Biotic Control of Skeletal Growth by Scleractinian Corals in Aragonite–Calcite Seas

Abstract

Modern scleractinian coral skeletons are commonly composed of aragonite, the orthorhombic form of CaCO3. Under certain conditions, modern corals produce calcite as a secondary precipitate to fill pore space. However, coral construction of primary skeletons from calcite has yet to be demonstrated. We report a calcitic primary skeleton produced by the modern scleractinian coral Acropora tenuis. When uncalcified juveniles were incubated from the larval stage in seawater with low mMg/Ca levels, the juveniles constructed calcitic crystals in parts of the primary skeleton such as the septa; the deposits were observable under Raman microscopy. Using scanning electron microscopy, we observed different crystal morphologies of aragonite and calcite in a single juvenile skeleton. Quantitative analysis using X-ray diffraction showed that the majority of the skeleton was composed of aragonite even though we had exposed the juveniles to manipulated seawater before their initial crystal nucleation and growth processes. Our results indicate that the modern scleractinian coral Acropora mainly produces aragonite skeletons in both aragonite and calcite seas, but also has the ability to use calcite for part of its skeletal growth when incubated in calcite seas.

Magnesium-Rich Nanometric Layer in the Skeleton of Pocillopora damicornis With Possible Involvement in Fibrous Aragonite Deposition

“…This has been explained by a possible role of Mg in stabilizing the amorphous calcium carbonate nanoparticles serving as the building blocks of CoC (Mass et al, 2017;Von Euw et al, 2017). This is further supported by reports that corals grown in seawater with low Mg/Ca ratios produce skeletons with a high proportion of calcite (Higuchi et al, 2014), indicating an important role of Mg in stabilizing the prevalent aragonitic form of modern-day corals. While Mg content in fibrous aragonite is distinctly lower than in the CoC, a clear banding pattern in Mg/Ca ratio in these structures has been demonstrated by microscale measurement in multiple coral species (Meibom et al, 2004(Meibom et al, , 2007(Meibom et al, , 2008Gagnon et al, 2007;Frankowiak et al, 2016), suggesting Mg is concentrated at the interfaces between adjacent fibrous aragonite layers (Frankowiak et al, 2016).…”

Aragonitic scleractinian corals in the Cretaceous calcitic sea

“…In the calcification process of scleractinian corals, the taxon-specific composition of such molecules may influence the threshold at which seawater Mg/Ca can induce polymorphic change, as observed in mollusks (e.g., Falini et al, 1996). Higuchi et al (2014) observed partially calcitic (~20% ) primary skeletons produced by juvenile Acropora incubated in low Mg/Ca seawater from the larval stage. Nevertheless, the majority of the initial and post-initial skeleton was still aragonitic, suggesting that either some corals have both aragonite-and calcite-forcing organic matrix genes (Higuchi et al, 2014), the expression of which is less controlled and modulated during early ontogenetic development (note that many bryozoans and molluscs are capable of controlling CaCO 3 polymorphism within a single shell; e.g., Sandberg, 1975;Marin et al, 2008), or the initial stages of Acropora calcification are generally less controlled, permitting essentially induced biomineralization of certain carbonate structures to take place (Gilis et al, 2015).…”

Peer Review #1 of "Differential gene expression in skeletal organic matrix proteins of scleractinian corals associated with mixed aragonite/calcite skeletons under low mMg/Ca conditions (v0.1)"

“…In this study, genes involved in carbonate chemistry of the calcifying fluid, such as Ca ATPase (Zoccola et al 2004) and bicarbonate anion transporter (Zoccola et al 2015), were in neither the top 50 up-or down-regulated genes, nor the full list of DE genes. Thus, we propose that change in mMg/Ca in seawater did not impact the carbonate chemistry of the calcifying fluid, although the calcification rate decreased with low Mg/Ca seawater (Higuchi et al 2014;Higuchi et al 2017).…”