"It is only by understanding how corals form their skeletons that we can understand and perhaps better predict how they will evolve as oceans become more acidic (as a result of climate change)," explains team leader Pupa Gilbert at Wisconsin-Madison.

Spherulites are polycrystalline structures in which acicular (or lath-like) crystals (also called fibres) radially grow out from a common centre to form a spherical-shaped structure. There are two types of spherulites: spherical spherulites in which the crystal fibres start from a point; and feather-like or plumose spherulites in which crystal fibres radiate out at an angle from a line (see image).

Model species for studying how coral calcifies

Gilbert and colleagues studied the skeleton of Stylophora pistillate coral – whose fibres are made of aragonite (CaCO3). S. pistillate is abundant in the Indo-Pacific and the Red Sea, and is a commonly used model species for studying how coral calcifies. They mapped the crystal orientations in the coral using a technique called polarization-dependent imaging contrast (PIC)-mapping that Gilbert developed a few years ago in her lab. The technique makes use of synchrotron light (in this case from the Berkeley-ALS) and a spectromicroscope (a photoemission electron spectromicroscope or PEEM in this case). It can provide detailed quantitative information about how the crystallographic c-axes of carbonate materials are oriented in 3D.

The researchers made measurements on coral skeletons that they had embedded in a substrate, polished and coated. They PIC-mapped them with various fields of view (from 10 to 60 microns) and with a spatial resolution down to just 10 nm.

Aragonite grows anisotropically

“We found that the corals grow their skeletons in bundles of aragonite crystals, with their c-axes and long axes oriented radially, thus precisely as expected for plumose spherulites,” Gilbert tells nanotechweb.org. “Interestingly, aragonite grows anisotropically, and 10 times faster along the c-axis than along the a-axis direction. Spherulites therefore fill the space they grow into faster than any other aragonite growth geometry, something that leads to an isotropic material being created from anisotropic crystals.”

This greater filling rate and the mechanical properties of the finished isotropic structure are key to the skeleton’s supporting function and therefore to its evolutionary success, she adds. “In this sense, spherulitic growth is nature’s 3D printing.”

According to the researchers, this type of growth might even be exploited in man-made 3D printing using aragonite spherulites, which can grow and fill space up to 1000 times faster than regular aragonite crystals.

The team, reporting its work in ACS Nano DOI: 10.1021/acsnano.7b00127, says that it would now like to better understand the mechanisms behind coral skeleton growth not only in S. pistillate but in other species of coral as well.