Calcium-Proton Exchange During Algal Calcification

Extracellular calcification by the giant celled alga Chara corallina may involve active Ca2+ extrusion from the cell in exchange for protons. The following evidence is presented: CaCO3 incrustations accrete largely along the inside, facing the cell, as revealed by X-ray microanalysis using Sr2+ and Mn2+ as tracers for new mineralization. Inward proton currents are inhibited by the Ca2+ transport antagonists Gd3+ and La3+. Low Ca2+ concentrations inhibit pH banding and photosynthesis, and solutions of low Ca2+ activity support more photosynthesis in the presence of additional buffered calcium. The ratio of calcification to photosynthesis in moderately alkaline solutions containing sufficient calcium remains stable at about 1.0 independent of solution Ca2+ concentration. Ion specific microelectrodes placed close to the calcified surface sometimes detect increases in Ca2+ activity coincident with decreases in proton activity. As the pCa of solution increases, the maximum pH observed at the alkaline surface increases, as does the maximum solution pH which supports electrochemical currents by the cell. Combinations of extracellular pH and pCa approach the calculated thermodynamic limits for ATP driven 2H+/Ca2+ exchange against the cytosol.