PRODUCTION OF LOW-MAGNESIUM CALCITE BY THREE SPECIES OF CORALLINE ALGAE GROWN IN ARTIFICIAL SEAWATER WITH THE MAGNESIUM/CALCIUM RATIO OF LATE CRETACEOUS SEAS
In 3 chemically distinct artificial seawaters, we grew 3 species of coralline algae that today secrete high-Mg calcite. The molar concentrations of Ca2+ plus Mg2+ (combined) and of other chemical constituents were the same in all 3 treatments, but the Mg/Ca mole ratios were 1, 2.5, and 5.8, respectively. We switched specimens between the second and third treatments after growth, to control the experiments. All 3 species incorporated into their skeletons a percentage of magnesium proportional to the ambient Mg/Ca ratio and close to the percentage found in nonskeletal calcite precipitated from water of the same composition. The species may employ organic templates to specify precipitation of calcite, but the calcites magnesium content is clearly governed by the ambient Mg/Ca ratio; photosynthesis apparently induces calcification from seawater without biotic pumping of Ca2+. In the treatment that mimicked Late Cretaceous seawater (Mg/Ca=1), all specimens died within four days, but there was a progressive decline in the mole % Mg in calcite impregnating the walls of cell rows representing daily growth. Before dying, each species produced low-Mg calcite, the imputed nonskeletal carbonate precipitate from Late Cretaceous tropical seas.