Hidden impacts of ocean acidification to live and dead coral framework

Abstract

Cold-water corals, such as Lophelia pertusa, are key habitat-forming organisms found throughout the world's oceans to 3000 m deep. The complex three-dimensional framework made by these vulnerable marine ecosystems support high biodiversity and commercially important species. Given their importance, a key question is how both the living and the dead framework will fare under projected climate change. Here, we demonstrate that over 12 months L. pertusa can physiologically acclimate to increased CO2, showing sustained net calcification. However, their new skeletal structure changes and exhibits decreased crystallographic and molecular-scale bonding organization. Although physiological acclimatization was evident, we also demonstrate that there is a negative correlation between increasing CO2 levels and breaking strength of exposed framework (approx. 20-30% weaker after 12 months), meaning the exposed bases of reefs will be less effective 'load-bearers', and will become more susceptible to bioerosion and mechanical damage by 2100.

Keywords: Lophelia pertusa; biomineralization; calcification; climate change; cold-water corals; ocean acidification.

Figure 1.

Chart showing locations and summarizing physiological results from research on projected future impacts of temperature and ocean acidification on Lophelia pertusa. ‘R’, Respiration; ‘C’, calcification; ‘↑’, an increase; ‘↓’, a decrease; ‘/’, no statistically significant change. Symbols represent experiment endpoint results, pH is recorded in the total scale.

Figure 2.

(a) EBSD of Lophelia pertusa calcification during the year-long experiment. Colours indicate grouped crystal organization and orientation. (b) Full-width half-maximum (FWHM) of aragonite peak spectra at ca 1085 cm⁻¹ of skeleton from living, or unprotected dead L. pertusa.

Figure 3.

Calcification rates ±s.e. of Lophelia pertusa expressed as µmol CaCO₃ g⁻¹ dry tissue h⁻¹ versus adjusted pCO₂ with linear trend ± 95% CIs at T + 12 months. Black triangles represent aspect ratio ±s.e., of newly formed corallites, with two example images of control and high CO₂ treatments that significantly differed. The shaded area represents water conditions Ω_Aragonite < 1. Asterisks (* and **) denote example corallites represented by the data.

Figure 4.

(a) Respiration rates ±s.e. of Lophelia pertusa expressed as µmol O₂ g⁻¹ AFDM h⁻¹ for each treatment at three, six and 12 month time points. Asterisks (* and **) denote significant differences at that time point between and within treatments, respectively. The arrow indicates a decrease in pH across treatments. (b) Respiration and calcification rates ±s.e. for all treatments at three, six and 12-month time points. Linear regression lines are fitted for each time point between treatments. Nine degrees Celsius 1000 ppm treatment results are excluded due to dissolution of exposed aragonite.

Figure 5.

Back scattered electron emission of Lophelia pertusa skeleton fragments maintained in Ω_Aragonite < 1. (a) The interface (dashed line) between tissue-protected skeleton (top) and exposed skeleton (bottom). (b) A site of tissue damage on L. pertusa, and subsequent dissolution of skeleton in an otherwise protected area. (c,d) Exposed and tissue-protected sections of skeleton, respectively, with close-up inset.