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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.

Limacina helicina—a pteropod mollusc Image used with permission by Russ Hopcroft, University of Alaska Fairbanks. More at Arctic Ocean Diversity

Unfortunately, baseline data among pteropod species are lacking and it is difficult to track the effects of ocean acidification at the population level in the field. Polar pteropods are adapted for metabolic activity in cold waters, but by trading off locomotor capabilities for increased aerobic capacity, they are even more vulnerable to changes in their environment (Rosenthal et al., 2009). Because pteropods are limited to the much more soluble aragonite for shell production, it is predicted their range will be severely limited over a short time – as little as 50-100 generations – vertically in the water column and then latitudinally as they are forced towards lower latitudes (Fabry et al., 2008). Even if they are able to adapt to warmer waters with lower aragonite-saturation depths they will necessarily be exposed to new planktonic communities, entering into new competitive interactions, and be displaced from their preferred prey.

The loss of the tiny pteropod is not without major repercussions in the polar food web. As a major food source in polar open waters (Pakhomov et al., 1997), decreased pteropod abundance will adversely affect the structure of food web (Seibel & Dierssen 2003). Some of the better-known predators of pteropods are whales, fish and seabirds. Pteropods located in northwest Pacific salmon fishing grounds can make up over 60% of juvenile salmon diets (Armstrong et al., 2005, Turley et al., 2010). There are few pteropod specialists. In Japanese waters a deep-water myctophid fish, Centrobrachus brevirostris, exclusively eats pteropods (Watanabe et al., 2002). Ironically, the most fierce-some predators that the thin-shelled pteropods have to be wary of are other larger, shell-less pteropods (Seibel et al., 2007). While few specialize on pteropods, they are an important, abundant, and high protein food source for a whole community of generalist predators. The disappearance of pteropods may cause these generalist predators to prey upon larvae of economically important fishery stocks, such as juvenile salmon, more frequently.

Pteropods are also the dominant grazers of polar phytoplankton, out-consuming copepods by up to 33% (Bernard & Froneman 2009) with ingestion rates at the upper end for zooplankton in general (Hunt et al., 2008). Fewer pteropods may relieve pressure on phytoplankton, though, and drive a small negative climate feedback by taking up and sequestering more CO2 in the polar latitudes. While lower pteropod abundances decrease export of carbon, via carcasses, fecal pellets and carbonate, to deep sea, marine snow will still accumulate to some extent because of the increased phytoplankton biomass.

This case study of the pteropod is but a small window into the effects of ocean acidification as a result of human input of CO2 into the atmosphere. While under high CO2 emissions scenarios in the lab, pteropods were still able to precipitate calcium carbonate for their shells, but shell production was highly contingent upon pH (Comeau et al., 2010). It is still unclear whether rate that new shells is created is equal to or greater than the rate at which older shell is dissolved as a net process operating at lower pH or high CO2 emissions (Comeau et al., 2010). Other calcifying organisms such as foraminferans, coccolithophores, other mollusks, shallow and deep-water corals, echinoderms, fish, crustaceans, coralline algae, and seagrasses are all physiologically susceptible to acidification (Doney et al., 2009). Pteropods appear to be most vulnerable species to ocean acidification, but the effects to economically important bivalve and corals, which often provide habitat for commercial fisheries, will likely be among the most severe socioeconomic and environmental outcomes.

Can we catch a fallen sea angel from the brink of decimation? That will be up to us to decide. If the old Once-ler can realize the errors of his ways, albeit nearly too late, perhaps we too can learn a lesson from The Lorax.


“But now,” says the Once-ler,
“Now that you’re here,
the word of the Lorax seems perfectly clear.
UNLESS someone like you cares a whole awful lot,
nothing is going to get better, Its not.” – Dr. Seuss, The Lorax


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About the author: Kevin Zelnio is a doctoral student at University of North Carolina, Wilmington, received his MSc at Penn State studying community of hydrothermal vents, and got his BSc in Evolution and Ecology at University of California, Davis. He studies the geographic extent of hybrid populations, molecular ecology of marine invertebrates and is a published taxonomist of hydrothermal vent invertebrates. He has described one new species of shrimp and 4 new species of anemones. Kevin is the Assistant Editor and Webmaster for Deep Sea News where he writes extensively about marine science. To learn more about him visit his homepage, where he occasionally writes about non-marine evolutionary ecology, and follow him on Twitter: @kzelnio. [Image used with permission by Anna Linda Photography]

The views expressed are those of the author and are not necessarily those of Scientific American.

The views expressed are those of the author and are not necessarily those of Scientific American.

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