Light and temperature effects on δ<sup>11</sup>B and B / Ca ratios of the zooxanthellate coral <i>Acropora</i> sp.: results from culturing experiments

Abstract

Abstract. The boron isotopic composition (δ 11 B) of marine carbonates (e.g. corals) is increasingly utilised as a proxy for paleo-pH, with the strong correlation between δ 11 B of marine calcifiers and seawater pH now well documented. However, the potential roles of other environmental parameters that may also influence both the boron isotopic composition and boron concentration into coral aragonite are poorly known. To overcome this, the tropical scleractinian coral Acropora sp. was cultured under 3 different temperatures (22, 25 and 28 • C) and two light conditions (200 and 400 µmol photon m −2 s −1 ). The δ 11 B indicates an increase in internal pH that is dependent on the light conditions. Changes in light intensities from 200 to 400 µmol photon m −2 s −1 seem to indicate an apparent decrease in pH at the site of calcification, contrary to what is expected in most models of light-enhanced calcification. Thus, variations in light conditions chosen to mimic average annual variations of the natural environments where Acropora sp. colonies can be found could bias pH reconstructions by about 0.05 units. For both light conditions, a significant impact of temperature on δ 11 B can be observed between 22 and 25 • C, corresponding to an increase of about 0.02 pH-units, while no further δ 11 B increase can be observed from 25 to 28 • C. This non-linear temperature effect complicates the determination of a correction factor. B / Ca ratios decrease with increasing light, consistent with the decrease in pH at the site of calcification under enhanced light intensities. When all the other parameters are constant, boron concentrations in Acropora sp. increase with increasing temperatures and increasing carbonate ion concentrations. These observations contradict previous studies where B / Ca in corals was found to vary inversely with temperature, suggesting that the controlling factors driving boron concentrations have not yet been adequately identified and might be influenced by other environmental variables and/or species-specific responses.

Primary Life Stage Boron Isotope and Trace Elements Incorporation in Aposymbiotic Acropora millepora Coral under Ocean Acidification and Warming

“…This is confirmed as our B/Ca ratio was only significantly affected by changes in pCO 2 concentration [ANOVA, F (1, 68) = 17.808, p = 1.24 × 10 −4 ; Table 6] and not temperature ( Figure 5; Table 6). Our primary polyp juveniles result is in contrast to studies that have found both significant inverse (Fallon et al, 2003) and direct (Dissard et al, 2012) relationships between B/Ca and SST in tropical corals.…”

“…Our observations confirm a previous study with cultured Acropora sp. coral nubbins (adult branches) with similar enrichment of skeletal δ 11 B ratio due to increasing temperature reflecting behavior changes at the calcification site possibly due to metabolic processes (Dissard et al, 2012). Our microstructure analysis indicates that biomineralization was compromised due to prolonged stress exposure in conditions 3 and 4 (Figure 4).…”

“…Moreover, a recent study of coral calcifying space pH revealed significant influence and modulation of internal pH due to photosynthesis and respiration (Comeau et al, 2017) that was alluded to by Dissard et al (2012). The effects of elevated pCO 2 on coral photosynthesis has been reported in previous studies, which have suggested a variety of effects that ranged from inhibitory to stimulatory (Reynaud et al, 2003;Anthony et al, 2008;Kaniewska et al, 2012).…”

“…For this study, we chose the fractionation factor α [B3−B4] of 1.0272 from the empirical study of Klochko et al (2006), which has been confirmed to be between the coral species-specific values range of 1.026-1.028 (Trotter et al, 2011;McCulloch et al, 2012) and applied in many studies (Allison et al, 2010;Krief et al, 2010;Rollion-Bard et al, 2011;Dissard et al, 2012).…”

Environmental controls on the boron and strontium isotopic composition of aragonite shell material of cultured <i>Arctica islandica</i>

“…This mechanism raises the pH of the calcification solution (Dissard et al, 2012;Rollion-Bard et al, 2011b). In this scenario, the activity of the enzyme is enhanced when a certain temperature has been reached, accelerating the proton removal process and resulting in a higher boron isotopic composition in the calcification solution with respect to the elevated pH.…”