Earthworms' Secret to Eating Dead Leaves Found

Earthworms manage to digest dead plant matter despite plants' toxic defenses. Now scientists know how they do it.

Worms may not be the most photogenic creatures, but they're essential to our planet as we know it. By munching on fallen leaves and other dead plant material, they reduce mounds of matter on the ground and return carbon to the ground, enriching the soil.

Now researchers at Imperial College London have figured out how the worms manage to digest dead plants despite toxic chemicals that deter most other herbivores. Plants make polyphenols, which act as antioxidants and give the plants their color. They also usually block digestion.

The scientists identified molecules in the earthworm's gut that counteract the plant's natural defenses. The molecules, named drilodefensins, allow an earthworm to eat up to one-third its body weight in a single day. The more polyphenols detected in an earthworm's diet, the more drilodefensins it produces in its gut, the researchers found.

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As earthworms burrow into the ground, they eat soil with their mouths, located in their first segment. They extract nutrients from decomposed organic matter, transporting nutrients and minerals from below to the surface through their waste - and their tunnels aerate the ground.

"Without drilodefensins, fallen leaves would remain on the surface of the ground for a very long time, building up to a thick layer," Jake Bundy from the Department of Surgery and Cancer at Imperial, said in a press release. "Our countryside would be unrecognizable, and the whole system of carbon cycling would be disrupted."

So much munching requires a lot of the earthworm's digesting molecule. Manuel Liebeke from Imperial College London estimates that for every person on Earth there are at least 1 kg (2.2 lbs) of drilodefensins present in the planet's earthworms. Even with such a quantity of the molecules, they are still in such high demand that earthworms recycle the molecules to keep on digesting.

The researchers identified the key to the worms' digestion by using modern visualization techniques based on mass spectrometry. Manuel Liebeke from Imperial College London explained that the technology has allowed scientists to zero in on animals' biology like never before.

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"We are now able to locate every molecule in, for example, an earthworm to a specific location. Knowing the location of a molecule can help us to figure out what it actually does," Liebeke said in a press release.

The study was published in today's edition of Nature Communications.

Earthworms may seem like humble subjects, but they are a source of food for many animals, like birds, rats, and toads, and they're often used in composting and as bait in commercial and recreational fishing.

Perhaps most importantly, they serve, in Charles Darwin's words, as "nature's ploughs."

Earthworms in rich organic soil. As they burrow, the worms consume soil in order to remove nutrients from decaying organic matter such as leaves and roots.

The mystery of why leaves take such different shapes is closer to being solved thanks to a new mathematical model that looks at the problem from the perspective of leaf veins. Since plants suck up more of the greenhouse gas carbon dioxide than anything else on the planet, understanding leaf veins is an important part of grappling with the global carbon budget puzzle.

"Across the world leaves take a very large amount of carbon out of the atmosphere each year," said Ben Blonder a doctoral student at the University of Arizona. Leaves absorb more than the oceans and about 10 times more than the amount humans put into the atmosphere. "To understand the carbon flux, you have to understand how leaves work, Blonder told Discovery News. "But not all leaves work the same."

There are basically three things at play in the workings of a leaf: the amount of carbon required to make it, how long the leaf lives and how fast or slow it processes sunlight -- or its rate of photosynthesis. These factors combine in different ways in different plants in different environments to create an incredible diversity of leaf shapes and structures. And veins are the basis of it all. "The really surprising thing is that these things relate to each other in ways that don't change across the globe," Blonder said.

Blonder developed a mathematical model to predict how leaves are balancing these three factors to best serve a plant, using three properties seen in the vein networks of leaves: density, distance between veins and the number regions of smaller veins that resemble capillaries in humans, referred to in this case as loops.

Vein density is a sign of how much a leaf has invested in the network. The distance between the veins is a measure of how well the veins are keeping the leaf supplied with water and nutrients. The number of loops shows how resilient a leaf is and is related to how long a leaf lives, since loops provide ways to re-route supplies in the case a leaf gets damaged.

Veins tell you a lot about a plant. For example, if a plant opens its leaf pores, called stomata, to absorb more carbon dioxide for photosynthesis, the leaf also loses a lot of water to evaporation. That requires lots of plumbing in the leaves to pipe in the water. That, in turn, means lots of big veins. If a plant requires lots of water all the time, it could favor certain geometrical arrangements of veins, which starts to suggest overall leaf shapes. So it's the veins -- the skeleton of the leaf -- which determine whether you will have a classic maple shape or a blade-like willow.

"Veins do all sorts of things," said Blonder. They provide structural support, resist damage, transport nutrients and even help send chemical signals to the plant, similar to nerves in an animal. "There are trade-offs for leaf patterns," he added. "What we've been able to do is synthesize these things so they all make sense on one big picture."

Blonder tested his model -- predicting relationships among photosynthesis rates, leaf lifespan, carbon cost and even nitrogen costs -- on more than 2,500 species worldwide. It worked. Then he and undergraduates assistants Lindsey Parker, Jackie Bezinson and David Cahler tested 25 leaves from the University of Arizona campus. Their initial results suggest the model works on a local scale, although they are expanding their tests to study leaves from species at the Rocky Mountain Biological Laboratory in Colorado. Blonder and his students have published their work on veins in the journal Ecology Letters.

Ultimately, a good understanding of leaves will become incorporated into climate models. This can help not only balance the carbon budget, but also predict evaporation rates and other weather and climate-related matters that are heavily reliant on plants. "It's of fundamental importance to understand how plants relate to global carbon cycles," said Blonder.