Biomineralization in Mediterranean Corals: The Role of the Intraskeletal Organic Matrix

Abstract

The precipitation of calcium carbonate was carried out in the presence of the intraskeletal organic matrix (OM) extracted from Mediterranean corals. They were diverse in growth form and trophic strategy, Balanophyllia europaea and Leptopsammia pruvotisolitary corals, only the first zooxanthellate coraland Cladocora caespitosa and Astroides calyculariscolonial corals, only the first zooxanthellate coral. The results showed that, although the OM marked differences among species, the diverse influence over the calcium carbonate precipitation was evident only for B. europaea. This OM was the most prone to favor the precipitation of aragonite in the absence of magnesium ions, according to overgrowth and solution precipitation experiments. In artificial seawater, where magnesium ions were present, this OM, as well the one from A. calycularis, precipitated mainly a form of amorphous calcium carbonate different from that obtained with SOM from L. pruvoti or C. caespitosa. The amorphous calcium carbonate from B. europaea was the most stable upon heating up to 100 °C and was the one that mainly converted into aragonite instead of magnesium calcite after heating at 300 °C. All this indicated a higher control of B. europaea OM over the calcium carbonate polymorphism than the other species. The influence of SOMs over precipitate morphology turned out to be also species related. In conclusion, this comparative study has shown that the influence of OM on in vitro precipitation of calcium carbonate was not related to the coral ecology, solitary vs colonial and zooxanthellate vs nonzooxanthellate, and suggested that the coral control over biomineralization process was species specific and encoded in coral genes.

The Origin and Role of Organic Matrix in Coral Calcification: Insights From Comparing Coral Skeleton and Abiogenic Aragonite

“…Similar results have been achieved on a variety of biogenic calcium carbonates (Pokroy et al, 2006a(Pokroy et al, ,b, 2009Stolarski et al, 2007;Zolotoyabko et al, 2007;Reggi et al, 2014;Zolotoyabko, 2017), including corals, and have been attributed to the degradation of intra-crystalline SOM by heating. While Raman spectroscopy cannot be used to quantify the unit cell parameters with the precision of XRD, we expect the unit cell changes to manifest in the Raman ν 1 peak as (1) a decrease in FWHM due to decreased disorder, and (2) an increase in wavenumber due to the shorter bond lengths, and thus higher frequency vibrations, in the contracted unit cell.…”

“…First, organic matter coats aragonite crystals in the skeleton (Clode and Marshall, 2002), and elevated organic matter content is closely associated with the relatively disordered centers of calcification (COCs; also called "rapid accretion deposits" or RADs) (Benzerara et al, 2011;Falini et al, 2013;Von Euw et al, 2017). Second, organic molecules extracted from the skeleton have been attributed to certain roles in crystal growth (Constantz and Weiner, 1988;Weiner and Addadi, 1991;Allemand et al, 1998;Goldberg, 2001;Cuif et al, 2008;Reggi et al, 2014;Takeuchi et al, 2016), with some proteins even capable of inducing spontaneous aragonite precipitation from seawater . Finally, the unit cell of biogenic aragonite contracts after annealing (i.e., heating and allowing to cool), presumably due to the removal of the SOM from the lattice (Pokroy et al, 2004;Reggi et al, 2014;Zolotoyabko, 2017).…”

“…Second, organic molecules extracted from the skeleton have been attributed to certain roles in crystal growth (Constantz and Weiner, 1988;Weiner and Addadi, 1991;Allemand et al, 1998;Goldberg, 2001;Cuif et al, 2008;Reggi et al, 2014;Takeuchi et al, 2016), with some proteins even capable of inducing spontaneous aragonite precipitation from seawater . Finally, the unit cell of biogenic aragonite contracts after annealing (i.e., heating and allowing to cool), presumably due to the removal of the SOM from the lattice (Pokroy et al, 2004;Reggi et al, 2014;Zolotoyabko, 2017). Together, there is compelling evidence that organic molecules are present within coral skeletons and that they are related, in some way, to the nucleation of aragonite crystals.…”

“…It is for this reason that studies of the organic matrix have relied heavily on characterizing its composition and location within the skeleton (in which it composes less than 0.1% by mass), rather than by direct observations during crystal growth (Allemand et al, 2004;Reggi et al, 2014;Wang et al, 2015). Techniques for observing and mapping the distribution of the SOM include scanning electron microscopy (Clode and Marshall, 2002), helium ion microscopy (Von Euw et al, 2017), transmission electron microscopy (Benzerara et al, 2011;Falini et al, 2013), and fluorescence as detected by a Raman spectrometer (Jolivet et al, 2008;Nehrke et al, 2011;Wall and Nehrke, 2012;Von Euw et al, 2017).…”

Shell properties of commercial clam Chamelea gallina are influenced by temperature and solar radiation along a wide latitudinal gradient

“…The constancy of OM content in C. gallina under different environmental conditions represents an important result, which adds relevance to the long recognized key role of OM in biomineral skeleton formation78545556. This despite the fact that still little information is known on varying OM content and composition in relation to environmental parameters.…”