Coral Reef Science:  Development Highlights

Habib Sekha

K. Schneider, J. Erez. 2006. Coral calcification response to carbonate ion varies with temperature. Geophysical Research Abstracts, Vol. 8, 00418, 2006.

Abstract:

Coral reefs are globally under two major stresses, temperature and atmospheric CO₂ increase, which lower the pH of the surface ocean. We studied the combined effect of temperature and carbonate chemistry in short (~1.5 hr.) laboratory experiments on the hermatypic coral Acropora eurystoma. The experiments were performed in sealed, temperature-controlled chambers equipped with a temperature probe, pH and oxygen electrodes. First set of experiments tested the carbonate system variation in constant temperature (24° C) as follows: pH changed (7.9-8.5) without changing DIC (dissolved inorganic carbon), DIC was changed keeping pH constant, and DIC varied keeping pCO₂ constant. The rates of calcification (in light and dark incubations) displayed a positive linear dependence on pH and the carbonate ion concentration (CO₃^2- ), with a slope of 25 percent increase for 0.1 pH unit. Unlike calcification, photosynthesis did not show any trend with pH, pCO₂ and DIC. Study of stable carbon isotopes of the DIC in these incubations suggests photosynthesis enhancement by calcification at high pH probably by proton supply that combines with bicarbonate to produce CO_2(aq). A second set of experiments tested the effect of seawater temperature changes (18-30° C). In this set of experiments, in addition to A. eurystoma the coral Stylophora pistillata was also examined. The corals were cultured in continuous stirred flow cells for six weeks and were assayed once or twice a week for their photosynthesis, respiration, and calcification. Calcification and respiration showed an optimum rate in a temperature range of 22-26° C. Photosynthesis in A. eurystoma showed the same optimum as calcification and respiration, while in S. pistillata there were similar rates in the temperature range of 18-26° C and a decrease in 30° C. Gross photosynthesis to respiration ratio (P_G/R) was negatively correlated with temperature in both coral species. The CO₃^2- in the water reservoir was varied artificially during the experiment and a general positive correlation with calcification was observed at all temperatures. The slope of the linear correlation was highest at the optimum temperatures (22-26° C). A third set of experiments tested the combined effect of seawater pH (7.9-8.5) at constant DIC and temperature changes (21-29° C). Temperature increase caused a decrease in net photosynthesis and an increase in respiration, while pH changes had no affect on these processes. P_G/R decreased in response to increasing temperature in agreement with the previous experiment. Calcification in the light showed an optimum at 24° C, while dark calcification was practically not affected by temperature. Light and dark calcification showed a positive correlation with pH changes, and the response of calcification to CO₃^2- concentration was temperature dependent, with the maximum sensitivity in the optimal temperature (24° C). This result is again in agreement with the log term experiment described above. In conclusion, calcification is much more sensitive to CO₃^2- in the optimum temperatures than above or below it while photosynthesis and respiration are sensitive to temperature but not to pH, showing an optimum at 22-26° C.

Comments:

The previous Science Notes & News column discussed an abstract by the same authors. The above study extends those results to one more coral (Stylophora pistillata) and also examines the influence of temperature on calcification, respiration and photosynthesis. Previously, it was shown that the calcification rate for Acropora eurystoma was directly proportional to the carbonate concentration. The same effect is, in this extended range of experiments, also seen for S. pistillata. The increase in photosynthesis with increasing calcification is attributed to the release of protons, which, together with bicarbonate, form carbonic acid. This carbonic acid would be a source of CO₂ for the zooxanthellae.

In my opinion, this implies that bicarbonate, and not only carbonate, is involved in calcification because carbonate can't supply protons, whereas bicarbonate would if used for calcification. Temperature variations showed that the optimum calcification rate was influenced by temperature during the light period and was almost not influenced by temperature in the dark period. The authors noted that the effect of increasing carbonate concentration on calcification was the greatest at the optimum temperature (determined experimentally). Photosynthesis and respiration rates depended on temperature but not on pH, unlike the carbonate concentration which depends on pH.