Paper Reviewed
Stemmer, K., Nehrke, G. and Brey, T. 2013. Elevated CO2 levels do not affect the shell structure of the bivalve Arctica islandica from the Western Baltic. PLOS One 8: e70106, doi:10.1371/journal.pone.0070106.

Living and fossil bivalve shells have long been utilized as bioarchives of past environmental conditions. However, according to Stemmer et al. (2013), a number of studies have shown that proxies derived from element and isotope-ratios, such as those from bivalves, "can be affected by shell growth and microstructure," making it "essential to evaluate the impact of changing environmental parameters such as high pCO2 and consequent changes in carbonate chemistry on shell properties [in order] to validate these biogeochemical proxies for a wider range of environmental conditions." And that is exactly what the team of three German researchers set out to do. Specifically, they investigated the impact of pCO2 on the shell microstructure of Arctica islandica in order "to evaluate the possible impact such changes would have on shell based proxies."

In February 2010, Stemmer et al. collected several young specimens (~4-5 years old) of A. islandica (ocean quahog), a long-living marine bivalve inhabiting a wide region of the northern Atlantic Ocean, from the Kiel Bight of the Western Baltic Sea (~54.53°N, 10.74°E). At this particular location, A. islandica lives below the thermohaline pycnocline and is subjected to large fluctuations in salinity (18-23) and pH, with pCO2 levels peaking at levels over 1000 µatm. The bivalves were transported from the Kiel Bight to the laboratory where they were acclimated for a period of 3 months following which time they were separated into three different treatment tanks of varying pCO2 levels (380, 760 and 1120 µatm). At the conclusion of the 90-day experiment the authors measured and compared shell growth and structure among the different treatments. So what did their analysis reveal?

Stemmer at al. report there was a high degree of variability in individual shell growth rates in each treatment, yet "pCO2 level had no significant effect on shell growth in height and thickness." Additionally, they found that crystal microstructure was "not altered by pCO2" either. Consequently, the three researchers concluded their results "indicate that A. islandica is in full physiological and chemical control of the shell formation process, including carbonate precipitation," even "when exposed to elevated proton concentrations, i.e. low pH."

In discussing the implications of their work, Stemmer et al. say these non-negative findings demonstrate that "we are still far from a consistent picture of the cause-and-effect mechanisms" that determine a marine organism's response to elevated pCO2 and lowered pH. And that reality suggests present policy discussions about what to do, or not do, about ocean acidification are putting the policy cart far ahead of the scientific horse.