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.
Highlights
In recent decades, marine biogenic skeletal formation and its response to global environmental change have become major research topics
Fossil records show that the reign of scleractinian corals was interrupted during the mid-Cretaceous Period when magnesium/calcium (Mg/Ca) ratio (mMg/Ca) dropped to its lowest levels (,0.5); such conditions are unfavorable for corals with aragonitic skeletons [5]
The growth rate of juveniles was significantly slower in low mMg/Ca water than growth under natural conditions (ANOVA, F4, 107 = 71.36, P,0.0001)
Summary
Marine biogenic skeletal formation and its response to global environmental change have become major research topics. The molar magnesium/calcium (Mg/Ca) ratio (mMg/Ca) of seawater has varied between 0.5 and 5.3 through the current Phanerozoic eon [1] During this period, three episodes of aragonite-facilitating conditions (high mMg/Ca) have occurred, along with two episodes of calcite-facilitating conditions (low mMg/Ca), which are known as aragonite and calcite seas, respectively [2]. Marine calcifying organisms that produce sediments and build reefs generally have skeletons and shells composed of either aragonite or calcite, and long-term changes in the mMg/Ca of seawater tend to correspond to changes in the prevailing mineralogy of these organisms [3,4]. High amounts of calcite in the primary skeleton have been reported in modern adult corals in artificial Cretaceous seawater with a low mMg/Ca value [12]. Some doubt still exists as to whether scleractinian corals are able to make calcitic primary skeletons under low mMg/ Ca conditions
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