Ocean acidification and observations – ITW Alexander Venn

© F. Benzami / Fondation Tara Expéditions

The oceans absorb a fourth of anthropic CO2 emissions released every day into the atmosphere, leading to changes in sea water chemistry and ocean acidification as a result of a decrease in the pH value. Coral, along with other calcified species, is directly affected by this acidification process. We met Alexander Venn, a research fellow at the Scientific Center of Monaco, at the National Taiwan Ocean University during Tara’s stopover in Keelung (Taiwan, China). He studies coral calcification and pH control. An opportunity to review with him a few observations on acidification and its effects.


© Ocean and Climate Platform


We have been talking since the 1990s about ocean acidification, can you explain what it is exactly?

Ocean acidification is a process where carbon dioxide that has been released into the atmosphere is making the ocean slightly more acidic. But the ocean is not actually “acidic”, it’s just becoming less alkaline because the seawater pH is going down. And that’s because carbon dioxide is an acidic gas and it’s dissolving into the ocean. We estimate that the acidity of the oceans has increased by 30% since the beginning of the industrial revolution. When scientific research into ocean acidification began in earnest about 20 years ago, people were already worried because the decreasing pH of water could affect corals. And now we know for certain that, in reaction to ocean acidification, many corals build their skeletons more slowly.

But we have also discovered that corals are actually quite good at controlling their internal pH. This means that corals can actually regulate their internal pH quite well. When we see a decreasing pH in seawater, we see less of decrease in pH inside the coral, where it’s calcifying. For all organisms, including all animals and plants, a very important part of their physiology is controlling internal pH. Indeed, we too must control the pH of our blood, because if our blood pH goes up or down too much we die.


ocean-climate-climate-change_3© Ocean and Climate Platform


Is this discovery a good news or bad news?

The good news is that corals are maybe a bit stronger than we thought, but the problem is their internal pH still decreases when the seawater acidifies and this results in impacts on calcification. We also hypothesize that corals have to invest more energy in regulating their internal pH when the seawater pH is decreases. And if they are diverting their energy to regulating pH, maybe they have less energy for other essential life processes.


What processes are concerned?

Calcification itself! Because we know that as the seawater pH goes down and the corals try to adapt fight to control their internal pH, some corals actually produce a more porous and potentially a more brittle skeleton, certainly with larger holes in it. Let’s say that the calcium carbonate of a coral skeleton is like cement or bricks for a house. These bricks become more expensive to produce, so if you compare the coral skeleton to a house, it will build a house with bigger corridors, bigger rooms, bigger windows, but then the walls are thinner. So, in a storm, the structure is less resistant.

Stylophora pisillata - Pistillate coral© Scott Mills / Wikimedia Commons

That’s our worry. And we have shown that process occurs with one important species of coral, that we have here in Taiwan: Stylophora pistillata. It’s a branching coral and this is an important coral found throughout the Pacific, the Red Sea and the Indian Ocean. It is also worth noting that Stylophora pistillata is quite a resistant species of coral and that although all coral species can control their internal pH, they probably vary in their ability to do this. By that I mean that some corals are more vulnerable to ocean acidification than other species.

We need to carry out further research on more coral species to understand this.


Interview by Noëlie Pansiot


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