Posted by jlwile on August 15, 2012
A few months ago, I discussed the acidification of the ocean. It is often called global warming’s “evil twin,” because it is caused by rising carbon dioxide levels in the atmosphere. Unlike global warming, however, the connection between carbon dioxide levels in the atmosphere and increasing ocean acidity is straightforward and has been confirmed by many observations. Thus, while it is not clear that increased levels of atmospheric carbon dioxide will lead to global warming, it is very clear that increased levels of atmospheric carbon dioxide lead to an increase in the acidity of the ocean.
The question is, “How will increased ocean acidity affect the organisms living there?” Many who call themselves environmentalists answer that question by saying increased ocean acidification will produce catastrophic results, threatening many species of ocean life. The reason? Many organisms that live in the ocean have shells made out of calcium carbonate. To make those shells, the organisms use carbonate ions that are dissolved in the seawater. However, as the acidity of ocean water increases, the concentration of carbonate ions in the water decreases. Thus, it is thought that increased ocean acidification will make it harder for these organisms to make their shells. Here’s how one publication from the National Academies puts it1
As ocean acidification decreases the availability of carbonate ions, these organisms must work harder to produce shells. As a result, they have less energy left to find food, to reproduce, or to defend against disease or predators. As the ocean becomes more acidic, populations of some species could decline, and others may even go extinct.
Now if that’s true, ocean acidification is a major problem. Indeed, if several shell-making organisms go extinct, we could be in real trouble.
However, this is a very simplistic way of looking at things. Yes, the availability of carbonate in the ocean will affect how easily shell-making organisms produce their shells. However, there are a host of other factors involved in the process. To single out one factor without considering the others is not very scientific. When all the factors are considered, the picture is not nearly as bad.
Consider, for example, coccolithophores. These single-celled algae are very important to ocean ecosystems because they perform photosynthesis. Thus, they not only affect the amount of oxygen that is in the atmosphere, they are also a very important food source for many ocean-dwelling organisms. The pictures at the top of this post are of a specific type of coccolithophore: Emiliania huxleyi. This species is rather important, as it is the most abundant species of coccolithophore in the ocean today.2 Obviously, if this species were to go extinct, we could be in for some serious problems. Why are some worried that it might go extinct? Look at the pictures. Those pretty designs on the coccolithophores are calcium carbonate plates. If the publication I quoted above is correct, these coccolithophores could be threatened with extinction.
Before I go on, however, notice that the two individuals in the picture look rather different. They are both from the same species (Emiliania huxleyi), but they appear different because their calcium carbonate plates are different. The one on the left used a lot of calcium carbonate to make its plates, and the one on the right used much less calcium carbonate to make its plates. As a result, the one on the left is called a “heavily calcified” version of the species. We don’t really know why some members of this species are heavily calcified and others are not. We just know that both types exist.
Because Emiliania huxleyi is such an important coccolithophore, and because there are different levels of calcification within the species, scientists have studied how it reacts to increased acidity. It was thought that the heavily-calcified version of the species should be less likely to survive conditions of increased acidity, because there is less carbonate with which to make plates. Usually, the studies done to test this idea involve taking the coccolithophores out of the ocean and exposing them to different levels of acidity in the lab. As I discussed in my previous post on the subject, it is not clear that this is a reasonable test of an organism’s reaction to ocean acidification. It is not surprising, then, that these studies have produced contradictory results.
A recent study was substantially more realistic. Rather than seeing how the coccolithophores respond to artificial changes in water acidity, the authors examined how the algae reacted to natural changes in acidity. They sampled the Emiliania huxleyi populations in the Bay of Biscay once a month for more than a year. Since the acidity of the bay’s water changes naturally throughout the year, it is an excellent test of how the coccolithophores respond to changing acidity in their natural environment.
The results were unexpected, to say the least. The heavily-calcified version of the species made up less than 10% of the population during the summer months, when the acidity was at its lowest. On the other hand, the heavily-calcified version made up more than 90% of the population in the winter months, when the acidity was at its highest!3 The authors are quick to make it clear that they don’t think the increased acidity was the cause of the heavily-calcified form becoming more predominant. Instead, they considered several explanations, most of which they rejected. The one they seem to like the most is the idea that the heavily-calcified individuals are more hardy and are therefore better at surviving through the winter.
Obviously, the explanation for these results cannot be determined without more research. However, the obvious conclusion is that despite increased acidity, Emiliania huxleyi had no problem increasing the amount of calcium carbonate in its plates. As a result, the idea that increasing ocean acidification will harm shell-forming organisms is overly simplistic. There are many factors at play in ocean chemistry, and as this study shows, singling out just one of those factors leads to errant conclusions.
In other words, It’s not that simple! The earth is incredibly well-designed and contains multiple negative feedback systems that tend to decrease the effect of changes in just one or two environmental variables. Until scientists start looking at the earth for what it is, an intricately designed and complex system, such overly-simplistic thinking will hamper our ability to understand it properly.
2. Biomineralization, Edmund Bäuerlein (Ed), Wiley VCH 2004, p. 197.
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3. Helen E. K. Smith, et. al., “Predominance of heavily calcified coccolithophores at low CaCO3 saturation during winter in the Bay of Biscay,” Proceedings of the National Academy of Sciences of the United States of America 109(23):8845-8849, 2012.
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