Coral bleaching occurs when corals are stressed. In response to environmental changes like increases in water temperature, the corals release algae, tiny organisms that live within them, into the surrounding waters. This process causes corals to lose their normally vibrant green, blue, pink, and even fluorescent hues and turn white. Though corals can grow back after succumbing to a bleaching event, they will only do so if they’re in safe, stable conditions.
New research has found that, much like humans who get sick when they’re rundown and overworked, bleaching may be a trigger for viral outbreaks of corals. Researchers from Oregon State University found a higher concentration of three strains of viruses, including one similar to herpes virus that causes coldsores in people, in a coral reef population from the Great Barrier Reef in Australia that was undergoing a bleaching event.
The study was published recently in Frontiers of Microbiology. Its findings were “really a case of serendipity,” Rebecca Vega-Thurber, a microbiologist at Oregon State University and lead author of the study, told Quartz.
She explained that she and her team had planned to go to Heron Island off the coast of Australia to study virus populations in coral reefs in 2011. During their trip, “there was this huge bleaching event at the reef we were working,” Vega-Thurber said. She thinks this was due to the corals being exposed to air during low tide, and warmer than usual water temperatures.
Vega-Thurber and her team got to work collecting samples of bleached coral and the surrounding water. As the corals bleached, the researchers also found up to four to six times the viral load had been recorded in corals previously, Vega-Thurber said. Using genetic sequencing and microscopy imaging, they identified the largest concentrations of viruses as being similar to the herpes virus, a retrovirus, and a megavirus.
Viruses can be a part of a healthy marine ecosystem, but when there are too many of them, they can be a threat to coral reefs. Vega-Thurber stresses that the viruses they found in the coral can’t infect humans.
Though higher viral loads were only associated—and not necessarily caused by—a bleaching event, Vega-Thurber thinks these two occurrences may be related, and because long-term bleaching can be lethal to coral, is concerned that the uptick in viruses could be contributing to wide-spread coral death. “It’s a total chicken and the egg question,” she said. “Did the bleaching event induce viral production, or did stress induce viruses which help induce bleaching? They’re probably going on at the same time.”
She compared it to the fact that at any given time, even healthy people have viruses that are circulating in our bodies, even if they aren’t making us sick. It takes a trigger, she said, like stress or exhaustion for us to actually become ill.
The large number of these viruses present in coral cells may inflict damage, although they haven’t been shown to directly kill coral just yet, Vega-Thurber said. She worries that the augmented number of viruses present during a bleaching event could infect other corals, damaging much more than a single location. In October 2015, the National Oceanic and Atmospheric Administration declared that “stressful conditions” caused by climate change had resulted in a global coral bleaching event, only the third on record, which means these viral outbreaks could be even more common.
Reefs provide food and shelter to a variety of marine life, like fish, rays, turtles, marine mammals, and sharks; without them, the entire ecosystem could be at risk of collapsing. But they are also resilient, Vega-Thurber said. “You can contaminate the environment and cause a whole bunch of nasty things to happen to the reef, but if you take that contaminate away, the reef can recover.”