I. INTRODUCTION A. Distribution of Calcium in Nature Calcium is the fifth most abundant element of the globe. It is found in a variety of rocks, such as aragonite or dolomite, and throughout most waters. The concentration of calcium in seawater varies from 1 to 10 roM (1); concentrations of calcium ion in fresh waters tend to be one to two orders of magnitude lower than in the oceans, but may reach the higher concentrations found in the ocean if the ground waters come from reservoirs in limestone. The latter are typically known as "hard" waters and are usually also high in magnesium. The average calcium concentration of the crust of the earth is almost 1 mol/kg, with more than 80% of the calcium found there occurring in the form of limestone (CaC03) deposits, which therefore constitute the principal forms of calcium in the earth. Kretsinger (1), in discussing these relationships, has called attention to the close link between carbon and calcium in living organisms. Moreover, since calcium phosphate is fairly insoluble and since phosphates and phosphorylation serve cells in a variety of ways (I)-to assure hydrophilicity for metabolites, as leaving groups in nucleophilic displacement reactions, as significant forms of energy storage (ATP), to link RNA and DNA-cells have to find ways to keep calcium from tying up phosphate and preventing its utilization.