Skeletal formation of scleractinian corals in response to seawater magnesium/calcium ratio fluctuation history

The molar magnesium (Mg)/calcium (Ca) ratio (mMg/Ca) of seawater has fluctuated throughout the Phanerozoic (541 Ma to the present), with three episodes favoring aragonite/high-Mg calcite formation (mMg/Ca>2) and two episodes favoring calcite formation (mMg/Ca<2), commonly referred to as aragonite and calcite seas, respectively. Although modern coral skeletons are generally composed of aragonite, seawater mMg/Ca fluctuation can lead to the formation of calcite skeletons under low mMg/Ca conditions. Temperature-dependent formation of aragonite and calcite by scleractinian corals in low-mMg/Ca seawater has also been documented, with corals producing more aragonite than in equivalent inorganic CaCO3 precipitation experiments. Thus, aragonitic scleractinian corals appear to be capable of biologically controlling their calcification under low-mMg/Ca conditions, regardless of temperature. Large-scale transcriptomic and proteomic studies have revealed significant expression of organic matrix and extracellular matrix-like proteins during the early stages of coral skeletal development under low-mMg/Ca conditions. Notably, downregulated genes associated with the organic matrix of aragonite skeletons were vastly outnumbered by upregulated genes, suggesting that corals actively initiate aragonite skeleton construction via the skeletal organic matrix under low-mMg/Ca conditions. However, coral growth rates significantly decreased at low mMg/Ca levels under all tested temperature conditions. Despite their potential to produce calcite skeletons, scleractinian corals did not become the dominant reef builders in calcite seas; instead, rudist bivalves remained the primary carbonate producers until the end of the Cretaceous.