But no matter how carefully Fuhrman counted his bacteria and protista and charted their growth and feeding rates, a discrepancy remained. Protista just weren't numerous or voracious enough to account for the number of bacteria that were disappearing. The other riddle was a visual one. Conventional wisdom of the time argued against the theory. In the open ocean, microorganisms float like isolated planets, surrounded by vast stretches of empty space.
Viruses are highly specific, designed to attack only certain types of bacteria.
The theory soon got a good experimental test. In a complex, six-step process, Fuhrman and Proctor accomplished this. The task involved taking 40-liter samples of seawater, concentrating down the portion that contained bacterial assemblages, filtering this to separate out the bacteria, fixing these bacteria in a block of plastic, making thin slices of this plasticized sample and then examining the slices under the electron microscope. The goal was to search for bacteria in the only phase of viral infection in which the infection is visible - the moment when an infected bacterium breaks open. Proctor and Fuhrman found in their samples that, at any given time, about 2 percent of the bacteria were in this phase - a figure that translated into an overall infection rate of about 50 percent.