Ocean acidification and its consequences for marine life pose several pressing problems that are of concern to scientists all over the world. One alarming scenario is that stony corals may lose the ability to produce calcium carbonate skeletons and may no longer be able to form reefs with their structures that offer protection and habitats for an extraordinary variety of species. But how likely is such a development?
In order to make informed predictions, scientists must first understand the processes of skeletal development in single organisms and how these processes are impacted by perturbation factors. Researchers of the Leibniz Center for Tropical Marine Ecology (ZMT) in Bremen have developed a mathematical model that simulates in detail the calcification of coral polyps on the organism level. “With such a model, various scenarios of ocean acidification can be simulated and their effects on stony corals examined,” says the ecologist Sönke Hohn.
Atmospheric CO2 dissolves in seawater and forms carbonic acid. Rising carbon dioxide concentrations ultimately cause seawater to acidify. While today the surface water of the oceans has a pH of about 8.2, it is expected that this value will decrease to 7.8 by the year 2100. An acidic environment dissolves calcium carbonate. It has been known for some time that increasing CO2 levels reduce the ability of many marine organisms to produce their calcium carbonate skeletons. A study published in Science not long ago showed images of "naked" coral polyps that were exposed to pH values of 7.4 and received global attention. However, thus far it has remained unclear how the calcification process works under perturbed conditions. Scientists in Bremen have elucidated this process in more detail.
At the underside of their skin, polyps secrete calcium carbonate thus creating skeletal structures. All polyps of a coral colony are connected with each other via a layer of tissue. This separates the surrounding seawater from the fluid in which the calcareous skeleton is formed. Using a mathematical model, the scientists from the ZMT were able to prove that CO2 penetrates through the layers of tissue into the calcifying fluid. This suggests that despite the fact that corals actively regulate ion transport mechanisms, an increase in CO2 levels makes them unable to fight against CO2 diffusion.
The model describes the complex biochemical processes of ion exchanges and calcification in four compartments: the surrounding seawater, the polyp tissue, the stomach, and the calcifying fluid beneath the polyp. The model was based on data from various studies. But in particular a series of experiments carried out at the ZMT in the past were used, in which pH and calcium ion concentrations were measured below the polyp tissue using microprobes.
“Not even sophisticated laboratory experiments are sufficient to completely understand the process of calcification" says Agostino Merico, who helped to develop the modelling approach. “By simulating all relevant physiological processes, our model elucidates the complex processes and quantifies the effects of ocean acidification at the organism level." Thus, calculations with increased CO2 levels in the atmosphere, as they are expected in a worst-case scenario in 2100, would result in a 10% reduction of the calcification rates in stony corals. In a next step, the combined impact of ocean warming and acidification will be studied using the model.
Hohn, S., Merico, A. (2012) Modelling coral polyp calcification in relation to ocean acidification, Biogeosciences, 9, 4441-4454. doi: 10.5194/bg-9-4441-2012.
Dr. Sönke Hohn
Leibniz-Zentrum für Marine Tropenökologie
Tel: 0421 - 23800 105