Viruses are the smallest and most abundant components of plankton. They range in size from 0.02 – 0.2um in diameter. In one liter of surface seawater you would find up to 10 billion viruses!
When most people think of a virus they associate it with sickness, but viruses are essential to the regulation of saltwater and freshwater ecosystems. Most marine viruses are bacteriophages, infecting the bacteria in aquatic microbial communities, and are harmless to plants and animals.
Virus particles (virions) are not alive in that they have no metabolism, no mobility with which to swim, and they do not grow. Instead, new viruses are assembled within a host cell. In order to replicate, a virus must contact an appropriate host cell and transfer its genetic material to the inside of that cell. Once inside, the viral genome directs the host cell to produce new virions. When these newly created virions exit the host cell (often killing the cell in the process), they are again lifeless particles drifting in the sea, waiting another chance encounter with the right type of cell.
Viruses are so small that they can only be viewed with a scanning electron microscope, or they can also be detected as small dots using epifluorescence microscopy with a sufficiently bright fluorescent DNA stain.