Corals and Carbon: The physiological response of a protected deep-sea coral (Solenosmilia variabilis) to ocean acidification

Calcifying corals provide important habitat complexity in the deep-sea and are consistently associated with a biodiversity of fish and other invertebrates. Little is understood about how deep-sea corals may respond to predicted scenarios of ocean acidification (OA), but any predicted changes will have wider impacts on the ecosystem. Colonies of Solenosmilia variabilis, a species of deep-sea coral found in the waters surrounding New Zealand, were collected during a cruise in March 2014 from the Louisville Seamount chain. Over 12-months, coral samples were maintained in temperature controlled (~3.5°C) continuous flow-through tanks. A control group of coral colonies was held in seawater with pH 7.88 and a treatment group in pH 7.65. These two pH levels were designed to reflect current pH conditions and end-of-century conditions, respectively. In addition to investigating changes in growth and morphology, measurements of respiration and intracellular pH (pHi) were taken after a mid-term (6 months for respiration; 9 months for pHi) and long-term (12 months for both respiration and pHi) exposure period. An established method used in measuring the pHi of shallow water corals was adapted for use with deep-sea corals for the first time. pHi was independent from the seawater treatment and ranged from 7.67 – 8.30. Respiration rate was not influenced by the reduced seawater pH tested here. Respiration rates were highly variable, ranging from 0.065 to 1.756 μg O2 g-1 protein h-1 and pHi ranged from 7.67 – 8.30. Yearly growth rates were also variable, ranging from 0.53 to 3.068 mm year-1, and again showed no detectable difference between the treatment and control colonies. However, a loss in the colouration of coral skeletons was observed in the treatment group and was attributed to a loss of tissue. This could indicate a reallocation of energy, allowing for the maintenance of those other physiological parameters measured here (e.g. growth and respiration rates). If this is indeed occurring, it would be consistent with the idea of phenotypic plasticity, where corals can alternate between soft-bodied and fossilizing forms, allowing them to survive past periods of environmental stress. This research is an important first step towards understanding the sensitivity of deep-sea corals to OA and the potential for acclimation, and suggests that in many respects, S. variabilis might not be susceptible to end-of-century projections of OA. Nevertheless, the observed tissue loss is interesting and warrants further investigation to assess its long-term implications. Furthermore, the impacts of greater levels of OA, and the interactive effects of other ecological parameters such as food availability, need to be tested.