Sea Stars on Acid

As an ecologist working in temperate climes, I’ve been following the ocean acidification field with some interest. It’s always been obvious to me how acidification has enormous ramifications for coral reefs and other tropical marine ecosystems. They exist in warm waters already, often close to their thermal maxima. Acidifying the water around them at any creatures using calcium carbonate seems like a recipe for disaster.

How not to do acidification research on Pacific salmon.  Photo from <a href=''>simple daily recipes</a>.

How not to do acidification research on Pacific salmon. Photo from simple daily recipes.

But what about up north in colder climes? There, what is the relative importance of acidification versus changes in temperature? Do changes in physiological rates due to warming compensate for costs of acidification due to CO2 increase? This is particularly interesting along the Pacific Coast of North America, as in many regions, upwelling already drives annual fluctuations in pH – sometimes to levels not predicted to be widespread until 2050 (Feeley et al 2005 Science).

The recent paper by Gooding, Harley, and Tang in PNAS puts an interesting spin on this. Their work shows that, under scenarios where both temperature and CO2 increase, the feeding and growth rates of sea stars actually increases.

Sadly, some folk in the non-science world seem to be taking this as evidence that either global warming is a lie, or will make the world a shiny happier place. The real answer is far from it.

The truth lies in the fact that the sea stars used here don’t have a ton of calcified body parts. Indeed, they may just be compensating with more wet tissue mass, although this currently remains unclear.

With respect to organisms that rely on calcified skeletons (e.g., sea urchins) we know that their larvae floating about in the ocean will react more poorly than expected to increased heat stress if they grow up in a high CO2 world (O’Donnell, Hammond, and Hoffman 2009 Marine Biology). And if hard-bodied prey (i.e. mussels) react more poorly to acidification than they gain from increased physiological rates due to heightened temperature, things get tricky. The particular sea star studied here, Pisaster ochraceus, for example, is already a voracious consumer of hard bodied prey. If it gets a boost while it’s prey is weakened, the consequences could be quite large.

The future of the intertidal?  Image modified from <a href=''></a>.

The future of the intertidal? Image modified from

I do wonder if there is hope, though. If in some regions there are already regular pulses of acidified waters, one would guess that organisms possess some machinery for dealing with this annual event. While they may not possess adaptations that allow them to deal with long-term acidification – not yet – perhaps these may serve as Gould’s Spandrels. While on average calcifying organisms may not perform well underacidified conditions – even with a boost from temperature – one wonders if the raw genetic variation is out there waiting to be tapped. A hopeful thought for some grim research.

Gooding, R., Harley, C., & Tang, E. (2009). Elevated water temperature and carbon dioxide concentration increase the growth of a keystone echinoderm Proceedings of the National Academy of Sciences, 106 (23), 9316-9321 DOI: 10.1073/pnas.0811143106

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