A species of coral has been found to build its calcium carbonate skeleton in an unexpected manner. The method not only allows for faster skeleton construction, but may provide protection against falling marine pH levels, known as ocean acidification. It is not known whether all coral species use the same method, but if this is widespread, the future of coral reefs may be a little less grim than previously thought.
“Coral reefs only cover one percent of ocean floors, but they host 25 percent of all marine species, so they're incredibly diverse and important from a biological point of view," said Professor Pupa Gilbert of the University of Wisconsin-Madison in a statement.
Gilbert reports in Proceedings of the National Academy of Sciences that Stylophora pistillata (hood coral) puts together pieces of calcium carbonate inside the coral's tissue, before adding it to their skeleton.
Biologists have been unsure whether corals grow through molecule-by-molecule additions of calcium carbonate, or by forming pieces of amorphous calcium carbonate particles that are subsequently used to build the skeletons.
Gilbert used high-powered X-ray light from the Berkeley Laboratory Advanced Light Source to view the molecules at the surfaces of the coral skeletons. She found unattached particles as large as 400 nanometers across – tiny by human standards, but 500 times the size of a single carbonate group. These particles attached to the ends of the coral skeleton, before gradually crystallizing over a period of several hours into aragonite (CaCO3) – from which the coral skeletons are formed – and becoming part of the coral structure.
From an evolutionary perspective, it makes sense that hard corals grow in this way, since it makes much more rapid growth possible. When building synthetic argonite structures, Gilbert found that the particle method was 100 times faster than molecule-by-molecule building. Moreover, it’s also thought to be the method used by sea urchins. While corals and urchins are widely separated on the evolutionary tree, the fact that it works for one species indicates its effectiveness.
The most exciting part of the work is that crystallization within tissues, where the chemistry can be controlled, offers some protection against acidic conditions in the wider ocean. In an era where rising carbon dioxide levels are producing acidification, this could be a lifeline for reefs. If so, it would explain how corals survived previous high-CO2 eras, but not why they have proven so vulnerable to acidic conditions in testing.
Even if resistance to acidity is possible, reefs face the global threat of rising temperatures, the primary cause of recent mass bleaching events, and more localized threats such as overfishing and nutrient run-off. Still, better to face two threats than three.