Coral reefs are like the Amazon rainforest of the seas as they are home to 25% of the world’s marine species. Not only do they provide a a home for these species, but they also play a huge role in their survival. However, human-induced climate change has made ecosystems like this vulnerable. The Great Barrier Reef in Australia, for instance, has been hit by its third mass coral bleaching event in the last five years. Scientists say that this is most likely driven by increasing global temperatures and other factors caused by climate change.
Coral Bleaching Has Become More Frequent
The tiny plant algae called zooxanthellae, which lives in coral, produces about 90% of the food the coral needs to grow. Bleaching happens when sea temperatures get too high or low, causing the algae to get ‘stressed out’ and leave the coral. This then turns the coral white and, with its main source of food gone, it is left very vulnerable. An increase of just one degree Celsius for four weeks can cause bleaching. If this continues for more than eight weeks, the coral can die.
In 2016, the Great Barrier Reef suffered one of the worst mass coral bleachings, killing more than half of the shallow-water corals. A recent study revealed that this was caused by a marine heatwave followed by a terrestrial one, both exacerbated by global warming. The researchers explained that the increase in sea surface temperature back then was caused by El Niño. After that, a landborne heatwave moved across eastern Australia and spilled out over the ocean just as the first phase of the marine heatwave was ending.
"It turns out that El Niño did play a role, and the eventual warmth was certainly higher because of the long-term trend, but the reason it lasted so long was actually this terrestrial heatwave lurking over eastern Australia until the marine warming event was just finally waning, and then: bang, the heatwave leaked out over the coastline," Kris Karnauskas, an associate professor of atmospheric and oceanic sciences at the University of Colorado Boulder, said.
A second mass bleaching in 2017 meant the coral could not recover. This year, the ecosystem has experienced its most widespread bleaching event on record, with the south of the reef bleaching extensively for the first time. Reports showed that last February saw the highest monthly sea temperatures ever recorded on the reef since records from the Australian Bureau of Meteorology began in 1900.
"We are all in shock really at how quick this has happened. Three severe bleaching events in five years is not something we anticipated happening until the middle of the century,” Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies at James Cook University, said.
A 2018 study revealed that bleaching events have become five times more frequent, with the global proportion of coral being hit by bleaching per year rising from 8% in the 1980s to 31% in 2016. According to Carbon Brief, a UK-based website designed to improve the understanding of climate change, both in terms of the science and the policy response, the average reef is affected once every 25 to 30 years in the 1980s and once every six years in 2016.
Prof. Nick Graham, a Royal Society university research fellow and chair in marine ecology at Lancaster University, said that this coral bleaching is a “modern phenomenon” driven by global warming. “All coral reef regions of the world are now experiencing more frequent severe coral bleaching events. Critically, the time between bleaching events is now as short as six years, which is insufficient time for full recovery of coral cover on damaged reefs,” he said.
If this continues, there’s a high possibility that our coral reefs won’t be able to survive because of the massive amounts of heat the world's oceans have already absorbed. While many researchers across the world have been exploring ways to revive reefs, experts say that these efforts will not be enough if we don't address the root cause of their demise: human-caused climate change.
Rising temperatures have made algae living inside the corals’ tissues stressed, causing them to break free. This is the mechanism behind the coral bleaching events that have plagued Australia’s Great Barrier Reef in recent years. In response, researchers from Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia's national science agency, the Australian Institute of Marine Science (AIMS), and the University of Melbourne developed a lab-grown strain of microalgae that is more tolerant to heat. The team hopes that this creation will help in addressing the worsening coral bleaching.
According to Phys.org, an internet news portal that provides the latest news on science, the researchers isolated the microalgae from coral and cultured them in the specialist symbiont lab at AIMS. Then, they exposed the cultured microalgae using a technique called "directed evolution" to increasingly warmer temperatures for over four years. The experiment assisted the corals to adapt and survive hotter conditions.
Prof. Madeleine van Oppen of the Australian Institute of Marine Science and the University of Melbourne explained that directed evolution refers to the exposure of an organism to controlled laboratory conditions aimed at accelerating adaption to those conditions and enhance certain traits.
"Our novel approach strengthens the heat resistance of coral by manipulating its microalgae, which is a key factor in the coral's heat tolerance," CSIRO Synthetic Biology Future Science Platform (SynBio FSP) science lead Dr. Patrick Buerger said.
Not only did the researchers establish that the coral-algal symbiosis was more heat-tolerant compared to the original one, but they also found out that the heat-tolerant microalgae are better at photosynthesis and improve the heat response of the coral animal. "These exciting findings show that the microalgae and the coral are in direct communication with each other,” van Oppen said.
The good news is that the researchers can level up their research. According to New Atlas, one of the world's largest independent science and technology publications, van Oppen said that this allows them to grow the heat-evolved algae strains in very high quantities in aquaculture facilities. “The benefits of this potential intervention is that even a few types of these heat-evolved algae can form a symbiosis with many different coral species. For instance, the microalgal species the work was done with is known to form a symbiosis with at least 100 coral species. Therefore, we can help with comparable low effort lots of different coral species,” she said.