Sea shells' beauty and peril

There's good news and bad news about sea shells. Studies of details as small as a billionth of a meter reveal a structure more sophisticated than any marine biologist had imagined. At the same time, a close look at what increasing carbon dioxide (CO2) concentration in the air is doing to the sea foreshadows a deadly threat.

Seawater becomes more acidic as it absorbs CO2. This gradually depletes the concentration of carbonate from which sea creatures build those marvelous shells. A new study predicts that large parts of the ocean may lack this essential nutrient within a few decades.

Professors Mary Boyce and Christine Ortiz and graduate student Benjamin Bruet at the Massachusetts Institute of Technology (MIT) in Cambridge are finding that the secrets of sea-shell strength lie in details so small that centuries of microscopic analysis have not revealed them. You have to look at how the shells' brittle calcium carbonate ceramic is put together on a structural scale measured in nanometers (billionths of a meter) - a feat that 21st-century technology makes possible.

They've been looking at the mother-of-pearl inner layer of mollusk shells in their laboratory at MIT's Institute for Soldier Nanotechnologies. They see millions of ceramic plates a few thousand nanometers across stacked up and glued together by a biological polymer.

Dr. Ortiz explains that the way the brittle carbonate is "designed," from macroscopic to microscopic levels, is what gives it strength and toughness. However, she adds that "understanding how the material is designed and functions at the smallest-length scales will be critical to learning how to create tough [synthetic materials]."

The structures are complex. For example, each of those millions of plates is divided into sectors like a sliced pie, according to the MIT report. Cylindrical beams run through the plates to give added strength. The hope is to mimic such design to give soldiers better body armor.

Materials scientists should find these clues while they can. James C. Orr and colleagues at the Laboratory of Climate and Environmental Sciences in Gif-sur-Yvette in France cast a shadow on sea shells' future in today's issue of Nature. They ran computer models showing how ocean water chemistry is affected by CO2 from the atmosphere. It makes the water more acidic and lowers the concentration of calcium carbonate marine animals use to make their shells.

The models project that: "Southern Ocean waters will begin to become undersaturated with respect to aragonite, a form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean."

Dr. Orr's team immersed pteropod snails in such aragonite undersaturated water. They report that the shells dissolved markedly within 48 hours. It would be hard for such creatures to form shells in the first place in this nutrient-poor water. The scientists note that this could also affect coral reefs. They warn, "Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries, as suggested previously."

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