.
They also appeared to be unique to earthworms. Using an advanced molecular microscopy technique, the scientists found drilodefensins in the guts of 14 different earthworm species but not in other closely related invertebrates such as leeches.
Co-author Dr Dave Spurgeon, from the Centre for Ecology and Hydrology in Wallingford, Oxfordshire, said: "We've established that earthworms, referred to as 'nature's ploughs' by Charles Darwin, have a metabolic coping mechanism to deal with a range of leaf litter diets.
"In this role, drilodefensins support the role of earthworm as key 'ecosystem engineers' within the carbon cycle."
The research is reported in the journal Nature Communications.