The Role of Saturation in Seashell Formation

From the corals of Australia’s Great Barrier Reef to the shellfish hatcheries of the Pacific Northwest, ocean acidification’s effects on shell-forming organisms are now well documented. However, ocean acidification research has tended to focus on just one aspect of the phenomenon’s chemistry: pH.

Now, a new study  published in Nature Climate Change suggests pH alone isn’t telling the whole story.

The study by Oregon State University researchers George Waldbusser, Burke Hales, Chris Langdon, and Brian Haley looks at the responses of Pacific oysters and Mediterranean mussel larvae under varying conditions of acidification. The team’s results suggest that past researchers’ penchant for pH has led to another important chemical measure being ignored: the saturation state.

Essentially, the saturation state is a way to quantify how much material — various forms of calcium carbonate ions — is available in seawater for organisms to build shells. The saturation state is closely linked to pH and alkalinity. (Low pH generally spells a lower saturation state with less calcium carbonate.) And both pH and the saturation state are closely linked to how much carbon dioxide has dissolved in the water.

This close link, or coupling, could be why past research had ascribed so much importance to low pH. Perhaps, the OSU researchers conjecture, previous investigations might have attributed to low pH the effects of the saturation state? If this error has been made, it’s easy enough to see why.

In the real world, ocean acidification occurs when carbon dioxide from fossil fuels dissolves in seawater. This lowers the water’s pH levels, rendering the water less basic and more “acidic.” Consequently, shell-forming creatures — be they mollusks or corals — have trouble forming their shells.

The reason for the organisms’ difficulties has long been blamed on dropping pH, with past research concluding that low pH waters make it difficult for some organisms to regulate their internal chemistry. (Effectively, the organisms themselves were becoming less basic and more acidic.) But the new study says that’s only a piece of it.

To discover what roles pH, the saturation state, and dissolved CO2 each play, the researchers set about decoupling the three factors experimentally. They did this by fiddling with the alkalinity of their seawater. Once decoupled, the researchers then subjected their larvae to differing levels of the three factors. Their data suggest that the saturation state was a significantly larger determinant of health and mortality for their larvae than pH or CO2. (Low pH was still a factor, but only at extremely low levels.)

The reason, the researchers conjecture, is that the tiny shell-makers were basically scrambling to build their shells before they ran out of energy. (Pacific oyster larvae, for instance, have a mere 48-hour window to form their initial shells — essential for growing swimming and feeding appendages — before the energy stored in the eggs runs out.) Consequently, if the saturation state is low enough, the energy needed to build shells before it’s too late becomes very high, and the larvae either fail to develop or become stunted.

Iria Gimenez, a student of Waldbusser and Hales, is now in the process of expanding this research into a working stress model of how oyster larvae are impacted by the variable conditions in coastal waters throughout the larval period in response to dissolved CO2, pH, and the saturation state.

Citation: Waldbusser, G.G., B. Hales, C.J. Langdon, B.A. Haley, P. Schrader, E.L. Brunner, M.W. Gray, C.A. Miller & I. Gimenez (2014). Saturation-state sensitivity of marine bivalve larvae to ocean acidification, Nature Climate Change, doi:10.1038/nclimate2479

Nathan Gilles is the managing editor of The Climate Circulator, and oversees CIRC’s social media accounts and website. When he’s not writing for CIRC, Nathan works as a freelance science writer. 



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