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