Reviews and syntheses: Revisiting the boron systematics of aragonite and their application to coral calcification

Abstract

Abstract. The isotopic and elemental systematics of boron in aragonitic coral skeletons have recently been developed as a proxy for the carbonate chemistry of the coral extracellular calcifying fluid. With knowledge of the boron isotopic fractionation in seawater and the B∕Ca partition coefficient (KD) between aragonite and seawater, measurements of coral skeleton δ11B and B∕Ca can potentially constrain the full carbonate system. Two sets of abiogenic aragonite precipitation experiments designed to quantify KD have recently made possible the application of this proxy system. However, while different KD formulations have been proposed, there has not yet been a comprehensive analysis that considers both experimental datasets and explores the implications for interpreting coral skeletons. Here, we evaluate four potential KD formulations: three previously presented in the literature and one newly developed. We assess how well each formulation reconstructs the known fluid carbonate chemistry from the abiogenic experiments, and we evaluate the implications for deriving the carbonate chemistry of coral calcifying fluid. Three of the KD formulations performed similarly when applied to abiogenic aragonites precipitated from seawater and to coral skeletons. Critically, we find that some uncertainty remains in understanding the mechanism of boron elemental partitioning between aragonite and seawater, and addressing this question should be a target of additional abiogenic precipitation experiments. Despite this, boron systematics can already be applied to quantify the coral calcifying fluid carbonate system, although uncertainties associated with the proxy system should be carefully considered for each application. Finally, we present a user-friendly computer code that calculates coral calcifying fluid carbonate chemistry, including propagation of uncertainties, given inputs of boron systematics measured in coral skeleton.

Investigating marine bio‐calcification mechanisms in a changing ocean with in vivo and high‐resolution ex vivo Raman spectroscopy

“…This advance can help to resolve the problem of differing temporal and spatial scales among techniques used in coral calcification studies (Holcomb et al, ). Although geochemical (i.e., isotopic and elemental) analyses of coral skeletons are effective at deriving the carbonate system of the calcifying fluid (DeCarlo, Holcomb, & McCulloch, ; McCulloch, D'Olivo Cordero, Falter, Holcomb, & Trotter, ), such studies have been difficult to reconcile with alternative approaches based on inserting micro‐sensors into the coral calcifying fluid (Al‐Horani, Al‐Moghrabi, & De Beer, ; Cai et al, ; Ries, ; Sevilgen et al, ), or imaging the calcifying fluid of corals exposed to pH‐sensitive dyes (Comeau et al, ; Holcomb et al, ; Venn, Tambutte, Holcomb, Allemand, & Tambutte, ). Conversely, Raman spectroscopy can now be applied in vivo at temporal and spatial scales comparable to the micro‐sensor studies (Figure ), as well as to bulk powders for comparison to isotopic measurements (e.g., DeCarlo et al, , DeCarlo, Comeau et al, ).…”

Long‐Term Impacts of the 1997–1998 Bleaching Event on the Growth and Resilience of Massive Porites Corals From the Central Red Sea

“…A permanent shift in the physiology of coral biomineralization was also manifest as a gradual increase in B/Ca over time; a shift that typically indicates a decrease in dissolved inorganic carbon (DIC) or carbonate ion concentration ([CO 3 2− ]) in the calcifying fluid (T. M. DeCarlo et al, 2018;Holcomb et al, 2016). Similarly, U/Ca, considered to be influenced by [CO 3 2− ] (T. M. DeCarlo et al, 2015), showed a significant change in the inner-shelf coral.…”

Morphological, elemental, and boron isotopic insights into pathophysiology of diseased coral growth anomalies

“…The ability to upregulate pH ECF is critical for coral resilience to changes in ambient seawater pH 28 , therefore, lower internal pH and porous skeleton structure in GAs indicate that these lesions will be particularly susceptible to the increasing global threat posed by ocean acidification. Under such conditions, GAs must either increase energy ] ECF is calculated from δ 11 B and B/Ca using method of DeCarlo et al 32 . Benjamini-Hochberg false discovery rate 80 was used to adjust p-values for multiple comparisons.…”

“…Some elements, such as boron or its isotopes (δ 11 B), can serve as useful markers of calcification. For instance, δ 11 B varies according to internal pH (pH ECF ) [27][28][29] , and recent studies suggest that an all-important second carbonate system parameter can be calculated (thus allowing full carbonate system computation) using paired coral δ 11 B and B/Ca ratios to estimate internal carbonate ion concentration ([CO 3 2− ] ECF ) [30][31][32] . Collectively, the elemental and boron chemistries have the potential to reveal key differences in calcification processes, as well as differences in wider holobiont physiological and biochemical activity, between healthy and diseased corals.…”

“…(1). In this way, a second carbonate system parameter [CO 3 2− ] ECF can be calculated from paired skeletal δ 11 B and B/Ca data using the method of DeCarlo et al 32 . Here we use D B/Ca formulations and dissociation constants of carbonic acid by McCulloch et al 31 .…”