Diurnal cycles of coral calcifying fluid aragonite saturation state
The sensitivity of corals to ocean acidification depends on the extent to which they can buffer their calcifying fluid aragonite saturation state (Ωcf) from declines in seawater pH. While the seasonal response of the coral calcifying fluid Ωcf to seawater pH has been studied previously, relatively little is known about Ωcf dynamics on shorter (daily) timescales, particularly whether it is sensitive to seawater pH. Here, we use alizarin dye to mark 4 days of skeletal growth in the corals Acropora nasuta and Pocillopora damicornis living in situ on Ningaloo Reef in Western Australia. Exploiting newly developed confocal Raman spectroscopy techniques, we imaged the alizarin stains and quantified Ωcf between them. We report the first observations of diurnal Ωcf cycles, which were found in both species. Our results are consistent with either external control of Ωcf by seawater pH or light, or alternatively that Ωcf follows an endogenous circadian rhythm.
Frazer McGregor and the Coral Bay Research Station provided field support that made this study possible. This study was funded by an ARC Laureate Fellowship (FL120100049) awarded to M.T.M., the ARC Centre of Excellence for Coral Reef Studies (CE140100020). The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflicts of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. The collection of corals for this study was conducted under permission from the Government of Western Australia Department of Parks and Wildlife (now Department of Conservation, Biodiversity and Attractions) with research permits and licenses to take fauna for scientific purposes (#SF010963) and the Government of Western Australia Department of Fishers exemption from the Fish Resources Management Act 1994 (#2944).
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