Microbes Lost from Corals Due to Global Warming Cause 90% of Great Barrier Reef to Bleach

The Great Barrier Reef in Australia is the largest living system on the planet. Yet more than 90% of the reef is bleaching because of the loss of a tiny algae that lives within the coral.

After three serious bleaching events in 1998, 2002, and 2016, the Great Barrier Reef is in trouble. Visible from space, the reef is home to the world's largest coral reef ecosystem, and a natural wonder encompassing over 100 islands, 1,600 miles, and a diversity of marine life.

Thermal stress caused by global warming has impacted most of the reef. In 2016, the high temperatures caused a "massive episode of coral bleaching," notes a recent study published in the journal Nature.

While coral bleaching sounds like coral that have been left out in the sun too long, it is actually a disastrous interruption of a mutually beneficial relationship between corals, which are marine invertebrates, and tiny algae cells called zooxanthellae.

But the conditions under which corals and algae have thrived unspoiled for eons are changing, and may spell the end of the Great Barrier Reef. Around the world, coral reefs form some of the most critical and diverse ecosystems on the planet. Coral reefs are densely populated, hosting more lifeforms per unit of measure than any other marine setting. Some scientists estimate millions of undiscovered species have yet to be explored in and around coral reefs. The biodiversity of animals and plants in reef systems drives development of new medicines that can challenge viruses, bacteria, and diseases like cancer.

In addition to tourism and fisheries, coral reefs buffer coastlines during violent storms, protect wetlands, and support coastal economies. Providing food, work, protection, medicine, and sheer beauty, coral reef systems like the Great Barrier Reef are an asset of almost incalculable value to humans. While the impacts of global warming are already painfully clear, on coral reefs the result is devastating.

Microbes Lost from Corals Due to Global Warming Cause 90% of Great Barrier Reef to Bleach — Scientists measure coral mortality following bleaching on the northern Great Barrier Reef, October 2016. *Image via Tane Sinclair-Taylor*

In this image, a field of bleached coral are collapsing down, with only a few living corals left. The once wildly vivid colors of this coral owed their beauty, and a lot more, to zooxanthellae.

Zooxanthellae, also known as Symbiodinium, are microorganisms that live inside coral animals, enjoying the protection provided by the coral structure. In return, the zooxanthellae, which are photosynthesizing algae, provide nutrients, oxygen, amino acids, and other important components on which the coral lives and expands. The coral and the algae are symbionts, meaning that they enjoy a relationship that benefits both organisms.

Studies suggest the symbiotic relationship between photosynthesizing algae and corals began more than 200 millions years ago, during the late-Triassic period.

Living in typically nutrient-poor waters, zooxanthellae help corals survive and thrive. Some of the nutrients passed on by photosynthesizing algae are used by corals to create calcium carbonate, the bony body of a reef system. Compared to corals that do not host algae, symbiont corals grow up to ten times faster than non-symbiont coral.

Coral bleaching happens when these algae leave their coral home in search of more suitable environments. A number of factors lead to coral bleaching, including high temperatures, problems with water quality, disease, or even too much sunlight. Corals and their resident zooxanthellae thrive in clear water that allows the photosynthesizing algae to get enough, but not too much, sunlight.

On the Great Barrier Reef, high temperatures have stressed most of the reef, setting off outcomes that may be irreversible. With increasing temperature, the relationship between the coral and its algae breaks down. Zooxanthellae lose their capability to photosynthesize, and may trigger a chemical reaction in the coral that causes it to expel the algae. Because it is the zooxanthellae that give corals their pigment, when they die, or are expelled, the dull skeleton shows through its transparent tissue of the coral itself.

Corals do not immediately die, and can be repopulated with zooxanthellae if environmental conditions change. If conditions do not improve, the coral invertebrates die, leaving behind their calcified, but empty, exoskeleton. Eventually, the entire coral collapses.

While earlier studies suggested phases of bleaching could provide a protective resilience to corals threatened again, the current study found that corals that recovered to be bleached again did not enjoy any adaptation to the stress caused by high water temperatures. The same holds true for the hope that heat-adapted zooxanthellae would recolonize stressed coral. Even fast-growing corals take upwards of 15 years to recover, and as authors note, "when long-lived corals die from bleaching their replacement will necessarily take many decades."

"We have now assessed whether past exposure to bleaching in 1998 and 2002 made reefs any more tolerant in 2016," Terry Hughes, of the ARC Centre of Excellence for Coral Reef Studies, who led the aerial surveys in the study, said in a press release. "Sadly, we found no evidence that past bleaching makes the corals any tougher."

It broke my heart to see so many corals dying on northern reefs on the Great Barrier Reef in 2016. With rising temperatures due to global warming, it's only a matter of time before we see more of these events. A fourth event after only one year is a major blow to the Reef.

— Terry Hughes (via press release)

Although a World Heritage site and global treasure, unless humans intervene to save the relationship between corals, and the tiny microorganisms that live within them, the beauty and diversity of the Great Barrier Reef will likely only live on in the photographs and videos of those lucky enough to have seen them.