Impact of Ocean Acidification on the geochemistry of the coralline algae Lithothamnion glaciale

Coralline algae are not only the most common but also the most wide-spread framework builders north and south of the low latitude coral belt. Their ability to withstand physical stresses in high energy environments relies on their skeleton which is composed of high Mg-calcite. High Mg-calcite is the most soluble form of calcium carbonate and therefore potentially vulnerable to the change in carbonate chemistry resulting from the absorption of anthropogenic CO₂ by the ocean. The cold water coralline alga Lithothamnion glaciale was cultured for 3 months under control conditions and high CO₂ concentrations (589 µatm). The experimental condition were set at 7±0.5°C with 20 µmol photons m^-2sec^-1 in 12 h light/ dark cycle. Electron microprobe and NanoSIMS analysis showed high Mg calcite bands around the algae cells in the control material. In contrast, under elevated CO₂ the banding is lost and overall Mg concentrations are lower. The loss of the Mg bands can be interpreted as loss of biological control on the biomineralization process. This reduction in Mg in the carbonate undermines the accuracy of the Mg/Ca ratio as proxy for past temperatures in time intervals with significantly different carbonate chemistry.