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Scientists studying the mineralized adhesive oysters use to tenaciously fuse together into clusters have determined that the material is chemically distinctive from that of other marine creatures, such as mussels and barnacles. The discovery, reported yesterday at the American Chemical Society national meeting in Boston, has inspired the research team to prepare synthetic mimics that could be used to replace surgical sutures or as marine antifouling coatings. The research is also contributing to efforts to help reintroduce oysters to native habitats.
Oyster reefs once were a dominant influence on Atlantic Ocean and Gulf of Mexico coastal ecosystems, noted Jonathan J. Wilker of Purdue University. The reefs help protect coasts from erosion during storms, provide habitat for other organisms, and filter large volumes of water, he said. Until the mid-1800s, intertidal oyster reefs were miles long and tens of feet deep, but now only about 2% of native reefs remain because of overfishing, pollution, and disease.
"We wanted to understand how oysters can attach to each other to construct these reef communities," Wilker said. "Surprisingly, very little is known about the adhesive materials."
With collaborator Paul D. Kenny of the University of South Carolina's Baruch Marine Field Laboratory, Wilker's group collected clusters of oysters, cut them into pieces to access the adhesive, and examined the interfaces between conjoined shellfish both visually and with fluorescence microscopy (J. Am. Chem. Soc., DOI: 10.1021/ja104996y).
The team isolated samples of the adhesive and found that it's an intractable material that doesn't dissolve in strong acid or with strong chelating agents, which made it a challenge to characterize. The researchers studied the adhesive by thermogravimetric analysis and by infrared spectroscopy and electron paramagnetic resonance spectroscopy.
The adhesive contains a mix of aragonite and calcite crystal forms of calcium carbonate, whereas the oyster shell is predominantly calcite, Wilker reported. The oyster adhesive also contains cross-linked phosphorylated protein similar to mussel and barnacle adhesives, but the oyster adhesive stands out for having a higher proportion of inorganic material and much less water. The adhesive that mussels and barnacles use to attach to surfaces is more like an organic, hydrated glue, Wilker said, whereas the oyster adhesive is more like mortar holding together bricks.
Wilker's group has made synthetic organic-inorganic composite mimics that have potential use as high-strength adhesives. The samples made so far are stronger than commercial adhesives in underwater tests, he said.
Scientists knew that young oysters settle on and attach to the shells of living adult oysters and start to grow, "but we never gave it any thought about how these animals maintain shell adhesion throughout their lifetimes," said Clemson University zoologist Andrew S. Mount, an expert in the cellular mechanisms of biomineralization and biofouling in marine organisms.
Mount recalled an oyster-collecting trip in which Wilker first showed him and Kenny how two adult oysters can fuse together along their entire lengths. "Wilker's contribution has shed light on how reef-building oysters fuse their shells together," Mount told C&EN. "This is a significant finding because oyster reefs have to survive frequent storm-generated high wave energies and therefore must have evolved the capability to build and maintain living reef structures."
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