Earthworms produce a unique class of compounds in their gut that provide protection against the damaging effects of plant chemical defences, finds a study in Nature Communications. These compounds act as surfactants, lowering the surface tension between compounds or disrupting their chemical properties, in a similar way to dishwashing liquid and other cleaning fluids and present a key adaptation to the challenge of recycling plant litter in the soil.
In order to deter aboveground herbivores from feeding on them, plants produce defensive protein-binding chemicals called polyphenols, which may inhibit the action of gut enzymes after ingestion. A consequence of this defensive strategy is that these chemicals are retained in leaf litter, presenting a dietary challenge for decomposer organisms like earthworms that feed below ground. Manuel Liebeke and colleagues use a variety of techniques to analyse the chemical composition of earthworm gut fluid after ingestion of plant polyphenols, visualising where in the digestive system these chemicals were most active. They identify a group of previously undescribed surface-active metabolites localised in the gut, which they name 'drilodefensins' after the Latin name for the order of invertebrates containing earthworms: Megadrile.
They find that these compounds are present in the guts of 14 different earthworm species, but not in other closely related invertebrate groups like leeches and sewage worms, suggesting they are unique to this group. Field populations of earthworms increase the concentration of these surfactant metabolites when fed a diet rich in polyphenols, indicating that it plays a key ecological role in the global turnover of an estimated 10 billion tonnes of plant carbon per year.