Reef building corals are the animals that provide the hard rock (calcium carbonate) skeletons that form coral reefs. This project investigated how some species of corals may be affected when the concentration of carbon dioxide (CO₂) in seawater, water temperature, and nutrient levels (e.g. nitrogen) increase. There is growing concern that in the next 100 years, increased CO₂ will lead to a lowering of ocean pH or "ocean acidification." Likewise, continued global climate change has already impacted seawater temperatures, and many species of corals are well known to lose their symbiotic algae when water temperatures approach 32°C (~ 90° fahrenheit) and begin to "bleach." Coral bleaching leads to a substantial decline in coral growth and reproduction and, in some cases, death, while ocean acidification may lead to a loss in coral growth or a loss of pre-existing coral skeleton.

We discovered a range of responses in four different species of Pacific reef corals when they were exposed to CO₂ concentrations and temperatures designed to simulate what a tropical ocean may be like in the next 50 to 100 years. Out of the four species tested, only one had a significant decline in growth rate (by ~53% when compared to our control of 28°C (~82°F)). When the animal biomass and energy reserves were examined (e.g. protein, carbohydrates, and lipids), we noted a minimal change in these parameters when collected near the growing edge of each coral colony where most new skeletal growth occurs. However, when whole coral fragments were examined, there was evidence that some coral species lost biomass (protein and lipids), while their symbiotic algae residing within their tissue were more resilient. As expected elevated temperature led to a slight reduction in photosynthetic efficiency in some coral species and this was not offset by excess CO₂. Hence, elevated CO₂ does not appear to protect corals from the damage of elevated seawater temperature.

We conducted additional experiments with a sea anemone that also contains symbiotic algae similar to those found in corals. For these experiments, excess CO₂ led to enhanced photosynthesis in the treated animals. In addition, elevated CO₂ led to greater reproduction and anemone growth. In contrast to many reef corals, this sea anemone appears grow better under ocean acidification conditions, and this is due to a higher level of photosynthesis in their symbiotic algae and a greater subsequent contribution of food (in the form of photosynthate from these algae) to the animal.

The overall conclusions of this project suggest that not all reef corals will be equally impacted by ocean acidification. Many corals are still in danger of losing vital symbiotic algae due to temperature stress, and some are not able to grow as well under simulated ocean acidification. With regard to ocean acidification, similar to recent results recorded from natural CO₂ seeps in the Mediterranean Sea where ocean acidification is already occurring, experiments with a tropical symbiotic sea anemone show that this animal may fair much better when compared to hard corals in the future ocean.

Last Modified: 01/15/2015
Modified by: Mark E Warner