Background
The authors write that "because viruses are obligate symbionts, they are generally perceived as parasitic and harmful to their hosts." However, as they continue, "evidence that viruses confer benefits to their hosts is growing and their role as mutualists is emerging."

What was done
In an effort to assemble what has been learned so far in this new field of study, van Oppen and her co-authors got together to review "both the detrimental and beneficial aspects of viral infections and argue that as the field of coral virology expands, in addition to their pathogenicity, the idea that viruses represent functionally beneficial components of the coral holobiont be considered [our italics]."

What was learned
After doing what they said they were going to do, the three marine biologists conclude that viruses and virus-like particles "have the potential to act as double-edged swords, conferring benefits to some hosts while being harmful to others." Hence, they say that just as "the coral animal lives in a symbiotic relationship with a diverse, metabolically active population of microorganisms (mostly bacteria) that change in response to environmental variation" and thereby "contribute to the capacity of the holobiont to acclimatize and/or adapt to environmental change," so too do they hypothesize that "eukaryotic viruses and phages are also important components of the coral holobiont and have the capacity to play fundamental roles in the basic biology of corals and their environmental thresholds."

What it means
In light of the many examples of such mutualism that van Oppen et al. present in the course of their review, there is ever more reason to believe that earth's corals may be well equipped to cope with whatever climate changes they may be forced to endure over both the near- and long-term future. They've been around for a long, long time already; and unless they succumb to the more direct environmental disturbances we create for them, they will likely persevere for a long, long time to come.