Foster Scholar Erin Arneson Examines Coral Resilience to Ocean Acidification

By Yaamini Venkataraman

January 2020

Diving at Gray’s Reef National Marine Sanctuary, one thing is obvious: it’s a hidden treasure. A patchwork of Oculina corals alternates with colonized rock to form the sanctuary’s hard bottom reef. Multitudes of marine organisms — from small zooplankton and jellies to larger predators like fish and sharks — inhabit the reef.

But this idyllic haven for marine fauna is not untouched by changing ocean conditions. The ocean constantly absorbs carbon dioxide from the atmosphere, and with carbon dioxide levels increasing, the ocean is becoming more acidic — a phenomenon called ocean acidification. Dr. Nancy Foster Scholar Erin Arneson devotes her time to determining how this ecosystem can stay healthy despite the dangers of ocean acidification.

For her master’s in biology at Georgia Southern University, Arneson runs laboratory and field experiments to see how ocean acidification impacts a species of coral called Oculina arbuscula. Her studies are supported by the NOAA Dr. Nancy Foster Scholarship Program, which provides support for master’s and doctoral degrees in sanctuary-related ocean sciences.

Not only is a more acidic ocean stressful for the organisms living in it, but chemical reactions also make the building block of coral skeleton, calcium carbonate, less available. Organisms that build calcified structures, like corals, shellfish, sea urchins, and some plankton, have to expend energy to build and maintain these structures. If acidification becomes too severe, the chemical conditions can make skeleton or shell production impossible, threatening the survival of some species.

“The problem with ocean acidification is that it reduces the amount of carbonate ions available for corals to calcify, usually resulting in slower [growth],” Arneson says. “In order for corals to continue calcifying, it’s an energetic cost.”

Aside from standard ocean acidification, water chemistry at Gray’s Reef National Marine Sanctuary can fluctuate due to other factors. The sanctuary is close enough to shore that runoff from land can alter the delicate chemical balance. Hurricanes may also stir up bottom sediments and throw the balance off. In the midst of this chemical turmoil, Oculina seems resilient.

Arneson believes that this resilience may come from a variation of the mutually beneficial relationship corals have with algae. Inside the cells of many coral species, there are colonies of algae. Safe from harm inside the coral, these algae create food from the sun that the corals use to create energy that drives skeletal production. In stressful conditions, these algae can be expelled by the corals, making the white coral skeleton visible. Many corals die after bleaching events because they don’t have the symbiotic algae to produce food and recycle wastes.

However, unlike most coral species, Oculina can be found naturally without algae. How do Oculina corals without symbiotic algae fare in stressful conditions? By examining calcification in corals with and without symbiotic algae, Arneson can determine if the algae provide “essential energy” towards calcification.

Understanding how Oculina fares with ocean acidification is crucial for maintaining a healthy ecosystem at Gray’s Reef National Marine Sanctuary.

“Corals are ecosystem engineers — they build habitat for other organisms. Oculina is the most structurally complex coral found in this region. You find little fish living within the branches, and crabs and a bunch of invertebrates using this species for refuge,” Arneson says. If Oculina colonies are unable to withstand worsening conditions, other organisms may be impacted.

So far, Arneson’s short-term experiments have shown that corals with and without algae can calcify in acidic waters. While this could be a glimmer of hope for Gray’s Reef National Marine Sanctuary, it’s unclear how Oculina corals experiencing ocean acidification for longer periods of time or along with other stressful conditions will fare. An undergraduate in Arneson’s lab found that calcification did decrease when corals were in both warmer and more acidic waters — a combination that could be more common as climate change progresses. Arneson’s goal is to get results the sanctuary can use to inform decisions that will help protect and manage this ecosystem.

As she runs laboratory experiments, Arneson is constantly reminded of the wonder of Gray’s Reef National Marine Sanctuary.

“Seeing the organism I’m studying in the natural environment and how it’s interacting with everything else really inspires me to do my lab work and try and share that with other people,” she says.

Through the STEM Institute at Georgia Southern University, Arneson brings the joy of seeing a hard bottom reef to science festivals for the whole family. She uses everything from activities and pictures of Gray’s Reef organisms to pH testing kits and live animals to look at under microscopes. In addition to engaging attendees in activities, she tries to build people’s connection to Gray’s Reef National Marine Sanctuary.

“I also try and get people interested by connecting them with this amazing place, especially for Georgia residents,” Arneson says. “This is your coast. This is your national marine sanctuary, and it’s amazing.”

Yaamini Venkataraman is a volunteer social media intern for NOAA’s Office of National Marine Sanctuaries and a graduate student at the University of Washington.