Objective: 

Corals have successfully been used as proxy recorders for a variety of environmental parameters. However, they are known for their strong vital effects since their trace element partitioning and isotopic fractionation is significantly different from inorganic carbonates precipitated under the same conditions. We studied ion transport across coral epithelia to investigate their role with respect to element and isotope partitioning of divalent cations.

Methods: 

A coral culturing facility was established at the IFM-GEOMAR Aquarium (T=26°C, S=35, 12h light/dark cycle) for rearing tropical reef corals of the genera Stylophora and Acropora. Patches of coral colonies were placed on perspex tiles or directly onto polycarbonate filters. They grew for several weeks until they completely and tightly covered the filters. Those "coral-filter units" were used for transepithelial measurements experiments in continuously perfused, open-circuit Ussing chambers. We measured transepithelial potential and resistance in response to different stimuli (pH, light and salinity).

Results: 

We found a resistance of ~950 Wcm² for intact coral tissues. Of this only a small fraction could be allocated to the skeleton. The coral showed no active transport potential under control conditions or in response to pH and light variations. Hyposaline-isoosmotic diffusion voltages indicated cation selectivity of the tissue.

Conclusion: 

We have established an assay for transepithelial investigation of coral tissue in vitro and describe the coral to form a high resistance and partially cation selective barrier between seawater and skeleton. The resistance is determined by the integrity of the coral tissue and the formation and permeability of tight junctions.