Nutrient uptake in corals: what is more important pH or temperature?

A review of the paper by Godinot, C. ,Houlbrèque, F. , Grover, R. ,Ferrier-Pagès, C. (2011) Coral Uptake of Inorganic Phosphorus and Nitrogen Negatively Affected by Simultaneous Changes in Temperature and pH, Plos one, volume 6, issue 9, e25024

Ocean acidification is an important aspect of climate change and just within the 20^th century the pH levels of the sea has dropped from 8.21 to 8.10 and is predicted to drop another 0.5 units in the next century. A decrease of pH is already known to affect the oceans carbon cycle and calcification of corals, and must also influence the other nutrient cycles such as phosphorous and nitrogen. The symbiotic dinoflagellates within corals (Symbiodinium) play an important part within coral nutrition and provide the coral with essential nutrients nitrogen and phosphorous in the form of amino acids. The ocean pH should therefore affect this process and the uptake of nutrients. The aim of this study is to test to see if the pH and temperature affect nutrient uptake in the coral Stylophora pistillata. This is important as nutrient depleted corals are known to be more susceptible to bleaching by expelling their dinoflagellate symbiont.

Colonies of S. pistillata were collected from the red sea and nubbins were cut off and kept in laboratory tanks. Three experiments were run alongside each other for 10 days: 1) three pH’s (8.1, 7.8 and 7.5) at standard 26°C, 2) three temperatures (26, 28 and 33°C) at standard pH 8.1, 3) three pH’s at “warm” 33 °C. After ten days, the nutrient uptake was studied by monitoring the nutrient levels in the water of a contained beaker containing nutrient enriched water. Measurements of photosynthesis and respiration were also taken using respirometric chambers and a diving PAM. Finally, chlorophyll, protein and zooxanthellae concentrations were measured and statistical analyses including t-tests and ANOVA were carried out.

There was found to be no effect of pH or temperature on zooxanthellae, protein or chlorophyll densities or on the rate of respiration. However, the rate of photosynthesis was affected by temperature and the combination of temperature and pH, but not by pH alone. It decreased twofold when the coral was exposed to temperature of 33°C or when kept at pH 7.8. There was no combined effect of pH and temperature. When the temperature was raised to 29°C , the uptake rate of ammonium increased 5-fold, however at 33°C there was a severe decrease. At 33°C and a low pH corals even excreted nitrate instead of absorbing. Phosphate was the only nutrient that was affected by a combination of pH and temperature and the uptake decreased four fold from pH 8.1 to 7.5.

This results show that short term ocean acidification has no major effect on nutrient uptake whereas an elevated temperature does. An elevated temperature increases the uptake of phosphorous, however corals cannot use the phosphorous without nitrogen, which is actually excreted during periods of high temperature. There was also a decrease of photosynthesis at 33°C, due to damages in the PSII in the zooxanthellae from thermal stress.

Despite such strong patterns in this paper, the results are converse to other papers such as Anthony et al 2008, but are similar to others. These variances may be due to other factors such as irradiance levels, showing that this is a complicated relationship and cannot be fully explained by simple experiments. However, if these results are the truth, it shows that climate change will severely effect nutrient supply to the both the dinoflagellate and the host coral, and can therefore alter the susceptibility of coral reefs to bleaching.