The Mineralogy of Biocalcifying Marine Organisms: Biological Control Versus the Influence of Seawater Chemistry

There has been much debate as to the degree to which aquatic organisms exert biological control over the mineralogyof their skeletal materials, for example by employing organic templates to specify the precipitation of a particular mineral or by controlling the chemistry of fluid occupying the space in which biomineralization occurs. Recent experiments reveal that most marine skeletonized taxa exert only partial control over their biomineralization. Nonskeletal precipitation in seawater yields low-Mg calcite when the Mg/Ca ratio is <1, high-Mg calcite when it is 1-2, and aragonite and/or high-Mg calcite when it is >2.

A variety of organisms that produce high-Mg calcite today (coralline algae, coccolithophores, serpulid worms, arthropods, and echinoids) incorporate progressively less Mg in their calcite as the Mg/Ca ratio of ambient artificial seawater is lowered from its present value of 5.2 to its imputed Cretaceous level of 1-1.5, with the fractionation pattern differing among taxa. Inasmuch as all studied calcitic organisms produce low-Mg calcite in “Cretaceous” seawater, it is likely that this was the mineralogy of all calcitic forms in the past when the Mg/Ca ratio of seawater was below 2. Because incorporation of Mg slows the rate of crystal growth of calcite, failure to exclude it effectively is presumably detrimental to many kinds of organisms. Mollusks have been found to suppress the Mg content of their skeletal calcite at Mg/Ca ratios below that of the modern ocean, but not above it, suggesting that they have evolved in concert with the rise of the ratio since the Cretaceous.

Calcareous green algae and scleractinian corals, though largely aragonitic today, also produce small quantities of calcite, and the percentage of calcite increases, along with its Mg content, as the Mg/Ca ratio of ambient seawater is lowered. Thus they exhibit only partial control over their biomineralization.