If, as the saying goes, “nothing in life is free,” corals may pay a price for being so adaptable to climate change. In fact, it was widely believed among scientists that corals should suffer reduced growth or other trade-offs when they partner with symbiotic algae that help them tolerate warmer waters. However, new research led by Penn State shows that some corals can have their cake and eat it too, and as a result, these partnerships with corals may dominate coral ecosystems in a climate-changing future.
“Our findings refute the general perception that reef-building corals with thermophilic algae grow poorly,” said Todd Lagunis, a professor of biology at Penn State. “Instead, these heat-adapted partnerships better tolerate extreme marine heatwaves and are more likely to ecologically expand and dominate coral reef ecosystems in the future. While the coral reefs of the future may not look pretty—with reduced diversity and greatly diminished ecosystem services—the resilient fauna left behind will likely continue to provide food and habitat for other animals, and some coral growth for the ecosystems that created them.”
LaJeunesse explained that corals are geological structures created by coral colonies consisting of tiny single polyps resembling sea anemones whose tissues contain dense clusters of photosynthetic algae, called “dinoflagellates.” These flagellates – which the researchers call “symbionts” – differ in their ability to withstand high temperatures. When the ocean gets too warm, many symbiont species die, and their coral hosts die with them. And when coral reefs collapse, fisheries, tourism, and ecosystem services, such as a hurricane barrier, are also at risk.
According to LaJeunesse, the first documentation of differences in coral mortality based on symbiont species found in the eastern Pacific Ocean after the 1997-1998 El Niño Southern Oscillation occurred when water temperatures were 2-4 °C warmer than average historical temperatures. After this event, he said, the corals that hosted the Durusdinium glynnii symbionts survived, while the corals that hosted the Cladocopium latusorum symbionts died.
Mark Warner, professor of marine science and policy at the University of Delaware, said, “Reefs associated with D. glynnii clearly had an advantage during this extreme thermal event, but does hosting these heat-tolerant symbionts come at a cost? Previous research has indicated that the costs of thermal tolerance manifest as reduced nutrient transfer from symbiont to growth and the significant negative physiological trade-offs that can occur if similar physiological trade-offs are desired, such as those that could occur in the fate of coral reef ecosystems.”
Kira Turnham, lead author of the study, published today (July 19) in the journal Proceedings of the Royal Society bexplained that to investigate potential trade-offs in thermo-tolerant partnerships, the research team compared the growth and reproduction of Pocillopora corals hosting the more thermo-tolerant D. glynnii symbiont and more sensitive C.
“These symbioses are common throughout the Indo-Pacific, and represent evolving, ecologically successful relationships,” she said.
Specifically, the team measured skeletal growth, total mass increase, and calcification rates—or the rate at which corals produce calcium carbonate, a measure of their growth. The team also measured reproductive output and response to heat stress to assess the functional performance of these partner groups.
“We found that D. glynnii provided the ability to tolerate water temperatures that compromise most forms of mutualism between corals and flagellates without noticeable trade-offs,” Turnham said. “This mix of partners grows and reproduces just like a more temperature-sensitive partnership.”
Turnham noted that the differences in performance and function between the two companies were only apparent during experimental heating, highlighting D. glynnii’s ability to withstand higher temperatures and provide heat tolerance to their hosts. The team is also studying similar equivalencies between corals and algae that include several coral species in the western Pacific nation of Palau to determine the breadth of these findings.
“This study highlights the contextual significance and fascinating biology of coral symbiosis,” Turnham said. “By investigating the shared evolutionary history of symbionts, providing a contextual perspective and using improved identification of symbiont species, we can make more meaningful predictions about the persistence of corals as the oceans are constantly warming from climate change.”
Other authors on the paper are Matthew Aschaffenburg, University of Delaware; Tai Petai, University of South Carolina Beaufort; David Paz-García, Centro de Investigaciones Biológicas del Noroeste; Hector Reyes Bunilla, Autonomous University of Baja California Sur; Jorge Pinzon, University of Texas at Arlington; Eli Timmins, Penn State; Robin Smith, University of the Virgin Islands; and Michael McGinley, University of Delaware.
The National Science Foundation supported this research.