Viruses, Viral Lysis of Bacteria, and the Viral
Shunt
(^) The ocean not only contains ∼ 106 bacteria per ml, it also contains ∼ 107 viruses per
ml. Counts are made by centrifuging particles from filtered (pore size, say, 0.45 μm)
seawater on to electron microscope grids (copper-wire grids covered with thin, plastic
film) and examining them by transmission electron microscopy (TEM; Fig. 5.12) or
by epifluorescent microscopy (Weinbauer & Suttle 1997; Chen et al. 2001). All
viruses live by parasitizing the cellular machinery of larger organisms. They consist of
a modestly complex protein coat covering a small strand of DNA or RNA as its
genome. Upon contact with a suitable host cell, say a marine bacterium, the protein
coat attaches to the cell surface and the viral genome is inserted through the cell wall,
often through a specific transport protein. The genome then redirects the cellular
machinery to generate multiple copies of the virus, which are eventually released into
the habitat by lysis of the cell to repeat the cycle. Most marine bacteriophages sustain
populations by such cycles of infection, multiplication in the host, then lysis.
Fig. 5.12 Transmission electron micrograph of viral plankton, probably mostly
bacteriophage, centrifuged from seawater on to an EM grid. Dark, roughly hexagonal
spots are virus “heads”. Some clearly show their attachment ducts. The web-like,
nearly circular objects are probably the collapsed protein aprons which surround the
attachment ducts of some bacteriophage.
(^) (Courtesy of K. Eric Wommack.)
Fuhrman and Noble (1995) studied bacteriophage activity in cultures of bacteria
collected at the end of Santa Monica pier, a decidedly neritic site. They examined
viral TdR incorporation by a method analogous to that for bacteria and determined
viral infection levels of bacteria (viruses per cell). They estimated protist bacterivory
by the FLB method (see above). They also examined the rates of disappearance of