The scaffolds that help hold together the world’s tropical reefs are at riskfrom acidification due to increased carbon dioxide in the world’s oceans,according to geoscientists at the University of Sydney.

Extensive sampling of the Great Barrier Reef fossil record has shown that thecalcified scaffolds that help stabilise and bind its structure become thin andweaker as pH levels fall.

Scientists have seen incidental evidence for this in the past, but a new studyled by Zsanett Szilagyi of the Geocoastal Research Group at the University ofSydney has shown that this is a global process, affecting reefs worldwide.

The research is published this week in Marine Geology.

“For the first time we have comprehensively shown that the thickness of thisgeologic ‘reef glue’ correlates with changes in ocean pH and dissolved carbondioxide,” said Ms Szilagyi.

The thickness of these crusts can now be regarded as a reliable indicator ofocean acidification going back tens if not hundreds of thousands of years.

“We haven’t had such a complete and high-resolution record before. And thisgeologic study shows that as oceans became more acidic, this is reflected inthe thickness of these reef crusts,” said Associate Professor Jody Websterfrom the School of Geosciences, who coordinated the study.


The ‘reef glue’ is made up of calcified deposits from microbes that livewithin reef formations around the world. Known as microbialites, thesestructures play an important role in many types of aquatic systems and areprobably best known from the ancient stromatolites that are built bycyanobacteria in Western Australia, which are billions of years old.

In some types of reef systems, including the Great Barrier Reef, microbialitecrusts likely formed by sulfate-reducing bacteria stabilise and bind the reefframework, forming a robust scaffold that can be used by corals and other reefbuilders to colonise and grow.

In the past these crusts have been more abundant than the corals and algaethat grow on and around them and they display variations in thickness overtime, while still performing their structural role.

“This means they are really good indicators of changes in environmentalconditions of our oceans,” Associate Professor Webster said.

The study found a variation in thickness from 11.5 centimetres 22,500 yearsago to about 3 centimetres in younger Great Barrier Reef sections, about12,000 years ago.

When combined with studies from 17 reef systems worldwide, the data shows thisthinning of the microbialite crusts coincides with pH dropping below 8.2 rightup to modern times.


The researchers gathered a dataset of microbial crusts from the Great BarrierReef as far back as 30,000 years. They compared a comprehensive three-dimensional analysis of samples to two-dimensional scans of the crustthickness.

The results from the Great Barrier Reef show that the two-dimensional analysisof crust thickness provides an accurate proxy for the more detailed three-dimensional method. Compiling 2D sample data from across the world, thescientific team built a global model of microbialite thickness through time.

The study found that the 2D technique gave results within 10 percent of the 3Danalysis.

“A real breakthrough here is that we are confident we can now apply a 2Danalysis to reefs and obtain reliable information about the history ofmicrobialite formations. This will give us substantial savings in time andresources,” Associate Professor Webster said.

“Previous studies have given us glimpses as to how these microbial crustsrespond to changes in their environment. What is new in our study is that wemeasured more than 700 well-dated microbialite samples from the InternationalOcean Discovery Program on the Great Barrier Reef and combined this with ameta-analysis of 17 other reef records from around the world,” he said.

“This allowed us to assess global-scale changes in microbialite developmentover the past 30,000 years. And, frankly, the findings are a stark warningsign for the dangers of rapid acidification of oceans.”

The study argues that in the present-day context of rapid global climatechange, changes in dissolved carbon dioxide, pH and temperature, could lead toreduced microbial crust formation, thereby weakening reef frameworks in thefuture.

The research was a global effort, with contributions from Eötvös LorándUniversity in Hungary, University of Tennessee in the US, Nagoya Universityand University of Tokyo in Japan, Goethe University in Germany, Aix-MarseilleUniversity in France, National Institute for Space Research in Brazil and theUniversity of Granada in Spain.

Image: Aerial view of Great Barrier Reef. Photo Pixabay

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