Deciphering the mysterious relationship between coral and algae

Across the world, once beautifully vibrant corals are turning ghostly white. 

In 2022, the Great Barrier Reef — the largest reef system in the world — was hit by its sixth mass bleaching event, severely bleaching 60% of the corals along hundreds of miles off the Australian coast. 

Over the last decade, as sea surface temperatures continue to rise, triggering physiological stress in corals, bleaching events have increased in frequency and intensity. Scientists are urgently working to find solutions to save this vulnerable resource, which is critical for preserving marine biodiversity, protecting coastlines and providing food for millions of people. 

Liza Roger, assistant professor in Arizona State University’s School of Molecular Sciences and an affiliate faculty in ASU’s School of Ocean Futures, is leading research to better understand coral bleaching and the phenomenon’s effect on the symbiotic relationship between coral and algae.

“With coral (research), one of the major things at the moment is their sensitivity to temperature,” said Roger, a marine scientist and geochemist. “We need to understand the dynamics of oxidative stress on corals better. We need to better understand the symbiosis with the algae that lives in their tissue.” 

Liza Roger

Coral and algae: A perfect match 

Corals and algae live in a mutually beneficial relationship with each other. Healthy corals are home to algae that photosynthesize, giving the coral energy and their bright color hues, while coral provide algae with shelter; both rely on each other for important nutrient exchanges for survival.

But as seawater temperatures rise, corals are hit with an onslaught of oxidative stress, causing algae to be expelled from coral tissue, leaving it transparent, showing the white underlying skeleton. 

It’s clear that the coral-algae symbiosis breaks down under these stressed conditions, but critical questions remain: How and why do these partners part ways, and who — the coral or algae — initiates the breakup? 

“We don’t know whether it's the host coral, or whether it's the algae partner, or whether it's both,” Roger said. “Is one poisoning the other? Are they poisoning each other? Is it just a mutual understanding that they've had enough of each other? You go your way, I go my way. We haven't figured that out yet.”

Roger is creating a new research lab at ASU that aims to bring together an interdisciplinary team of marine biologists, computer scientists, physicists and chemical engineers to better understand oxidative stress on corals and formulate conservation solutions. The one-of-a-kind lab will grow corals, in vitro and in vivo, and other marine organisms.

“We’re trying to look at how they are handling this stress and understand it at the molecular level,” Roger said.

Hidden clues from other marine life 

Some clues about the breakdown of coral-algae symbiosis may already exist in other marine species. 

The upside down jellyfish Cassiopea, sea anemones and giant clams all have similar types of symbiotic relationships with algae as coral do, yet have different temperature thresholds.

Cassiopea live in tropical mangroves under warmer sea temperatures with low seawater flow and lower oxygen; sea anemones live in rocky pools with varying tides and drastic temperature and oxygen level fluctuations; and giant clams can live right beside a coral under the same stressful conditions, yet each in their own unique way manage to keep its symbiosis with algae. 

“These organisms are doing something that the coral is not managing to do,” Roger said. “So the idea is to look at those organisms and learn how they do it. Is it a pathway? Is it an enzyme? What is it? How can they do it? Then we can package that to give it to coral.”

For Roger, her work today examining oxidative stress on corals is the product of a lifelong fascination with the ocean and the organisms that live within it. 

“I knew how to pick up a crab without getting nipped before I knew how to tie my shoelaces,” said Roger, who grew up along the coastline of Normandy, France, playing in shallow rock pools at low tide with her family. 

From producing academic work in marine biology, coastal management and geochemistry, to a stint educating tourists on whale species and migration as a guide in Iceland, to introducing people to the vast life and world beneath the waves in Greece and Thailand as a scuba diving instructor, Roger’s affinity for the ocean only continued to grow with time.  

Today, Roger stays focused on the ocean and is driven to save the underwater spaces and its breath of biodiversity in which she is so intimately tied.  

“This is not just about spoiling your tourist destination. It's about the whole ocean and the whole ecosystem,” she said. “I always try to be very solution-oriented. I think that's something I've always had; finding the right type of problem is always the thing. This is a big deal.”

Roger is currently recruiting graduate students and postdocs for her new marine biochem research lab. To learn more, contact her at lroger@asu.edu. 

Top photo: Coral colony on the Great Barrier Reef, Australia. Courtesy Liza Roger