ENCORE – A Step on the Path Toward More Resilient Corals

Jurassic Park’s Dr. Ian Malcolm was right. Against seemingly impossible odds, life often does “find a way.” Fictional dinosaurs notwithstanding, creatures large and small display impressive abilities to adapt to changing conditions. Sometimes, though, the changes are so rapid and profound that “life” starts losing its way. Sometimes, “life” needs a hand.

Such is the case as coral reefs around the planet are threatened by surging ocean temperatures. Now in its second year, ASU BIOS’s ENCORE project (Enhancing Coral Resilience Against Climate Warming) is adding to scientific knowledge about how human intervention might give corals a leg up in their struggle to succeed in warmer waters.

“Can we make reef-building corals stronger? That is what the ENCORE project is about,” said marine benthic ecologist Yvonne Sawall. “We’re trying to train them to become more thermally resistant.”

Recent research aimed at saving coral communities include studies that focus on identifying corals that are already highly thermal-resistant and attempting to propagate them in larger areas, or studies such as ENCORE that test strategies for increasing the thermal-resistance of corals by “pre-conditioning” them to tolerate more heat, said Sawall, who leads ENCORE with three other principal investigators: ASU BIOS marine biologist and associate scientist Samantha de Putron, marine ecophysiologist Hollie Putnam of the University of Rhode Island and coral molecular ecologist Gretchen Goodbody-Gringley of the Central Caribbean Marine Institute. Two Bermuda Program students, three other university students and three PhD candidates have also assisted the project, with support from the Heising-Simons Foundation International.

Work began last year with thermal performance tests and environmental conditioning exposures on several Bermudian coral species in aquaria and at the Bermuda Marine Mesocosm Facility, a newly updated outdoor facility that mimics near-natural conditions and is especially suited for thermal manipulation experiments. Initial “stress tests” performed with adult corals in 2022 found that the impact of higher temperatures on the photosynthesis of coral-dwelling algae that provide corals with food and underpin their survival is species-specific. As in the wild, marine heat waves can reduce photosynthesis and cause coral bleaching (loss of coral-dwelling algae) – a condition that is potentially fatal but from which corals can recover if the habitat sufficiently improves.

A 2020 United Nations Environment Programme (UNEP) report found a progressive loss of about 14 percent of the world’s coral between 2009 and 2018, “primarily due to recurring large-scale coral bleaching events”. The UNEP report stated that, “Almost invariably, sharp declines in coral cover corresponded with rapid increases in sea surface temperatures, indicating their vulnerability to marine heatwaves,” adding that during the preceding decade “the interval between mass coral bleaching events has been insufficient to allow coral reefs to recover.”

As global ocean temperatures continue rising, the race is on for science-based solutions to improve the thermal resistance of corals. So far, de Putron said, ENCORE researchers “do see potential for corals to adjust to a warmer ocean…but we don’t know how long they keep that resiliency.”

This summer, ENCORE explored whether corals that are exposed to and survive heat stress last year retained a higher resiliency by exposing them to another heat stress this year. This experiment is currently underway. The team also embarked on experiments with “coral babies” by collecting adult corals, allowing them to spawn in the mesocosm and then conducting thermal manipulation experiments with resulting coral larvae. The hypothesis is that corals that are exposed to thermal stress in their very early life stages are more adaptable to heat stress events.

“It was a busy summer at the mesocosm with several visiting scientists and students and it was great to see collaborative efforts across the concurrent coral projects, which contributed to the many successes,” said de Putron. “The PIs on the grant all traveled to meet at BIOS at the end of July for a busy few days of important meetings that allowed us to finalize the research plans and strategies.”

As of mid-August, Diploria labyrinthiformis (brain coral) specimens had yet to spawn, but experiments were underway with an unexpectedly prolific spawn of Porites astreoides (mustard hill coral). The project’s postdoctoral scientist Brett Jameson said the team began with two groups of Porites astreiodes larvae – one cohort that were exposed to heat (30 degrees C) and the other kept in water of ambient temperature (28 degrees C). All were given substrate on which to attach after two days, and then transferred to a “common garden” at ambient temperature two weeks later, where growth and mortality were monitored.

After the specimens spent two weeks in the common garden, ENCORE scientists repeated the thermal manipulation experiment. In this way, the study mimics a potential natural scenario in Bermuda, where an early summer marine heat wave is followed by another more intense one in August. This time, half of the recruits previously exposed to heat stress were exposed again, as were half that had remained in ambient temperature in the first phase. Likewise, half the ambient group in phase one remained in ambient conditions during the second phase, while half were subjected to warmer water in this round.

“We will end up with four groups: Ambient/Ambient, Ambient/Heat, Heat/Ambient, Heat/Heat,” Jameson said. “We will test if the previous stress has been beneficial or detrimental and if we can  distinguish between the effects at different levels of coral development.” While photosynthetic performance, growth and mortality rates were recorded throughout the experiment, samples were also sacrificed for biochemical and molecular analyses.

ENCORE’s 2023 agenda also includes thermal stress tests (like those in Bermuda last year) on corals collected off Oahu, Hawaii in September and the Cayman Islands in October.

Next year, the team plans to investigate whether heat-resilient coral specimens are passing those traits along to the next generation. Attempting to answer the question, “what’s the legacy,” researchers will also conduct a new experiment next summer - collecting adult corals, exposing them to heat to see if they produce more heat resistant offspring.

Noting that specimens collected today are already more thermally resistant than previous generations of corals due to natural selection, Sawall said scientists have discovered that some corals are tolerating higher temperatures because the symbiotic algae they host have themselves adapted to handle more heat.

“There is hope that through adaptation of the algae…they may actually be able to adapt fairly quickly,” Sawall said. “At least some of them…that’s the hope.”