Molecular and mineral responses of corals grown under artificial Calcite Sea conditions

Abstract

The formation of skeletal structures composed of different calcium carbonate polymorphs (aragonite and calcite) appears to be regulated both biologically and environmentally. Among environmental factors influencing aragonite and calcite precipitation, changes in sea water conditions- primarily in the molar ratio of magnesium and calcium during so-called "Calcite" (mMg:mCa below 2) or "Aragonite" seas (mMg:mCa above 2) - have had profound impacts on the distribution and performance of marine calcifiers throughout the Earth's history. Nonetheless, the fossil record shows that some species appear to have counteracted such changes and kept their skeleton polymorph unaltered. Here, the aragonitic octocoral Heliopora coerulea and the aragonitic scleractinian Montipora digitata were exposed to Calcite Sea-like mMg:mCa with various levels of changes in magnesium and calcium concentration, and both mineralogical (i.e.,CaCO3 polymorph) and gene expression changes were monitored. The two species exhibited different responses: no calcite deposition could be detected in H. coerulea, while M. digitata presented considerable amounts of calcite under mMg:mCa of 1.5. Nonetheless, aragonite was the main skeleton constituent in both species and under both treatment conditions. Despite a strong variability between experimental replicates, expression for a set of putative calcification-related genes, including known components of scleractinians skeleton organic matrix, was found to consistently change when mMg:mCa was lowered. These results are consistent with the hypothesis of an involvement of the skeleton organic matrix in counteracting decreases in seawater mMg:mCa. On the other hand, no clear changes in expression for calcium and magnesium transporters were detected. Nonetheless, different calcium channels were found downregulated in H. coerulea when exposed to mMg:mCa of 2.5, pointing to an active calcium uptake regulation by the corals under altered mMg:mCa to maintain aragonite deposition.

Effects of Excess Atmospheric CO2 on Calcium Carbonate Producers along the Red Sea Coast of Yemen: Its Risk and Socio-economic Impacts.

“…Aragonite, a calcium carbonate variant, is formed under speci c conditions of high alkalinity and magnesium concentrations in marine environments (Mucci and Morse 1984;Rushdi et al 1992; Wendt 2020). Typically found in shallow, warm waters, aragonite can be found in the structure of mollusk shells, major types of corals, and certain organisms (Moya et al 2008;Lebrato et al 2013;Conci et al 2022). Calcite is a stable calcium carbonate that forms under a wide range of conditions in both shallow and deep-sea waters (Ridgwell and Zeebe 2005).…”

Fibrous or Prismatic? A Comparison of the Lamello-Fibrillar Nacre in Early Cambrian and Modern Lophotrochozoans

“…However, long-standing debate exists with respect to whether and to what extent did changing seawater chemistry influence carbonate polymorphs in sophisticated BCM skeletons, particularly at the time when metazoans first acquired the ability to biomineralize skeletons [4,[17][18][19]. Laboratory experiments have been conducted to decipher the effects of seawater chemistry on skeletal mineralogy in modern taxa, e.g., [20,21]. However, many questions fundamental to unraveling the Cambrian metazoan skeletonization bio-event remain unresolved, primarily because the "basic data" regarding biomineralization of very early animal skeletons (e.g., primary mineralogy and skeletal microstructures) are difficult to obtain.…”