Coral reefs today face many threats: increasingly acidic oceans, rising temperatures, over-fishing, and pollution, to name a few. As if these pressures weren’t enough, corals and the animals that live on them can get sick, just like humans, and ocean acidification, global warming, and pollution may make reefs more vulnerable to disease.
In a paper published this month in the Proceedings of the Royal Society B, Gareth Williams, a postdoctoral researcher in the Marine Biology Research Division of Scripps Institution of Oceanography, UC San Diego, and colleagues found that ocean acidification and rising temperatures could affect marine diseases in some surprising ways.
The study combined field observations on Palmyra Atoll and experimental manipulations to observe coralline fungal disease, a fungal disease that affects crustose coralline algae (CCA), the pink algae that holds coral reefs together, encourages the settlement and growth of young corals, and discourages growth of harmful seaweed on reefs.
“Crustose coralline algae play very essential roles on coral reefs,” Williams said, “and diseases that kill them represent a very real threat to the health and resilience of coral reefs worldwide.”
This study focused on one episode of coralline fungal disease that infected CCA at Palmyra Atoll in the Pacific between 2009-2010, and how environmental changes—rising temperatures and increasing acidity—affected the spread of the disease and the hosts’ ability to survive the infection.
“We found that when it gets very warm, this disease becomes more common. Our oceans are also becoming more acidic and we found that this increased acidity at certain levels may actually slow the disease spread across the reef,” Williams said.
Unfortunately, the increased acidity slows the growth of the CCA as well, and diseased corals still lost nearly twice as much mass as healthy individuals. So even though under more acidic conditions the disease spreads more slowly, hosts were less likely to survive and recover from their infection.
Williams has been studying the coral reefs at Palmyra Atoll since 2007. This reef is the site of intense study by Scripps because it remains almost completely unaffected by humans and is protected under federal law. Although some of the nearby islands have small native populations, Honolulu, the nearest major city, is over a thousand miles away. Probably because of this distance from any direct human influences, Palmyra was able to recover rapidly from the fungal outbreak it experienced between 2009-2010, despite the detrimental effects of warmer and more acidic conditions. “Reefs that are more heavily impacted might not be so resilient,” Williams said.
Unlike human diseases, there are no “medicines” that can be administered to reefs to cure them. As the oceans continue to heat up and acidify, Williams’s results show that these disease outbreaks are likely to become increasingly common and damaging. However, temperature and pH aren’t the only factors affecting marine diseases. The prevalence of some fungal diseases on coral reefs has been linked to increased levels of nutrients in the seawater. Nutrients are chemical foods upon which algae and other marine species depend on, but if levels get exceptionally high, or “enriched” as scientists say, they can disrupt ecological balance. Nutrient enrichment is often caused by human activities such as fertilizer runoff from farming or dumping of human waste.
The National Geographic Society, the Gordon and Betty Moore Foundation, and Victoria University of Wellington in New Zealand funded the research. In addition to Williams, report co-authors include Nichole Price, Maggie Johnson, and Jennifer Smith from Scripps and colleagues from the University of Hawaii at Manoa, the Hawaii Institute of Marine Biology, Victoria University of Wellington, NOAA’s Pacific Islands Fisheries Science Center, and the U.S. Geological Survey’s National Wildlife Health Center. Williams said his collaboration with researchers with different areas of expertise, especially Price, ensured the success of the project.
“I went into this project with a good working knowledge of coral disease ecology and the effects of temperature, but I had zero knowledge on the inner workings of how to actually test the effects of ocean acidification,” he said. “Fortunately for me, there are several others here at Scripps with that exact expertise that I was able to tap into. There's always somebody here that knows how to tackle a scientific problem, and they are often just down the corridor.”
Williams said that without global action on climate change, the management of nutrient pollution may be the most effective way to reduce the impact of fungal diseases on local reefs. “I feel it’s urgent we take action to remove impacts to reefs and reinstate resilience at the local scale. The coral reef management community urgently needs some success stories.”
– Mallory Pickett is a master’s student in the lab of chemical oceanographer Andreas Andersson at Scripps Institution of Oceanography, UC San Diego