Sensitivity of Corals to ocean acidification: the Influence of pH at the Site of Calcification in Modulating Rates of Calcification

Chair: Janice Lough

Comeau S.(1), Cornwall C.E.(2), McCulloch M.T.(3)

1 ARC Centre of Excellence in Coral Reef Studies, The University of Western Australia, School of Earth & Environment & Ocean’s Institute, Western Australia 6009, Australia.
2 ARC Centre of Excellence in Coral Reef Studies, The University of Western Australia, School of Earth & Environment & Ocean’s Institute, Western Australia 6009, Australia.
3 ARC Centre of Excellence in Coral Reef Studies, The University of Western Australia, School of Earth & Environment & Ocean’s Institute, Western Australia 6009, Australia.

Background
Central to evaluating the effects of ocean acidification (OA) on corals is understanding why calcification is affected by declining pH in some coral species but not in others. One of the hypotheses to explain the range of responses to OA reported is that species insensitive to OA are able to maintain elevated calcifying fluid pH (pHcf) and DIC levels, and hence elevated carbonate saturation states at their site of calcification, largely independent of the carbonate chemistry conditions in the surrounding seawater.

Methods
To test this hypothesis, we designed an experiment in which one fast calcifiying coral (Acropora yongei), which is potentially sensitive to OA, and one slower calcifying coral (Pocillopora damicornis), which is generally much less sensitive to OA, are incubated during 8-weeks. Experiments are being conducted under three pCO2 levels (400, 750 and 1200 μatm), and calcification rates of the organisms are being monitored during the incubation period using both alkalinity anomaly and buoyant weight methods. Determination of the pHcf at the site of calcification requires post-mortem measurements using boron isotope systematics (11B) measured on the skeletal growth formed during the experimental period.

Findings
In this first still ongoing phase of experiments, corals are exhibiting the expected pattern of behaviour with respect to rates of calcification, decreasing as pCO2 increased. The largest effects are found in the more sensitive A. yongei relative to P. damicornis. We will report the relationships between pHcf at the site of calcification and rates of calcification following post-mortem measurements of 11B on both corals. P. damicornis is expected to maintain more elevated levels pHcf at the site of calcification, largely independent of changing pCO2.

Conclusions
Understanding the key mechanisms controlling the coral mineralization process is critical to making further progress in determining the mechanisms that explain the species-specific coral sensitivities to OA.