Boosting coral heat resistance

Australian scientists may have found a way to help corals become more tolerant to warmer waters by fast-tracking heat adaptations of microalgae that live symbiotically with them.

While coral reefs – the ocean’s most diverse ecosystems – are adapting to climate change, they can’t keep up with increasing heat waves, leading to their widespread and rapid decimation.

“Coral reefs are in decline worldwide,” says first author Patrick Buerger from the CSIRO, Australia’s national science agency.

“Climate change has reduced coral cover, and surviving corals are under increasing pressure as water temperatures rise and the frequency and severity of coral bleaching events increase.”

Buerger is first author of a paper published in the journal Science Advances.

To bolster the corals’ resilience, he and colleagues turned to their microalgal friends (Symbiodinium) that provide them with oxygen, nutrients and energy, and used directed evolution to strengthen the algae’s heat resistance.Directed evolution draws from naturally occurring adaptation mechanisms, rather than genetic engineering, Buerger notes. Senior researcher Madeleine van Oppen, from the University of Melbourne, instigated the process in the lab four years ago.

First, her team isolated the microalgae from Great Barrier Reef coral and cultured them in the lab to create replicates, some of which they then exposed to increasingly warm temperatures up to 31 degrees Celsius and others they kept at 27 degrees.

“Every time a cell divides, random mutations occur in the genome,” van Oppen explains. “Many of these have no effect on algal performance, some have detrimental effects and are lost, some may have a beneficial effect.

“So, by exposing the algae to elevated temperature we selected for mutations that are beneficial under these conditions, as such adapting the algae to increased temperature.”

Four years later, they compared each strain’s thermal tolerance and found all 10 that had heat-evolved showed increased tolerance to the warmer environment.

They reintroduced each of those into coral larvae that had not yet acquired algal symbionts, producing a variety of different coral-alga combinations, and exposed the larvae to heat.

A third of the heat-evolved strains increased the coral larvae’s thermal tolerance, which the team found was a result of lower photosynthesis and more genes associated with producing sugars – many of which are passed onto the coral.

The coral, in turn, showed higher activity of genes known to be involved in its heat response.

The process shows promise for future reef restoration, but more work needs to be done to explore its success in a range of adult coral species and ensure that other factors like growth rates aren’t impacted.

Even if successful, van Oppen notes this potentially valuable method is essentially a band aid solution.

“Assisted evolution approaches may buy time by helping corals to survive over the next few decades until climate warming has been addressed,” she says.

“Without strong action on climate change and reduction of greenhouse gases in the atmosphere, there is little hope for coral reefs.”