Emma Versteegh

A new paper in Applied Geochemistry: Carbon isotope fractionation between amorphous calcium carbonate and calcite in earthworm-produced calcium carbonate

• Earthworms produce granules of calcium carbonate that form from an amorphous calcium carbonate suspension.
• The microspherulites of amorphous calcium carbonate coalesce and recrystallize.
• Fractionation of C isotopes occurs as the ACC recrystallizes with ε_calcite-ACC = −1.20 ± 0.52%.
• This is consistent with a dissolution-reprecipitation pathway rather than solid state rearrangement.
• This may be important for the interpretation of CaCO₃-based C isotope environmental proxies.

In this study we investigate carbon isotope fractionation during the crystallization of biogenic calcium carbonate. Several species of earthworm including Lumbricus terrestrissecrete CaCO₃. Initially a milky fluid comprising micro-spherules of amorphous CaCO₃(ACC) is secreted into pouches of the earthworm calciferous gland. The micro-spherules coalesce and crystalize to form millimetre scale granules, largely comprising calcite. These are secreted into the earthworm intestine and from there into the soil. L. terrestriswere cultured for 28 days in two different soils, moistened with three different mineral waters at 10, 16 and 20 °C. The milky fluid in the calciferous glands, granules in the pouches of the calciferous glands and granules excreted into the soil were collected and analysed by FTIR spectroscopy to determine the form of CaCO₃ present and by IRMS to determine δ¹³C values. The milky fluid was ACC. Granules removed from the pouches and soil were largely calcite; the granules removed from the pouches contained more residual ACC than those recovered from the soil. The δ¹³C values of milky fluid and pouch granules became significantly more negative with increasing temperature (p ≤ 0.001). For samples from each temperature treatment, δ¹³C values became significantly (p ≤ 0.001) more negative from the milky fluid to the pouch granules to the soil granules (−13.77, −14.69 and −15.00 respectively at 10 °C; −14.37, −15.07 and −15.18 respectively at 16 °C and −14.89, −15.41 and −15.65 respectively at 20 °C). Fractionation of C isotopes occurred as the ACC recrystallized to form calcite with the fractionation factor ε_calcite-ACC = −1.20 ± 0.52‰. This is consistent with the crystallization involving dissolution and reprecipitation rather than a solid state rearrangement. Although C isotopic fractionation has previously been described between different species of dissolved inorganic carbon and various CaCO₃ polymorphs, this is the first documented evidence for C isotope fractionation between ACC and the calcite it recrystallizes to. This phenomenon may prove important for the interpretation of CaCO₃-based C isotope environmental proxies.

Typical FTIR spectra for milky fluid, a granule recovered from one of the pouches (pouch granule), and a granule recovered from the soil (soil granule); all spectra relate to samples from the same earthworm. Reference spectra for synthetic calcite and ACC are also shown. Spectra are vertically offset on the absorbance axis for clarity. The major calcium carbonate peaks (υ₁ to υ₄) are labelled. Calcite shows peaks at ∼714 cm⁻¹ (υ₄), ∼866 cm⁻¹ (υ₂), ∼1090 cm⁻¹ (υ₁) and 1420–1470 cm⁻¹ (υ₃); amorphous calcium carbonate lacks the ∼714 cm⁻¹ peak.

Shrimp called Rimicaris hybisae at deep hydrothermal vents in the Caribbean seem to have different dietary habits depending on the proximity of other shrimp. Those who live in dense clusters like this one live off bacteria primarily, but in areas where the shrimp are distributed more sparsely, the shrimp are more likely to turn carnivorous, and even eat each other. Credit: Courtesy Chris German, WHOI/NSF, NASA/ROV Jason ©2012 Woods Hole Oceanographic Institution

At one of the world’s deepest undersea hydrothermal vents, tiny shrimp are piled on top of each other, layer upon layer, crawling on rock chimneys that spew hot water. Bacteria, inside the shrimps’ mouths and in specially evolved gill covers, produce organic matter that feed the crustaceans.

Scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, are studying this mysterious ecosystem in the Caribbean to get clues about what life could be like on other planetary bodies, such as Jupiter’s icy moon Europa, which has a subsurface ocean.

“For two-thirds of the Earth’s history, life has existed only as microbial life,” said Max Coleman, senior research scientist at JPL. “On Europa, the best chance for life would be microbial.”

The particular bacteria in the vents are able to survive in extreme environments because of chemosynthesis, a process that works in the absence of sunlight and involves organisms getting energy from chemical reactions. In this case, the bacteria use hydrogen sulfide, a chemical abundant at the vents, to make organic matter. The temperatures at the vents can climb up to a scorching 750 degrees Fahrenheit (400 degrees Celsius), but waters just an inch away are cool enough to support the shrimp. The shrimp are blind, but have thermal receptors in the backs of their heads.

“The overall objective of our research is to see how much life or biomass can be supported by the chemical energy of the hot submarine springs,” Coleman said.

Hydrogen sulfide is toxic to organisms in high concentrations, but the bacteria feeding the shrimp need a certain amount of this chemical to survive. Nature has worked out a solution: The shrimp position themselves on the very border between normal, oxygenated ocean water and sulfide-rich water so that they and the bacteria can coexist in harmony.

“It’s a remarkable symbiotic system,” Coleman said.

Coleman was part of a team led by Chris German at the Woods Hole Oceanographic Institution, in Woods Hole, Massachusetts, that discovered these vents in 2009, off the west coast of Cuba. This research, funded under NASA’s Astrobiology Science and Technology for Exploring Planets program, detected the vents by picking up the chemical signals of their plumes of water in the ocean.

The researchers returned in 2012 on the RV Atlantis with a robotic vehicle called Jason, supported by the National Science Foundation. Scientists collected extensive specimens from two hydrothermal vent fields: The Von Damm field at 7,500 feet (2,300 meters) and Piccard at more than 16,000 feet (4,900 meters), which is the world’s deepest.

Coleman and collaborator Cindy Van Dover, marine biologist at Duke University, Durham, North Carolina, examined the shrimp for the first time when the same team returned in 2013 on the RV Falkor, provided by the Schmidt Ocean Institute in Palo Alto, California. Van Dover returned soon after using the robotic vehicle Hercules aboard the Exploration Vessel Nautilus, and did more collections and studies.

A bonus finding from studying this extreme oasis of life is that some of the shrimp, called Rimicaris hybisae, appear to be cannibalistic. The researchers discovered that when the shrimp arrange themselves in dense groups, bacteria seem to be the main food supplier, as the shrimp likely absorb the carbohydrates that the bacteria produce. But in areas where the shrimp are distributed more sparsely, the shrimp are more likely to turn carnivorous, eating snails, other crustaceans, and even each other.

Although the researchers did not directly observe Rimicaris hybisae practicing cannibalism, scientists did find bits of crustaceans in the shrimps’ guts. And Rimicaris hybisae is the most abundant crustacean species in the area by far.

“Whether an animal like this could exist on Europa heavily depends on the actual amount of energy that’s released there, through hydrothermal vents,” said Emma Versteegh, a postdoctoral fellow at JPL.

The group received funding for shrimp-collecting expeditions from NASA’s Astrobiology Science and Technology for Exploring Planets (ASTEP) program, through a project called “Oases for Life.” That name is especially appropriate for this investigation, Coleman said.

“You go along the ocean bottom and there’s nothing, effectively,” Coleman said. “And then suddenly we get these hydrothermal vents and a massive ecosystem. It’s just literally teeming with life.”

This research was conducted in collaboration with the Woods Hole Oceanographic Institution and Duke University. The Schmidt Ocean Institute provided technical and financial support for marine and underwater robotic operations during the 2013 RV Falkor cruise. The California Institute of Technology in Pasadena manages JPL for NASA.

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