In vivo pH measurement at the site of calcification in an octocoral

Abstract

Calcareous octocorals are ecologically important calcifiers, but little is known about their biomineralization physiology, relative to scleractinian corals. Many marine calcifiers promote calcification by up-regulating pH at calcification sites against the surrounding seawater. Here, we investigated pH in the red octocoral Corallium rubrum which forms sclerites and an axial skeleton. To achieve this, we cultured microcolonies on coverslips facilitating microscopy of calcification sites of sclerites and axial skeleton. Initially we conducted extensive characterisation of the structural arrangement of biominerals and calcifying cells in context with other tissues, and then measured pH by live tissue imaging. Our results reveal that developing sclerites are enveloped by two scleroblasts and an extracellular calcifying medium of pH 7.97 ± 0.15. Similarly, axial skeleton crystals are surrounded by cells and a calcifying medium of pH 7.89 ± 0.09. In both cases, calcifying media are more alkaline compared to calcifying cells and fluids in gastrovascular canals, but importantly they are not pH up-regulated with respect to the surrounding seawater, contrary to what is observed in scleractinians. This points to a potential vulnerability of this species to decrease in seawater pH and is consistent with reports that red coral calcification is sensitive to ocean acidification.

Growth Kinetics and Distribution of Trace Elements in Precious Corals

“…In this case, differences of composition between sclerites and skeleton can be tentatively ascribed (at least partly) to differences of calcifying fluid, themselves related to differences of mechanism of formation of the skeleton and the sclerites. Indeed, while the formation of the skeleton takes place extracellularly, the early stages of formation of sclerites take place intracellularly in a primary scleroblast, then in a highly confined extracellular medium formed by only two secondary scleroblasts (Le Goff et al, 2017).…”

Activation Profile Analysis of CruCA4, an α-Carbonic Anhydrase Involved in Skeleton Formation of the Mediterranean Red Coral, Corallium rubrum

“…In this context, we have here investigated in vitro the effect of CAAs (amino acids and amines) on the activity of the recombinant CruCA4 from Mediterranean red coral, Corallium rubrum . CruCA4 belongs to the α-CA class and is an enzyme with a significant hydratase activity involved in coral skeleton formation [ 36 , 37 , 38 ]. At the site of calcification, CruCA4 generates bicarbonate which is then converted into carbonate [ 37 , 38 ].…”

ATP Supply May Contribute to Light-Enhanced Calcification in Corals More Than Abiotic Mechanisms

“…The identification of energy demanding processes related to calcification in corals is complicated by the fact that not all corals seem to have evolved the same physiological pathways to deliver the skeleton building blocks to the site of calcification (Barott et al, 2015a). Even the up-regulation of pH at the site of calcification appears not to be a universal feature in corals (Le Goff et al, 2017). Finally, here we are not considering the cost for OM synthesis.…”

“…Barott et al (2015a) however, also suggest that HCO − 3 transport trough BATs may be driven by the co-transport of Sodium that may reach the ECM through an active Na/K-ATPase pump, thus making also carbon delivery costly. However, there is also evidence that not all corals may have evolved the same physiological pathways and mechanisms to control calcification (Barott et al, 2015a;Le Goff et al, 2017), so a complete description of all the processes involved in calcification in corals is still missing.…”