Are Molluscan Shells Important Metabolically-Derived Carbon Dioxide Sinks And Are They Net Carbon Sinks?

This paper addresses two aspects of molluscan shell formation. Firstly, the question of whether deposition of calcium carbonate increases atmospheric CO2 and ocean acidity as according to Ware and colleague’s hypothesis or do they form a net carbon sink? We conclude that alkaline moieties coming from a variety of sources, such as surface runoff and subsurface venting, compensate for any possible contribution of calcium carbonate formation to atmospheric CO2 and ocean acidification, but we do recognize that the current fossil fuel production of CO2 is driving both atmospheric CO2 and ocean acidification. The second issue addresses the problems that molluscs have in getting rid of metabolically produced CO2. These include the dual function of the gills (food gathering and gas exchange), the low diffusion coefficient of CO2 in water, and the low partial pressure differential between tissue pCO2 and water pCO2. We suggest that these problems are overcome, at least in part, by sequestering tissue CO2 in shell calcium carbonate. This hypothesis can be tested by examining the effect of interferences with shell formation on molluscan pCO2 in hemolymph.