In garden ponds and in oceans, in desert soil and in industrial water-cooling towers, matters of life and death are playing out unseen by the human eye. Here, giant viruses prey on single-celled hosts such as amoebas or algae. This microscopic bloodbath can happen on such a large scale that massive algae blooms visible on the ocean surface turn white, as dead algae fade to reveal their colorless skeletons.
Giant viruses, a group discovered only in 2003, are mysteriously large and complex, seemingly between bacteria and the tiny, simple viruses of classical biology. Scientists still don’t know much about what giant viruses do, other than kill amoebas and algae. Leave it to viruses, however, to keep surprising us: Giant viruses don’t just kill their hosts. In some cases, according to a recent study, they can keep their hosts alive and become part of them.
A couple of years ago, Monir Moniruzzaman, a postdoctoral researcher at Virginia Tech, was trying to unravel the evolutionary history of giant viruses. He stuck a particular viral gene into a large genomic search engine to scrounge up similar viruses, which he would then assemble into an evolutionary tree. To his surprise, his top match wasn’t a virus at all: It was algae. As he kept searching for more viral genes and kept getting more algae hits, he and his adviser, Frank Aylward, noticed a strange pattern. The viral genes in the algae samples had been subtly altered, as if they were being passed down from generation to generation as part of the algae genome. Giant viruses weren’t simply infecting and killing algae, it seemed; sometimes, they were integrating their DNA into the living algal cell’s DNA.