Osteoporosis in the world's oceans: Bioeroding sponges are threatening coral reefs

The study was published in the online journal PLoS ONE.

Increasing concentrations of carbon dioxide are not only changing the atmosphere, but also the oceans: Carbon dioxide reacts with water and forms carbonic acid, which acidifies the seas. The pH decreases simultaneously with this acidification, having mostly negative effects on many organisms in our oceans.

"Especially organisms that form calcareous skeletons, such as corals, will have to expend more energy in more acidic conditions," states Dr. Max Wisshak from Senckenberg am Meer in Wilhelmshaven. "In contrast -- so our hypothesis -- organisms that bioerode calcareous skeletons by biochemical etching should have an easier job in future and should thus be counted into the small circle of winners of the global change."

To test this hypothesis the team of scientists in collaboration with the Australian Institute of Marine Science and GEOMAR -- Helmholtz Centre for Oceanography in Kiel conducted elaborate experiments at the largest coral reef on earth, the Australian Great Barrier Reef, working with the bioeroding sponge Cliona orientalis. This demosponge is widely distributed, belongs to the most aggressive bioeroders, and will in future very likely contribute even more to the erosion of many coral skeletons in the Great Barrier Reef.

"We were able to confirm a clear relationship between the pH of the seawater and the bioerosion rate of these sponges," says Wisshak. "Our data predict an up to 25 percent increase of sponge bioerosion until end of this century!"

As bioeroding sponges often contribute the lion share of coral bioerosion on tropical reefs, with increasing ocean acidification the reefs will be exposed to a duplicate burden: calcification will be more difficult, and existing skeletons will be more strongly weakened by bioerosion. "This may lead to a shift from present-day positive reef growth to future negative budgets with stronger reef bioerosion. In consequence our reefs would be threatened by yet another stress factor," explains the marine geologist from Wilhelmshaven.

Testing effects of rising sea surface temperatures in the world's oceans -- another consequence of the present global change -- revealed a comparatively small influence on bioerosion rates of the sponges. What the sponges do not mind has a serious outcome for corals: Rising temperatures have a large impact on corals: Heat stress causes "bleaching" in corals, and in consequence they can die in dramatic mass mortality events.

The outcome of severe bleaching events is disastrous as reefs are home to many marine organisms and the basis of a complex food chain -- all the way up to humans.