Do viruses even fit the definition of something alive? While they are top predators of the microbial world, they lack the ability to reproduce and so must take over the cell of a host — called an infection — and use its machinery to replicate. The virus injects its own DNA into the host; sometimes those new genes are useful to the host and become part of its genome.

Researchers recently identified an ancient virus that inserted its DNA into the genomes of four-limbed animals that were human ancestors. That snippet of genetic code, called ARC, is part of the nervous system of modern humans and plays a role in human consciousness — nerve communication, memory formation and higher-order thinking. Between 40 percent and 80 percent of the human genome may be linked to ancient viral invasions.

Viruses and their prey are also big players in the world’s ecosystems. Much research now is aimed at factoring their processes into our understanding of how the planet works.

“If you could weigh all the living material in the oceans, 95 percent of it is stuff is you can’t see, and they are responsible for supplying half the oxygen on the planet,” Dr. Suttle said.

In laboratory experiments, he has filtered viruses out of seawater but left their prey, bacteria. When that happens, plankton in the water stop growing. That’s because when preying viruses infect and take out one species of microbe — they are very specific predators — they liberate nutrients in them, such as nitrogen, that feed other species of bacteria. In the same way, an elk killed by a wolf becomes food for ravens, coyotes and other species. As plankton grow, they take in carbon dioxide and create oxygen.

One study estimated that viruses in the ocean cause a trillion trillion infections every second, destroying some 20 percent of all bacterial cells in the sea daily.