Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual polyps. The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton. Scientists have discovered two viruses that appear to infect the single-celled microalgae that reside in corals and are important for coral growth and health, and they say the viruses could play a role in the serious decline of coral ecosystems around the world. These viruses, including an RNA virus never before isolated from a coral, have been shown for the first time to clearly be associated with these microalgae called Symbiodinium. If it’s proven that they are infecting those algae and causing disease, it will be another step toward understanding the multiple threats that coral reefs are facing. The research was published today in the ISME Journal, in work supported by the National Science Foundation.