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Earthworms churn out calcite crystals

In 1881, Darwin discovered that earthworms leave behind granules of calcite in the soil.

Earthworm granules (as seen in this closeup of a Lumbricus terrestris granule) are aggregates of calcite crystals.

Any gardener can tell you that earthworms play a major role in soil ecology. But that information hasn’t always been common knowledge. Charles Darwin was one of the first to study earthworms, and in 1881, he discovered the curious fact that many species leave behind calcite crystals as they work their way through the soil. Now, new research might shed some light on the enduring mystery of how and why earthworms produce the crystals.

“Darwin observed that the crystals formed, but no one has looked closely at their mineralogical composition before,” says Mark Hodson, a geochemist at the University of Reading in England and co-author of a new study published in Geology. “We found the crystals are composed of finely zoned calcite wrapped around a central grain of quartz.”

Hodson and colleagues analyzed scanning-electron-microscope images and found that the calcite starts out in the worm as amorphous calcium carbonate and is then converted into crystalline calcium carbonate, or calcite, before it is excreted. “Amorphous calcium carbonate is thermodynamically unstable,” Hodson says. “So it’s not clear if the earthworm is intentionally converting the amorphous stuff to calcite or whether it’s just thermodynamics taking its toll. It all ties back into why this occurs in the first place, which we don’t yet know.”

“There are a number of theories,” Hodson says. “But each has proved unsatisfactory in one way or another.” Earthworms may use the calcium carbonate to regulate the pH or carbon dioxide levels in their intestines, or perhaps they inadvertently ingest large quantities of calcium carbonate as they move through the soil and the crystals are the most efficient way of getting rid of excess calcium, which can be toxic in high concentrations, he says.

What is known is that calcite granules are produced by most species of earthworms in many different soils. Laboratory studies have shown that an individual worm can produce 2.2 milligrams of calcite per day, or as many as 30,000 granules per year. And soil studies have recorded as much as 56 kilograms of earthworm-produced calcite per hectare of soil. “These worms produce a tremendous amount of calcite. It’s a wonder we’re not knee deep in the stuff,” says Matt Canti, a geoarchaeologist with the English Heritage Commission in England, who has investigated the possibility of using earthworm-derived calcite crystals to radiocarbon date ancient soils. Such techniques are possible, he says, but difficult and very expensive.

“There’s a lot to be discovered about earthworms,” Canti says. Not only are the crystals themselves still a biological mystery, he says, but the role such massive loads of calcite play in soil ecology and the carbon cycle is still largely unknown. Hodson plans to tackle the ecology angle next by studying how soil type affects the crystalline structure and composition of the granules.

“Not many people know how interesting earthworms are,” Hodson says. “But [when] working with worms, more and more fascinating questions in all sorts of diverse fields keep popping up all the time.”

Mary Caperton Morton
Tuesday, February 10, 2009