Abstract
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The mature skeletons
of hard corals, termed stony or scleractinian
corals, are made of aragonite (CaCO3). During their formation,
particles attaching to the skeleton’s growing surface are calcium
carbonate, transiently amorphous. Here we show that amorphous particles
are observed frequently and reproducibly just outside the skeleton,
where a calicoblastic cell layer envelops and deposits the forming
skeleton. The observation of particles in these locations, therefore,
is consistent with nucleation and growth of particles in intracellular
vesicles. The observed extraskeletal particles range in size between
0.2 and 1.0 μm and contain more of the amorphous precursor phases
than the skeleton surface or bulk, where they gradually crystallize
to aragonite. This observation was repeated in three diverse genera
of corals, Acropora sp., Stylophora pistillata—differently sensitive to ocean acidification (OA)—and Turbinaria peltata, demonstrating that intracellular particles
are a major source of material during the additive manufacturing of
coral skeletons. Thus, particles are formed away from seawater, in
a presumed intracellular calcifying fluid (ICF) in closed vesicles
and not, as previously assumed, in the extracellular calcifying fluid
(ECF), which, unlike ICF, is partly open to seawater. After particle
attachment, the growing skeleton surface remains exposed to ECF, and,
remarkably, its crystallization rate varies significantly across genera.
The skeleton surface layers containing amorphous pixels vary in thickness
across genera: ∼2.1 μm in Acropora,
1.1 μm in Stylophora, and 0.9 μm in Turbinaria. Thus, the slow-crystallizing Acropora skeleton surface remains amorphous and soluble longer, including
overnight, when the pH in the ECF drops. Increased skeleton surface
solubility is consistent with Acropora’s vulnerability
to OA, whereas the Stylophora skeleton surface layer
crystallizes faster, consistent with Stylophora’s
resilience to OA. Turbinaria, whose response to OA
has not yet been tested, is expected to be even more resilient than Stylophora, based on the present data.
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