The single greatest threat facing coral reefs today is ocean acidification caused by human activities. It has been well documented that the burning of fossil fuels elevates carbon dioxide (CO2) concentrations in the atmosphere. This excess CO2 is absorbed by the ocean, and its dissociation in ocean water leads to decreased pH (higher acidity) and decreased concentration of carbonate (CO3-2), an essential ion for coral skeleton formation. Ocean acidification can lead to coral bleaching, large-scale coral fatality that degrades coral reefs. The other impending effects of ocean acidification can be speculated upon; however, it is also useful to examine marine environments that exhibit naturally occurring conditions similar to those that have been predicted to appear as a result of ocean acidification.
A recent paper published in Science Advances documents a study that examined one of those such environments. Scientists from Woods Hole Oceanographic Institution studied coral reefs around the Rock Islands of Palau, an archipelago in the western Pacific Ocean. Due to a combination of biological and hydrographic processes, the waters surrounding Palau’s Rock Islands have a pH that is lower than most of the world ocean. The concentration of carbonate in these waters—measured by calcium carbonate saturation state (Ω)—is lower as well. Both of these values are equivalent to predicted pH/Ω conditions in the western tropical Pacific open ocean by the year 2100, making Palau an ideal place to study the likely effects of future ocean acidification on coral reefs.
The scientists surveyed eight sites ranging from barrier reefs around the archipelago (with a pH/Ω of 8.05/3.7) to reefs in lagoons closer to land (with a pH/Ω of 7.84/2.3). Despite the large decline in pH and calcium carbonate saturation state moving inland, the scientists found no significant decline in species diversity or coral cover.
In fact, they observed just the opposite. Coral communities in the lower pH areas around the Rock Islands were found to host a greater number of coral species and to cover a greater portion of the seafloor. This is fascinating as it contradicts various laboratory studies that have shown a range of negative effects on corals living in low pH conditions. Such effects include reduced species diversity, increased algal growth, lower rates of calcification, and difficulty constructing skeletons in juvenile corals. Amazingly, corals living in low pH conditions around Palau’s Rock Islands were actually outperforming those living in higher pH conditions further from the islands.
Additionally, the coral communities changed significantly with pH and Ω in their species composition. For example, more Acropora were seen in the high pH areas while more Porites were seen in low pH areas. This may suggest that certain species of coral are better adapted to deal with low pH conditions than others. This discovery is very significant as pH levels in our oceans continue to plunge. By studying coral species that seem to thrive under low pH conditions, we can identify what mechanisms allow them to do so. This will enhance our understanding of corals’ response to ocean acidification and will leave us better equipped to salvage as many coral reefs as possible.
Another finding from this study relates to the ability of coral skeletons to withstand, not chemical pressures imposed by ocean conditions, but biological pressures inflicted by organisms that inhabit reefs. The Woods Hole scientists discovered that decreased pH seemed to promote increased bioerosion: destruction of coral by marine organisms that burrow directly into their skeletons. This result is supported by a number of previous studies.
It’s difficult to determine exactly why lower pH leads to destruction of coral by bioerosion, because many factors other than pH could be at play. One hypothesis is that coral skeletons that form under low pH conditions are less dense and therefore more susceptible to penetration by bioeroders. Another is that low pH conditions favor the biochemical dissolution processes that bioeroders use to excavate coral skeletons. No matter what the cause, it is clear that ocean acidification leads to enhanced bioerosion, a harrowing prospect for the long-term structural integrity of coral reefs.
One major positive that can be taken from this study is that certain coral species do indeed seem to be capable of thriving in low pH conditions. This is important as we continue to burn fossil fuels and our oceans become more and more acidic. However, as the scientists who conducted this study say, it is critical that this research be continued to identify the reason for these coral species’ resiliency. Understanding as much as we possibly can about coral reef biochemistry will ultimately put us in a better position as we strive to protect them.
Coral reefs cover less than one percent of the ocean floor, but they are home to over a quarter of all marine life. They prevent erosion by protecting shorelines, sustain a variety of essential fisheries worldwide, and display inherent beauty seen nowhere else on earth. For these reasons and many others, it is imperative that we do all we can to protect coral reefs from our own negative environmental impacts.
Barkley, H.C., Cohen, A.L., Golbuu, Y., Starczak, V.R., DeCarlo, T.M., Shamberger, K.E.F. (2015). Changes in coral reef communities across a natural gradient in seawater pH. Science Advances, 1(5), e1500328.
National Science Foundation. “Coral reefs defy ocean acidification odds in Palau.” ScienceDaily. ScienceDaily, 10 June 2015.