Researchers have already predicted that a more acidic ocean will make it more difficult for corals to build their calcium carbonate skeletons. The new finding suggests that the reef's broader structure may also suffer because a lower pH reduces the formation of the reef's cement binder. The binder is made from calcium carbonate that precipitates out of ocean water when it rushes through the pores of coral skeletons.
"Until now, we've mostly addressed acidification in terms of what it does to the living organism," said study author Joan Kleypas of the National Center for Atmospheric Research in Boulder, Colo.
"Here we're finding that the reef structure itself can certainly feel the effect of ocean acidification, even if the biology somehow finds a way to cope with acidification. This is mainly an inorganic process, so we're looking at something that will happen regardless of what the biology does."
The researchers made their findings by comparing places around the world where CO2 levels in the ocean vary naturally.
The eastern tropical Pacific Ocean, near the Galapagos Islands, has particularly high CO2 concentrations because it is an upwelling spot where deeper waters, enriched in CO2 by microbial degradation of organic matter down below, rise to the surface.
"The eastern Pacific is in essence a natural laboratory to study how coral reef ecosystems are structured and function under these acidic conditions," said study author Derek Manzello of the University of Miami.
The researchers made measurements of the seawater and the amount of coral cement present in reefs near the Galapagos, and compared those with levels found near the Pacific coast of Panamá, and with those in the Bahamas.
Reefs in the Galapagos had the highest levels of carbon dioxide, which corresponded to water less saturated with carbonate ions -- and only trace amounts of cement on the reefs.