Mapping coral calcification strategies from in situ boron isotope and trace element measurements of the tropical coral Siderastrea siderea

Abstract

Boron isotopic and elemental analysis of coral aragonite can give important insights into the calcification strategies employed in coral skeletal construction. Traditional methods of analysis have limited spatial (and thus temporal) resolution, hindering attempts to unravel skeletal heterogeneity. Laser ablation mass spectrometry allows a much more refined view, and here we employ these techniques to explore boron isotope and co-varying elemental ratios in the tropical coral Siderastrea siderea. We generate two-dimensional maps of the carbonate parameters within the calcification medium that deposited the skeleton, which reveal large heterogeneities in carbonate chemistry across the macro-structure of a coral polyp. These differences have the potential to bias proxy interpretations, and indicate that different processes facilitated precipitation of different parts of the coral skeleton: the low-density columella being precipitated from a fluid with a carbonate composition closer to seawater, compared to the high-density inter-polyp walls where aragonite saturation was ~ 5 times that of external seawater. Therefore, the skeleton does not precipitate from a spatially homogeneous fluid and its different parts may thus have varying sensitivity to environmental stress. This offers new insights into the mechanisms behind the response of the S. siderea skeletal phenotype to ocean acidification.

Porites Calcifying Fluid pH on Seasonal to Diurnal Scales

“…This suggests that approximately half of the variability in each of the δ 11 B transects from LAA-1 results from sampling different components of the coral skeleton macrostructure. Similar δ 11 B heterogeneities have been found in other coral δ 11 B LA-MC-ICP-MS studies (Chalk et al, 2021) and are likely linked to spatial variations of pH cf in the coral skeleton as shown in studies using pH-sensitive dyes or microelectrodes (Cai et al, 2016;Sevilgen et al, 2019;Venn et al, 2011). These macrostructural heterogeneities in skeletal δ 11 B may also explain the short-term differences between bulk and LA-MC-ICP-MS derived δ 11 B in some intervals (Figure 3c), as well as some of the differences between individual LA δ 11 B transects L1-6.…”

The calcifying interface in a stony coral’s primary polyp: An interplay between seawater and an extracellular calcifying space

“…This should be considered in risk assessment and management of coral-reef ecosystems around the globe. By exploiting new experimental approaches, our observations add up to several recent studies ( 15, 37, 68 ), revealing a larger and more versatile biological toolkit exploited by corals for their biomineralization process than previously recognized.…”

“…This should be considered in risk assessment and management of coral-reef ecosystems around the globe. By exploiting new experimental approaches, our observations add up to several recent studies (15,37,68), revealing a larger and more versatile biological toolkit exploited by corals for their biomineralization process than previously recognized. Collected larvae were acclimated overnight under ambient conditions (~25 °C and ~pH 8.2) in a flow-through outdoor aquarium exposed to natural lighting with fresh seawater filtered to 60 µm.…”

“…While we provide here further evidence that seawater is a component of the ECM, it is important to stress that this does not mean the ECM directly reflects ocean chemistry or that (68,69). The concentration of Ca 2+ ions in seawater is roughly fifty times higher than CO3 2concentration (69).…”

“…Previous studies and our cryo-SEM observations reported here show that ECM pockets are largely delimited spaces, in which internalized seawater is prone to strict biological control and modification induced by the calicoblastic cells. The control and alteration of ECM chemistry by the coral tissue is clearly evident from previous work showing elevated concentrations of Ca 2+ , CO 3 2- , pH levels and thus aragonite saturation state (Ω arag ) inside ECM pockets relative to the external seawater ( 68, 69 ). The concentration of Ca 2+ ions in seawater is roughly fifty times higher than CO 3 2- concentration ( 69 ).…”