There's a new paper out in this month's Geology that's worth a mention.

It's a study looking at the biological effects of ocean acidification on a suite of organisms which secrete skeletons of calcite or aragonite.

The scientists grew 18 different species of calcifying organisms (across a broad taxonomic swath representative of both oceanic diversity and styles of calcification). Over a 90 day period, they jacked up the level of CO2 in the growing chamber, which the water absorbed and thus converted to carbonic acid. The levels tested ranged from ~400 ppm CO2 to 2850 ppm. This lowered the pH of the water through the same mechanism as is occuring in the world ocean due to anthropogenic CO2 emissions. The lower pH resulted in changes in the saturation state of calcite and aragonite (calcite being slightly tougher than aragonite: it stands up to dissolution by lower pH better). One of the organisms tested is the sea urchin Eucidaris tribuloides, shown in the image above. This image was the one chosen to grace the cover of this issue of Geology. The scale bar is 1 cm.

My hypothesis going into such an experiment would be that all the organisms would experience lower rates of calcification, or even dissolution, under the more acidic regime. However, that is not what the researchers found. While most of the organisms did follow that expectation, there were a significant minority that did not. Here's the only figure in the paper, a graph showing the results for each species, comparing calcification rate to the saturation state of aragonite (lower omega values = more acidic). So each graph should be read as "more acid to the left, more neutral to the right"):


Ten of the 18 species (temperate corals, pencil urchins, hard clams, conchs, serpulid worms, periwinkles, bay scallops, oysters, whelks, soft clams) had their net calcification rate decrease. Of those, six of the ten negatively impacted species (pencil urchins, hard clams, conchs, periwinkles, whelks, soft clam) actually lost shell material through dissolution. Four of the 18 species (limpets, purple urchins, coralline red algae, calcareous green algae) net calcification increased under intermediate conditions of acidification, and then declined at the highest levels. In three species (crabs, lobsters, and shrimp), net calcification was greatest under the highest level of acidification. One species, the blue mussel, exhibited no response to elevated CO2 levels.

What this means is that there's NOT just one simple rule for how calcifying organisms respond to increases in CO2-induced ocean acidification. Some organisms predictably do worse at building calcitic skeletons, while others seem to do a better job than ever. The authors offer some possible explanations for why this might be the case: "These varied responses may reflect differences amongst organisms in their ability to regulate pH at the site of calcification, in the extent to which their outer shell layer is protected by an organic covering, in the solubility of their shell or skeletal mineral [i.e. aragonite and high-Mg calcite are more soluble], and in the extent to which they utilize photosynthesis [because the CO2 is actively absorbed by the organism to do photosynthesis]."

This information will be useful not only in terms of better predicting the effects of anthropogenic CO2 on marine biota and ecosystems, but also in terms of sorting out ancient mass extinctions. Because the study identifies both positive and negative responses to elevated CO2-induced acidification for a wide range of organisms, the results offer "a unique, polyphyletic fingerprint for identifying such CO2-induced extinction events in the fossil record." In other words, we could go and look for winners and losers in various past mass extinctions and see if they match this "CO2-induced acidification" pattern. If so, then that would give us higher confidence in attriuting a particular mass extinction to elevated CO2 levels.

Reference:
Ries, J., Cohen, A., & McCorkle, D. (2009). Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification Geology, 37 (12), 1131-1134 DOI: 10.1130/G30210A.1

Labels: CO2, invertebrates, ocean acidfication

A Swim Through the Ocean's Future.

Hat tip to the Volcanism Blog for alerting me to this.

Labels: ocean acidfication, volcano

There's a new study out I read about today in New Scientist which took squid and put them in a tank of ocean water that was equilibrated to simulated atmospheric concentrations of carbon dioxide predicted for the year 2100. The oceans were also warmer in temperature, again simulating predicted future conditions. In these acidic oceans, the squid's metabolic levels dropped by 31%, and the time they spent contracting their muscles dropped by 45%. I didn't get to read the full study, which is behind a Proceedings of the National Academy of Sciences paywall, but the abstract online hints that these mini-oceans were about 0.3 pH units lower than modern ocean values. The abstract doesn't say how much warmer the experimental tanks were, but notes that water's ability to hold oxygen decreases with warmer temperatures. The lack of oxygen may be the prime reason for the squid's diminished activity.
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Journal reference: DOI: 10.1073/pnas.0806886105

Labels: CO2, critters, mollusks, news, ocean acidfication, oceans

Okay, so we all know that carbon dioxide has this property of being selectively transparent, and that it is accumulating at greater concentrations in Earth's atmosphere because the rate that it is being produced by human activities greatly exceeds the rate it is removed by natural processes. That's the global warming issue in a nutshell. But there's another aspect to climate change that hasn't gotten as much press: ocean acidification.

Much of the carbon dioxide produced since the Industrial Revolution has been absorbed by the oceans. Global warming would have been as noticeable as it is today much earlier had the oceans not acted as a "carbon sink" in this fashion. But the oceans can't absorb CO2 forever without consequence. When CO2 dissolves in H2O, it produces H2CO3, also known as carbonic acid. (top image)

Caldeira and Wickett published a study in 2003 in which the explored the consequences of adding all this extra acid to the oceans. The oceans are large, so changes to their pH take place slowly, but it looks like the ocean's pH is dropping (becoming more acidic) as it absorbs the extra CO2 from the atmosphere. They made some predictions (second image) about how projected emissions of CO2 will influence the amount of CO2 in the atmosphere (shown here as pCO2, which translates as the "partial pressure of carbon dioxide in the atmosphere), and then how that would influence the ocean's pH over a range of depths over the next millennium. As you might expect, their model shows surface waters becoming acidic first, because they are in direct contact with the CO2-rich atmosphere. Oceanic mixing propagates the acidic waters to the depths over longer timescales. They predict a maximum reduction of 0.7 pH units in surface waters, starting around the year 2200.

How will this effect marine life? Remember that lots of marine creatures make their skeletal material (hard parts) out of the mineral calcite, and calcite dissolves in acid. (In my classes, we put a drop of hydrochloric acid on a rock sample to determine if it is calcite.) Consider the effects on two kinds of plankton: coccolithophores and pteropods. The third and fourth images here show scanning electron micrographs of how skeletal material reacts to acidified conditions. The third image is from a study by Ulf Reibesell of the University of Norway, who grew coccolithophores in a series of model "ocean" tanks that had equilibrated to an "atmosphere" containing 300 ppm and 800 ppm CO2. For reference, pre-Industrial CO2 values were about 280 ppm, and today's CO2 values are about 380 ppm. You can see that the calcareous plates of the coccolithophores are smaller, thinner, and more degraded in the more acidic water. The fourth image shows the results of a similar experiment on a pteropod, by Orr, et al. in 2005. (A pteropod is a kind of planktonic snail.) The pteropod was placed in a tank of water undersaturated with respect to aragonite (a polymorph of calcite) for 48 hours. Sub-images b, c, and d show degradation of the snail's shell in those acid waters, and sub-image e shows a the surface of a normal pteropod shell for comparison.

Here's some model predictions of ocean pH from Scott Doney in a 2006 paper in Scientific American. Note that the northern Pacific Ocean becomes marginally saturated with respect to aragonite by the end of the century, and the Southern Ocean will be undersaturated by then. The skeletons of organisms with calcareous shells in those waters will begin to dissolve! So far, the pH drop has been only about 0.1 pH unit, but it is expected to hit around 0.3 pH units by 2100. It's hard to imagine how fundamental a change this will be to oceanic ecosystems!

Now, a new study in Science by the Coral Reef Targeted Research Group concludes that it's not just these high-latitude ocean water. Global warming kills tropical coral reefs, too. They consider the effects of ocean acidification as well as the effects of "bleaching" (when warm corals eject their symbiotic algae).

Labels: climate change, CO2, ocean acidfication