Marine viruses.
The term femtoplankton refers to plankton less than 0.2 microns across. However, the prefix femto-, although the next order of measure down (from pico-, which describes the larger picoplankton), is a misnomer, as the size of femtoplankton is measured in nanometers (if you want to use whole numbers), not femtometers (one femtometer is approximately the radius of a single proton).
Although marine viruses have been known about since the beginning of the 20th century, it was only technological advances of the 1980s and 1990s that allowed scientists to even see the structures of marine bacteria and viruses, describe them, and begin to estimate their numbers in the ocean.
Marine viruses may be the most abundant biological entities in the ocean: you can find them everywhere, from the poles to the equator, from the surface, down to the ocean floor.
Their role in the marine food web is still being explored, but they affect the cycling of nutrients in two main ways: one, they return some of the carbon captured by photosynthesis into the water, instead of letting it progress up to the next trophic level; and two, they keep the marine bacteria population in check.
Sources:
Doug Escribano. "Microbial Ecologies: the role of viruses in the the marine environment." 16 August 2002. <http://jrscience.wcp.muohio.edu/fieldcourses01/PapersMarineEcologyArticles/MicrobialEcologies.TheRol.html> (30 December 2003)
"Cycling Through the Food Web." Bigelow Laboratory for Ocean Sciences Web Site. <http://www.bigelow.org/bacteria/size.html> (30 December 2003)