Internal carbonic anhydrase activity in the tissue of scleractinian corals is sufficient to support proposed roles in photosynthesis and calcification

Author:

Hopkinson Brian M.1,Tansik Anna L.1,Fitt William K.2

Affiliation:

1. Department of Marine Sciences, University of Georgia, Athens, GA, USA

2. School of Ecology, University of Georgia, Athens, GA, USA

Abstract

Reef-building corals import inorganic carbon (Ci) to build their calcium carbonate skeletons and to support photosynthesis by the symbiotic algae that reside in their tissue. The internal pathways that deliver Ci for both photosynthesis and calcification are known to involve the enzyme carbonic anhydrase (CA), which interconverts CO2 and HCO3−. We have developed a method for absolute quantification of internal CA (iCA) activity in coral tissue based on the rate of 18O-removal from labeled Ci. The method was applied to three Caribbean corals (Orbicella faveolata, Porites astreoides, and Siderastrea radians) and showed that these species have similar iCA activities per unit surface area, but that S. radians had ∼10-fold higher iCA activity per unit tissue volume. A model of coral Ci processing shows that the measured iCA activity is sufficient to support the proposed roles for iCA in Ci transport for photosynthesis and calcification. This is the case even when iCA activity is homogeneously distributed throughout the coral, but the model indicates that it would be advantageous to concentrate iCA in the spaces where calcification (the calcifying fluid) and photosynthesis (the oral endoderm) take place. We argue that because the rates of photosynthesis and calcification per unit surface area are similar among the corals studied here, the areal iCA activity used to deliver Ci for these reactions should also be similar. The elevated iCA activity per unit volume of S. radians compared to the other species is probably due to the thinner effective tissue thickness in this species.

Publisher

The Company of Biologists

Subject

Insect Science,Molecular Biology,Animal Science and Zoology,Aquatic Science,Physiology,Ecology, Evolution, Behavior and Systematics

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