Production of Calcite by the Green Alga Halimeda in Artificial Cretaceous Seawater

The codiacean green alga Halimeda contributes 20-30% of the carbonate sediment in lagoonal areas adjacent to modern Caribbean and Indo-Pacific coral reefs. This alga is syncytial, lacking cell membranes, so that an individual thallus functions as a giant, multinucleate cell. The thallus grows as branching chains of segments interconnected by tubular filaments. A segment is formed in a single day and then filled with calcium carbonate over several days. Aragonite crystals grow within segments in the form of needles, but in some regions of a segment these are subsequently dissolved and their calcium carbonate is reprecipitated as microgranular aragonite. Some of the needles grow in a spherulitic pattern similar to that of inorganic aragonite precipitates. It has been debated whether Halimeda employs organic templates to secrete the aragonite polymorph of calcium carbonate or simply induces precipitation by taking up carbon dioxide for photosynthesis. We have found that Halimeda incrassata segments grown in seawater of modern ionic composition (Mg/Ca molar ratio = 5.2) actually contain an average of about 8% high-Mg calcite (mean 16 mol % Mg substituting for Ca). As the Mg/Ca ratio of ambient seawater is stepped down, calcite constitutes an increasing percentage of the calcium carbonate produced, and, as we have found for numerous other kinds of organisms, the Mg content of the calcite declines. For segments grown in seawater with the imputed Cretaceous Mg/Ca molar ratio of 1.5, calcite constituted, on average, 46% of the calcium carbonate (maximum, 67%) and contained about 6 mol% Mg. Experiments show that in artificial seawaters having different Mg/Ca molar ratios but otherwise having the ionic strength and chemical composition of modern seawater, aragonite can precipitate inorganically when the Mg/Ca molar ratio is above 2. The fact that Halimeda produces slightly more aragonite than calcite when the ambient Mg/Ca molar ratio is 1.5 indicates that it does exert a degree of biological control over its calcium carbonate production, but that the control is incomplete. Our growth experiments showed that there is a correlation between rate of production of carbonate and rate of production of organic matter by Halimeda, apparently because, as has been previously demonstrated, this alga's photosynthesis is enhanced by the carbon dioxide produced by its calcification. Productivity was highest in seawater of modern composition. Experiments in which either the ambient Mg/Ca ratio or the absolute concentration of Ca was held constant while the other was varied showed that an increase in either one led to a higher rate of production of both organic matter and calcium carbonate. Nonetheless, rates of production were higher in imputed Cretaceous seawater (Mg/Ca molar ratio = 1.5) than in imputed Oligocene seawater (Mg/Ca molar ratio = 2.5), presumably because in the Oligocene treatment, both of the minerals produced (high-Mg calcite and aragonite) were favored by the ambient Mg/Ca ratio, whereas in the Cretaceous treatment, 56% of the calcium carbonate was aragonite, which was not favored by the ambient Mg/Ca ratio.