Climate change is not the only factor causing mass deaths of coral. Two recent studies from the Oregon State University reveal the role of bacteria and viruses in the coral loss as well.
The first study in the journal Environmental Microbiology focuses on corals that appear healthy but get infected by parasitic bacteria. The team focused on coral reefs in the Caribbean and discovered that disease resistance of staghorn coral is essential in restoring coral in the region, said Becca Maher.
Although coral reefs exist in less than 1% of the ocean, they are the home of about 25% of marine species. They regulate the oceans' carbon dioxide levels and also provide researchers with insight for new medicines.
By 2017, global warming has caused nearly 75% of coral reefs to suffer from coral bleaching. Bleaching occurs when the symbiotic relationship between corals and algae is destroyed, causing corals to turn white and become more vulnerable.
"New studies in diverse host systems reveal the profound influence of the microbiome on animal host fitness and survival after disturbance," wrote the authors. Microbes can also help the host's metabolisms and defense against pathogens or changing environmental conditions.
Last year, microbiologist Rebeca Vega Thurber discovered new bacteria, Candidatus Aquarickettsia, that thrive when coral reefs are polluted with nutrients. The bacteria make coral more vulnerable to disease.
Grace Klinges said that the newly identified bacteria are found around the world, affecting multiple species of corals, particularly in the microbiomes of Caribbean staghorn coral. The abundance of Ca. Aquarickettsia is a sign that seemingly healthy corals are susceptible to disease.
When the corals are stressed due to warmer temperatures, the microbiome dominance of Ca. Aquarickettsia decreased "because the bacteria rely on host nutritional resources that become scarce during periods of stress," said Vega Thurber. An opportunity then arises for pathogens to attack the coral with a disease.
Read Also: The Great Barrier Reef Has Declined By Half, New Study Shows
In another paper published in the journal Frontiers in Marine Science, researchers discovered that coral bleaching was associated with a viral infection. They studied the 2016 bleaching event in Mo'orea, French Polynesia by analyzing viral metagenomes of coral colony pairs.
Bleached and non-bleached pairs were genetically analyzed to see if there was a difference in their viruses. Bleached corals were discovered to have more eukaryotic viral sequences while non-bleached corals had more bacteriophage sequences, explained Adriana Messyasz.
There was also an abundance of giant viruses in bleached corals. Giant viruses have double-stranded, complex DNA called nucleocytoplasmic DNA (NCLDV).
The giant virus they sequenced was closely related to a marine flagellate-associated virus, shared Messyasz. NCLDV is also present in healthy corals but in smaller numbers. The study shows how giant viruses are an essential part of how severe coral bleaching is.
The authors also wrote that when corals are stressed because of warmer temperatures, "eukaryotic viruses may take advantage of the weakened coral" and affect the symbiotic relationship with algae.
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