Response of coral calcification and calcifying fluid composition to thermally induced bleaching stress

Abstract

Severe, global-scale thermal stress events like those of 1998 and 2016, are becoming more frequent and intense, potentially compromising the future of coral reefs. Here we report the effects of the 1998 bleaching event on coral calcification as well as the composition of the calcifying fluid (cf) from which corals precipitate their calcium carbonate skeletons. This was investigated by using the Sr/Ca, Li/Mg (temperature), and boron isotopes (δ11B) and B/Ca (carbonate chemistry) proxies in a Porites sp. coral. Following the summer of 1998 the coral exhibited a prolonged period (~18 months) of reduced calcification (~60%) and a breakdown in the seasonality of the geochemical proxies. However, the maintenance of elevated dissolved inorganic carbon (DICcf; >×2 seawater) and pHcf (>8.3 compared to seawater ~8.0) even during severe stress of 1998 indicate that a minimum threshold of high aragonite saturation state (Ωcf) of ~14 (~×4 seawater), is an essential pre-requisite for coral calcification. However, despite maintaining elevated levels of Ωcf even under severe stress, coral growth is still impaired. We attribute this to reductions in either the effective active volume of calcification and/or DICcf as bleaching compromises the photosynthetically fixed carbon pool available to the coral.

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

“…The outstanding question, however, is what role the SOM plays throughout the entire skeletonbuilding process. Arguments that skeletal accretion is entirely biologically controlled, independent of calcifying fluid pH or Ar (Von Euw et al, 2017), cannot be reconciled with the abundant evidence that calcification is sensitive to seawater carbonate chemistry (Langdon et al, 2003;Langdon and Atkinson, 2005;Cohen et al, 2009;Chan and Connolly, 2013;Comeau et al, 2013Comeau et al, , 2017Drenkard et al, 2013;Venn et al, 2013) or with geochemical evidence that coral skeletons are precipitated from a seawater-like solution (McConnaughey, 1989;Adkins et al, 2003;Cohen and McConnaughey, 2003;Gaetani and Cohen, 2006;Gagnon et al, 2007Gagnon et al, , 2012Trotter et al, 2011;McCulloch et al, 2012McCulloch et al, , 2017D'Olivo and McCulloch, 2017). Yet, that SOM may be involved as a template for initial crystal nucleation in COCs, as a buffer of calcifying fluid pH, or as a means to control the size and orientation of crystals, is consistent with the available evidence and with the original hypothesis of Barnes (1970).…”

Assessing carbon dioxide removal through global and regional ocean alkalinization under high and low emission pathways

“…Despite large changes regionally, these cannot compensate for the large changes associated with RCP8.5. Even targeted AOA in the tropical ocean can preserve only a tiny area of the ocean conducive to healthy coral growth; and even then the concomitant large warming is likely to be a stronger influence on coral growth than ocean chemistry (D'Olivo and McCulloch, 2017).…”

Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

“…Briefly, the core was collected near Havannah Island, an inshore reef in the central GBR where corals bleached during 1998. The skeleton was cut in a slab and cleaned with bleach as described by D'Olivo and McCulloch (2017), but no additional preparation was required. D'Olivo and McCulloch (2017) present δ 11 B and trace element/Ca ratios from multiple tracks in the skeleton, and they show anomalies to the skeletal geochemistry corresponding to the 1998 thermal stress event.…”