A Tale of Two Cold-Water Coral Reefs

Human industrial and agricultural activity has released massive quantities of carbon dioxide (CO2) into our atmosphere, with significant effects on our climate and oceans. About one-third of this atmospheric CO2 is absorbed by our oceans1, where it chemically interacts with seawater to reduce its pH – a scale that measures how acidic (lower pH values) or basic (higher pH values) water is. Since the industrial revolution, increasing levels of carbon dioxide have already reduced the mean pH of the ocean by 0.1 units, which corresponds to a decrease of approximately 30%. By the year 2100, further pH decreases of between 0.3-0.5 units are expected – around a 150% decline2. This process, referred to as ocean acidification, will considerably decrease the rate at which calcifying organisms are able to secrete shells, skeletons, and other calcium carbonate structures.

Cold-water corals are predicted to be among the most susceptible groups to ocean acidification. These corals thrive in deep, cold waters hundreds of feet below the surface of the ocean; they support high levels of biodiversity by calcifying skeletons used as habitat by thousands of associated species3. Despite their remoteness, cold-water corals will not be isolated from the effects of ocean acidification. These corals already grow under very low pH conditions compared to tropical coral reefs4. As deep waters become increasingly acidified, these fragile cold-water coral communities could be lost in the coming decades, in many cases before they have even been discovered.

Currently, there is no clear scientific consensus on how cold-water corals will respond to ocean acidification. Some studies have found that cold-water corals have a similar response as tropical corals – they demonstrate a reduced capacity to calcify skeletal material and their existing skeletons may even begin to dissolve under sufficiently low pH conditions5. In contrast, other studies have documented surprising resilience to reduced pH, with cold-water corals retaining their ability to calcify new skeleton even under low pH conditions6. It is possible that the explanation for these contrasting results is that separated populations exhibit different responses, a well-documented phenomenon in a number of other organisms. However, this effect has never previously been tested in cold-water coral populations.

In a recent publication, I assessed the response of two biogeographically separated populations of the cold-water coral Lophelia pertusa (pictured) to ocean acidification. Coral colonies were collected from the Gulf of Mexico and the Tisler Reef in Norway, and grown under a range of pH conditions. The results were strikingly contrasting. Corals collected from the Gulf of Mexico underwent a classic stress response to low pH conditions – they exhibited significantly lower calcification rates, a lower respiration rate, and a lower feeding rate. In contrast, corals from Tisler Reef were able to largely maintain calcification rates under low pH conditions, and had elevated respiration and feeding rates. These findings demonstrate that some cold-water coral populations may prove to be more resilient to ocean acidification than others, providing a potential avenue for the continued success of these vital species in an increasingly acidified ocean.

A close-up photograph of the cold-water coral Lophelia pertusa in the experimental aquarium used to test its response to ocean acidification. Photo: Dr. Samuel Georgian
A close-up photograph of the cold-water coral Lophelia pertusa in the experimental aquarium used to test its response to ocean acidification. Photo: Dr. Samuel Georgian

Marine Conservation Institute is dedicated to protecting cold-water coral ecosystems, and actively works to support the development of deep-sea protected areas around the world. The institute’s largest initiative, the Global Ocean Refuge System (GLORES), is designed to catalyze strong protection for 30% of the ocean in each unique marine region, including cold-water coral ecosystems. The Global Ocean Refuge System is made possible by contributions from ocean advocates like you. Thank You.

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References

1-Le Quéré C, Raupach MR, Canadell JG, Marland G, Bopp L, Ciais P, Conway TJ, Doney SC, Feely RA, Foster P, Friedlingstein P. 2009. Trends in the sources and sinks of carbon dioxide. Nature Geoscience 2(12):831-836.

2-Caldeira K, Wickett ME. 2005. Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean. Journal of Geophysical Research: Oceans 110:C09S04

3-Henry L, Roberts JM. 2007. Biodiversity and ecological composition of macrobenthos on cold-water coral mounds and adjacent off-mound habitat in the bathyal Porcupine Seabight, NE Atlantic. Deep Sea Research Part I: Oceanographic Research Papers 54:654-672.

4-Georgian SE, DeLeo D, Durkin A, Gomez CE, Kurman M, Lunden JJ, Cordes EE. 2015. Oceanographic patterns and carbonate chemistry in the vicinity of cold‐water coral reefs in the Gulf of Mexico: Implications for resilience in a changing ocean. Limnology and Oceanography 61:648-665.

5-Lunden JJ, McNicholl CG, Sears CR, Morrison CL, Cordes EE. 2014. Acute survivorship of the deep-sea coral Lophelia pertusa from the Gulf of Mexico under acidification, warming, and deoxygenation. Frontiers in Marine Science 1:78.

6- Form AU, Riebesell U. 2012. Acclimation to ocean acidification during long‐term CO2 exposure in the cold‐water coral Lophelia pertusa. Global Change Biology 18(3):843-853.

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