Alizarin deposition by corals

The hydroquinone dye, alizarin red S, was employed to visualize sites of calcification in reef corals under controlled laboratory and aquarium conditions using the reef coral Pocillopora damicornis Lamarck and ten other species found in Kaneohe Bay, Oahu, Hawaii. Alizarin is taken up by living corals most effectively if added to the intake of a running seawater aquarium giving a final concentration of 10ppm. Reef corals deposit alizarin in a pattern corresponding to the calcium deposition during the test period, to produce a visible magenta stain. Areas of uptake were erratic and did not follow polyp boundaries. Deposition was often most marked in areas far removed from zooxanthellae concentrations. The deposited alizarin was redissolved from the coral skeleton with 10% EDTA solution which had been adjusted to the pH of seawater and quantitative measurements were made spectroscopically. Simultaneous uptake studies employing alizarin and radio-calcium in the form of 45Ca indicate a high correlation between these methods and that alizarin uptake is an accurate measure of calcification in reef corals. Statistical regression analysis of the two methods gave r = .987, p = <.01. Incorporation of the dye reflects the biological activity of the coral and was deposited most actively by the youngest growth forms, followed by a plateau of activity which indicated that radius of growth was about equal in all medium sized colonies studied. In light-dark experiments, heads of coral were split and half was preconditioned in total darkness for 48 hr. Both halves were then subjected to alizarin during the following 48 hr. The dye uptake in the halves subjected to dark almost equaled and at times exceeded the alizarin uptake in the halves subjected to light. Alizarin incorporation was assessed in P. damicornis subjected to incremental increases in inorganic phosphate. The dye uptake decreased linearly with the logarithmic increase in the ambient phosphate but was still apparent to a concentration of 10^4 times the normal for seawater. Various reef corals were compared as to their ability to deposit alizarin over a given period of time. They could then be ranked as to growth potential. Some specimens were marked and returned to the reef for long term growth records. A large colony of p. damicornis showed a 1 cm linear growth in five months. Calcification was observed in newly settled polyps of P. damicornis. In laboratory experiments the available calcium in seawater was decreased. Active deposition of alizarin occurred when calcium level was lowered to one third of seawater normal. Coral polyps can live for 48 hr. but do not deposit alizarin in seawater diluted to 50%. Normal alizarin deposition occurred in seawater diluted or concentrated 10% from normal but was decreased with dilutions or concentrations of 20% normal.