To catch a fallen sea angel: A mighty mollusk detects ocean acidification

“What’s more,” snapped the Lorax. (His dander was up.)
“Let me say a few words about Gluppity-Glupp.
Your machine chugs on, day and night without stop
making Gluppity-Glupp. Also Schloppity-Schlopp.
And what do you do with this leftover goo?…
I’ll show you. You dirty old Once-ler man, you!
“You’re glumping the pond where the Humming-Fish hummed!
No more can they hum, for their gills are all gummed.
So I’m sending them off. Oh, their future is dreary.
They’ll walk on their fins and get woefully weary
in search of some water that isn’t so smeary.”

The fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) concluded “the response of organisms to ocean acidification is poorly known and could cause further changes in the marine carbon cycle with consequences that are difficult to estimate” (Bindoff et al. 2007). In the intervening three years since its publication, ocean acidification has risen to become a major research area in marine science. While the oceans buffer the planet against rising CO2 concentrations, it does so a cost to its own chemistry.

In case you have yet to noticed the ocean is REALLY big, 1.3 billion cubic kilometers (312 million cubic miles) big in fact. The oceans hover near a pH around 8.1. Since the ocean is HUGE, it takes A LOT to move the pH up or down. At a pH of 8.1, the carbonate system is composed of 90% bicarbonate, 9% carbonate, and only 1% as dissolved CO2. While we put our Gluppity-Glupp and Schloppity-Schlopp into the atmosphere, the ocean does its best to buffer the planet by balancing its chemistry with the air. As a consequence the acid balance is tilted lower because while the concentration of each component of the carbonate system increases, the increase in hydrogen ions comes at a cost to carbonate ions, which is what marine calcifiers need to create shells (Doney et al. 2009).

The fate of carbon dioxide in the ocean.

Calcifying organisms exist in all regions of the ocean from the deep seafloor to the pelagic open waters, from near-shore to far offshore, and at a wide range of depths. Carbonate dissolves faster at shallower depths where most of the carbonate-secreting animals live, such as corals and bivalve mollusks. Carbonates also dissolve a greater rate in frigid polar waters, which are home to large populations of the planktonic calcifiers like pteropod mollusks and formaniferans. Calcium carbonate exists as two forms when utilized by organisms – calcite and aragonite. The chemical differences may be subtle, but the results are dramatic. Aragonite dissolves at a much shallower depth, depending on where in the ocean you are 0.5 to 3 kilometers deep, while calcite dissolves between 4.2 and 5 kilometers deep. It is the aragonitic form that used by many animals, such as mollusks and corals. The IPCC’s “business as usual” CO2 emissions model projects that high latitude waters will be undersaturated with respect to aragonite near the end of the century (Orr et al. 2005). This model assumes that we do not change our emissions behavior and lessen our current rate of CO2 input to the atmosphere.

To grasp how our input of CO2 feeds back upon polar foods webs we can use the unassuming pteropod mollusk, commonly called the sea angel because of its modified wing-like (ptero-) foot (-pod), as a case study. Pteropod mollusks are particularly susceptible to ocean acidification because their carbonate shells are very thin and composed of aragonite, which is 50% more soluble in seawater than calcite, the other form of calcium carbonate. Hence, they are considered a “canary” in the climate change coal mine. Orr and colleagues (2005) examined the fate of the pteropod’s fragile shell under “business as usual” CO2 emissions. After 48 hours, shells edges were already acid-pitted (Orr et al. 2005). Calcification is a physiological process and organisms exert some degree of control over the enzymatic constituents, but it is an energetically expensive process (reviewed in Fabry et al. 2008). When shells get damaged, animals must exert even more precious energy to repair the damage.

Kevin Zelnio, Scientific American blog, 5 November 2010. Blog article.

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