Student’s research to help endangered coral

Think of a coral’s skeleton as a house in which tiny algae live and produce nutrients that feed the coral.

That cooperative living arrangement, known as symbiosis, is what Rachel Geyer is studying. As part of her Summer Undergraduate Research Fellowship, she is trying to understand why some corals grow more complex skeletons, possibly making the coral more resilient. She hopes her findings may help tropical coral species threatened by rapidly warming waters and excess carbon dioxide in the atmosphere.

“Having more skeleton is like increasing the square footage of the house so that more symbionts can live in those nooks and crannies, which is a good thing,” said Geyer. Scientists are looking for traits that increase coral resiliency. “At coral nurseries, when selecting coral to transplant in new coral colonies, they try to repopulate corals with those good traits. The better we can understand what those good traits are, the better those efforts will go.”

Geyer is working in the lab of Karl Castillo, associate professor in the College of Arts and Sciences’ Earth, marine and environmental sciences department, under the guidance of biology doctoral student Stephanie Peak. The senior from Waxhaw, North Carolina, is majoring in environmental science with minors in journalism and geology.

Corals are in the same phylum, or group of related organisms, as jellyfish and sea anemones. People may think of corals as weirdly shaped rocks, but they are actually colonies of individual tube-shaped animals called polyps. Polyps range in size, depending on the coral species. The biggest is about the size of a nickel; the smallest, a pinhead.

Senior Rachel Geyer works in the Castillo Lab at Murray Hall on July 11, 2024. (Johnny Andrews/UNC-Chapel Hill)

Coral polyps strongly resemble sea anemones with specialized stingers, called nematocysts, hidden among their wavy tentacles.

“Picture it like a little cup with lots of little tentacles at the top, which can sting enemies and prey. It’s usually an orangish-brownish color,” she said. When polyps react to warm water or other stressors, they expel the algae that provide nutrients to them and turn white.

“They cluster together and secrete calcium carbonate, which is like what seashells are made of,” she said. “The secretions build a skeleton slowly. Because it takes so long, the corals can be hundreds or thousands of years old. It’s important to protect them because they’re dying rapidly now.”

But some corals aren’t dying.

Geyer is studying samples of the temperate coral Oculina arbuscula that Peak gathered off North Carolina’s coast. Oculina arbuscula is not threatened, so researchers can take more samples. She is measuring the complexity of the samples’ skeletons and whether depth affects this.

She’s been on Castillo lab trips to coral colonies at Radio Island near Morehead City, North Carolina, and visited a coral nursery at Key Largo, Florida. As part of the Maymester 2022 course Coral Reef Ecology + Management taught by Brian Naess, Geyer traveled with a class to Saint John Island in the U.S. Virgin Islands. They snorkeled at coral reefs, sailed around the island and observed marine life. A marine biology course in spring 2024 completely hooked her on coral reef research.

“I fell in love with it. I know for sure that I want to go into this,” Geyer said.