Aragonite

Lophelia pertusa utilizes calcium carbonate (CaCO3) in the form of aragonite from the water to form their outer skeleton. As the coral grows, a complex framework of skeleton is left behind creating reefs up to 30m tall (Hovland and Thomsen, 1997). When L. pertusa dies, the hard skeleton remains leaving the framework behind (fig 6). L. pertusa can only

Dead Lophelia pertusa skeleton framework. Collected from Rockall bank 2009. © David Price

Fig. 6. Dead Lophelia pertusa skeleton framework. Collected from Rockall bank 2009. © David Price

lay their skeleton down easily when the water is “saturated” with aragonite. Saturation state (Ω) refers to the solubility of aragonite in sea water. Therefore when Ωarag (saturation state of aragonite) is larger than 1, the water is supersaturated and aragonite will precipitate from seawater and can be used by coral. When Ωarag is less than 1, the water is unsaturated resulting in the dissolution of aragonite dominated calcium carbonate structures like the skeleton of L. pertusa. More than 97.3% of L. pertusa global locations are in supersaturated waters (Ωarag>1) (Davies et al., 2011).

In short term laboratory experiments, growth rate decreased when exposed to unsaturated waters with respect to aragonite (Form and Riebesell, 2012). Interestingly when exposed for a longer period of time (six months), growth rate actually increased (Form and Riebesell, 2012). However increased growth rates under stressful conditions may incur costs that may be detrimental to survivorship (Wood et al., 2008). To cope with unsaturated water, large energetic inputs are needed which need to be subsidized by food. Furthermore, the dead coral framework underneath the live coral is susceptible to dissolution due to ceased calcification and may collapse. The consequences of such physical damage is unknown but may have significant consequences to the reef at the top of the framework.

Acidification alters calcification rates, respiration rates, growth rates and may cause energetic imbalances in cold-water coral (Maier et al., 2009; Form and Riebesell, 2012, Hennige et al., 2014). As a further consequence of ocean acidification, the aragonite saturation horizontal (ASH) is rising up the water column (Orr et al., 2005). The ASH is the point at which the Ωarag changes from supersaturated to unsaturated, so from habitable to inhabitable conditions for L. pertusa. By 2099, only 30% of cold-water coral locations will be in waters supersaturated with respect to aragonite (fig 7; Guinotte et al., 2006). The importance of aragonite saturation can be observed in the Pacific Ocean where the ASH is relatively shallow (50-600m) compared to the North Atlantic (>2000m), which may be the cause of the scarcity of L. pertusa.

Fig. 7. Gif image showing the change of the aragonite saturation horizontal from 1765 to 2099. Adapted from Guinotte et al., 2006.

Fig. 7. Gif image showing the change of the aragonite saturation horizontal from 1765 to 2099. Adapted from Guinotte et al., 2006.

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