Some Like It Hot: Foster Scholar Elise Keister Investigates Why Some Coral Reefs Can’t Withstand Warmer Ocean Temperatures While Others Can

By Yaamini Venkataraman

November 2020

Is 80ºF hot? If you live in the southeast, 80ºF can be a pitstop to even warmer days. But in the temperate Pacific Northwest, that same temperature can be scorching. Like people, different coral reefs can have dramatically different tolerances to warmer ocean temperatures. In Florida Keys National Marine Sanctuary, researchers have noticed that when ocean temperature rises—even by 1 or 2 degrees—some reefs struggle while others are resilient. Why are certain reefs in the same region doing better than others?

Dr. Nancy Foster Scholar Elise Keister studies why some reefs in the sanctuary are better at coping with warmer waters. Her studies are funded by NOAA’s Dr. Nancy Foster Scholarship Program, which provides support for master’s and doctoral degrees in sanctuary-related ocean sciences. As a third year Ph.D. student at the University of Alabama at Birmingham, Keister studies two coral reefs in the Florida Keys sanctuary: one closer to shore than the other. These reefs have the same three species: Orbicella annularis (boulder star coral), Orbicella faveolata (mountainous star coral), and Porites astreoides (mustard hill coral). The reef closer to shore appears to be more tolerant to warming water temperatures than the reef farther offshore. Keister wants to know why.

When coral reefs are healthy, they teem with colorful and diverse marine life, and hard corals create a structure that protects shorelines from waves and storm surges. But due to their location and heavy human use, the coral reefs of the Florida Keys are hit hard by many stressors. So, understanding why these two reefs respond differently to warmer waters may help sanctuary managers ensure that both marine life and people can continue to reap the benefits of coral reefs.

Why does higher temperature tolerance matter? If coral reefs cannot withstand warmer ocean temperatures, they are more likely to suffer coral bleaching. Reef-building corals depend on a partnership with algae that live in their tissue, producing food and recycling waste. When the corals are stressed, chemical changes within the tissue cause the loss of the algae, leaving behind a pale or white coral. Losing an ally like algae can leave the coral in critical condition.

“The offshore reef is more susceptible to bleaching events, and the corals are also declining at much higher rates than their inshore counterparts,” Keister says. “My research is looking at the mechanisms these inshore corals use to withstand stress.”

Keister suspects that corals at the inshore reefs have more energy available to them because they take advantage of multiple food sources. While corals rely on energy from the algae in their tissues, they can also feed on plankton, providing energy reserves for stressful conditions.

“[Coral] relies on algae for a major source of sugar during high temperature events. The coral expels the algae, [and] if the temperature remains high, the coral will just die because it’s going to starve,” Keister says. But if corals are feeding on high quantities of plankton, they may be better able to withstand a bleaching event.

One kind of molecule a coral uses to store energy is a lipid, or fat. While Keister’s work shows no difference in the total lipid content—the totality of coral energy reserves—between the two reefs, there appear to be some differences in the composition of the reserves. The types of lipid molecules differed between the two reefs. This may indicate the reefs store and use energy-rich lipid molecules in different ways.

Just as not all corals are the same, not all symbiotic algae are the same. Coral species have different relationships with various species of algae, some of which are more beneficial to the coral than others. Corals on the offshore reef seem to associate with one particular species of algae from the Indo-Pacific that was introduced to the Caribbean. While this algae may be more tolerant to warmer temperatures, as some parts of the Indo-Pacific can get warmer than the Caribbean, it’s possible that the algae-coral relationship may not be ideal because this particular algae species didn’t evolve with Caribbean corals.

“We’re thinking about a coral with algae that is more thermally tolerant, but potentially an algae that’s more selfish. There has been research that shows there are fewer carbon molecules being given from the algae to the coral to build up energy reserves or to put into gametes,” Keister says. Without adequate energy from algae, the eggs may not have enough lipid reserves for larval and juvenile corals to survive, meaning fewer new corals for the reefs. “It’s not like the algae is being parasitic—it’s just not potentially passing on as much as other algal species because it hasn’t co-evolved with these species of corals.” This unequal partnership, and the changes to egg energy reserves and coral survival that come with it, may help explain declining coral cover on the offshore reef.

The dynamics between corals and algae have fascinated Keister long before she started graduate school. “[Symbiosis] is just so integral to [coral] survival—you can’t have one without the other. This research has been conducted for over 40 years and there’s still so much we don’t know.”

From resilient shorelines and biodiversity hotspots to recreational fishing and diving, healthy coral reefs are an integral part of Florida Keys National Marine Sanctuary’s identity. Coral reefs have declined more than 90 percent over the past 40 years due to hurricanes, pollution, overuse, disease, bleaching, and other factors. In 2019, NOAA and partner organizations launched Restoring Seven Iconic Reefs: A Mission to Recover the Coral Reefs of the Florida Keys, an ambitious program to aggressively restore reefs within the sanctuary.

When considering coral restoration, sanctuary managers need to understand how corals will respond to differing environments, and if certain corals will be more successful than others when outplanted. Keister hopes her research will shed light on some of those unknowns in a way that’s meaningful and keeps corals as part of the sanctuary’s story.

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.