Tracking the Early Events of Mineral Formation during Coral Development

Abstract

In the tropical and subtropical oceans, corals provide a literal and figurative ecological framework that retains nutrients, supports high rates of primary production, and permits extensive biological diversity. These fragile ecosystems are threatened with extinction in the coming century, in part due to the acidification of the ocean [1][2]. The change in the acidity of the ocean may affect the coral skeleton formation, which is composed of calcium carbonate mineral. Elucidating the mechanism of calcium carbonate mineral formation in corals may help us to understand how environmental changes affect the process of coral biomineralization. Although various aspects of biomineralization in corals have been studied for decades, mostly on adult corals, the basic mechanism responsible for the precipitation of calcium carbonate in the form of aragonite remains enigmatic [reviewed by 3].Corals have a biphasic life cycle with planktonic larval stages and benthic adults. These two phases are separated by settlement and metamorphosis, which are critical stages in coral development during which planulae undergo intense changes in morphology, building of new tissues, initiation of calcium carbonate precipitation, and in some cases uptake of symbionts. The planktonic free-swimming planulae deposit minerals almost immediately after settlement [4]. This suggests that immature mineral phases (presumably amorphous calcium carbonate -ACC) is present in pre-settled planulae.To elucidate the key mechanism that facilitates the initial, rapid calcification in the early stages of the coral development, we correlate cryo-scanning electron microscopy (SEM) with cryo-energy-dispersive X-ray spectroscopy (EDS) and cryo-fluorescence techniques on coral planulae frozen samples [5]. Using this approach and thus, avoiding the process of chemical fixation, dehydration and chemical staining with heavy metals, each planula is rapidly frozen at high pressure. As a result, the water is vitrified while keeping the sample as close as possible to its native state. We observed freeze-fractured surfaces of the sample using the cryo-SEM with both secondary electron and backscattered electron (BSE) detectors. The elements that are present in the mineralized regions are detected by EDS under cryogenic conditions. The cryo-fluorescence platform helps to identify auto-fluorescent symbionts and to detect mineralized regions in the developing planula via calcein blue staining.Our results show that first mineral deposition in corals already starts at the pre-settlement (Fig. 1). Immature minerals can vary in shape, Mg content and crystallinity. After settling, the aboral epidermis is attached to the substrate and begins skeleton formation. The first calcareous elements after settling are circular platelets and rod-shaped granules, which can also vary in Mg and Ca content and aggregate to form the primary septa, followed by the formation of the basal disk [6,7].

Minerals in the pre-settled coral Stylophora pistillata crystallize via protein and ion changes

“…This larva subsequently changes into a benthic (settled) stage of a primary polyp (Fig. 1c ), in which the formation of the aragonite exoskeleton is thought to be initiated 6 – 9 . Studies of skeletogenesis typically consider the primary polyp stage as the time point of the calcification onset 6 .…”

“…c The primary settled polyp (bottom view). Under polarized light, the birefringent crystals formed on the circumference appear as bright spots, while the mineralized septa 9 , covered by organic layers of tissue, appear as the long dark streaks (the six primary septa are marked by asterisks), scale bars: a , c 500 µm; b 200 µm …”

Geochronology of Cadmium (Cd), Cuprum (Cu), and Arsenics (As) in Annual Band of Coral Porites lutea at Pantai Kondang Merak, Malang

“…This is because coral skeleton had more intragranular porosity that made the amount of trace metals released through a porous structure (Jiang et al, 2017). Bioaccumulation process of heavy metals in coral skeleton depends on biomineralization process on endodermis layer that took a long time process (Akiva et al, 2016). Lastly, coral has a selective mechanism of heavy metals absorption in which not all types of metals can be transported from coral tissue into a coral skeleton (Darvishnia et al, 2016).…”

Coloring coral larvae allows tracking of local dispersal and settlement

“…The toxicity of neutral red to Acroporidae larvae is counter to that reported from other benthic marine invertebrate larvae (e.g., oysters) that exhibit greater sensitivity to Nile blue [ 27 ] and are unaffected by neutral red [ 28 ]. While calcein blue and alizarin red have been successfully used in staining adult benthic invertebrates [ 27 – 29 ], as well as mineral deposits found within brooded coral planulae [ 30 ], the absence of visual staining in spawning coral larvae due to a lack of calcium binding potential limits their application to post-settlement life history stages ( Fig 1 ) following the onset of early skeletal formation [ 31 ].…”