Intracellular pH distribution in foraminifera determined by the fluorescent probe HPTS

Fossil foraminifera are excellent tools to reconstruct past marine environments since the chemistry of their calcium carbonate reflects many physical and chemical parameters of seawater. However, trace elemental/calcium ratios in foraminiferal calcite vary between species and within specimens, usually deviating from ratios found in inorganically precipitated calcite. Biological control on CaCO3 precipitation is most likely to proceed by modification of the pH at the site of calcification or by removal of Mg2+ from the mother solution. In this study, we show the application of the nontoxic fluorescent probe 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) in analyzing foraminiferal intracellular pH. In all analyzed taxa, high-pH vesicles with a size of 3-5 mu m were found in the cell, commonly in higher densities toward the ultimate chamber. Vesicles with the highest pH (>= 8.5) were found in the miliolid species Quinqueloculina costata: hyaline species displayed vesicles with a pH of 8.0-8.5. These results show the potential of the application of HPTS in providing fundamental insights in foraminiferal control on the pH during calcification and thereby on trace elemental partitioning.