Most viruses are much smaller than the critters we feel comfortable calling "organisms." Since 2003, scientists have been finding enormous exceptions to this trend — viruses so large and complex that they even get infected by other viruses.
The world of these virus-infecting viruses known as virophages — which literally means, "things that devour viruses" — is still largely a mystery. So far, it looks like most giant viruses infect single-celled organisms such as amoeba, and scientists recently learned that they may play a key role in regulating the relationship between giant viruses and their prey. A species known as the Sputnik virophage may actually confer a kind of immunity to giant viruses upon amoebas, they report in the journal Proceedings of the National Academy of Sciences.
Most known viruses reproduce by injecting their genetic material into a host. That's a major reason that classifying them as living or nonliving gets messy.
It's also why classifying giant viruses gets even messier. Giant viruses, unlike their smaller counterparts, have all of the molecular machinery they need to reproduce — virophages wouldn't be able to infect them if they didn't, because they wouldn't be able to hijack their replication machinery.
When Sputnik infects a giant virus such as the mamavirus, shown above, it injects its DNA into the virus so that it becomes integrated with the giant virus's genome. When the giant virus then infects the host, Sputnik's DNA becomes integrated into the prey's DNA, too.
The researchers found that the genes from Sputnik are highly expressed — in other words, used a lot — in the the genome of a species of algae called Bigelowiella natans. This suggests that the genes are serving some function in the algae, and it turns out that algae with Sputnik DNA tend to be less susceptible to infection by giant viruses.
Taken together, this evidence points toward Sputnik providing the algae with some sort of defense against their own prey, giant viruses. However, the field of virophage research is still brand new, and it has already been full of surprises, so scientists aren't sold on the idea just yet.