Many interesting interactions between microbes take place in the ocean. As single-celled organisms lacking complex sensory organs, many such interactions and communications are mediated by chemical signals. Some bacteria, for example, each produce small amounts of certain chemicals and release them into the environment. When the concentration of the chemical signal builds up to a certain point, the bacteria change their behavior to take advantage of their high numbers that must be present to produce so much of the signal. This process is called quorum sensing.
Some of these chemical signals can affect the behavior of organisms other than bacteria also. In this study, a common marine algal species was found to stop growing in response to a certain bacterial signal. This chemical inhibits an enzyme required for the algae to produce nucleotides to replicate their genomes. As a result, the algae are able to resist destruction by a virus that would otherwise decimate their populations.