Aquatic viruses unexpectedly abundant.

Aquatic Viruses Unexpectedly Abundant

Using a high-speed centrifuge and a sensitive electron microscope, scientists have discovered that even pristine marine and freshwater environments harbor astonishing numbers of aquatic viruses.

The newly discovered viral concentrations exceed by up to 10 million times those previously recorded in aquatic environments, suggesting these minuscule microbes -- some as small as 60 nanometers -- represent a much bigger piece of the ecological puzzle than scientists believed. Moreover, although the viruses themselves appear incapable of infecting humans, they may create a health threat by injecting disease-causing genes into common bacteria.

Gunnar Bratbak and his colleagues at the University of Bergen in Norway subjected filtered water samples to 100,000 times the force of gravity and analyzed the resulting sediment. Among other findings, they determined that 1 teaspoon of North Atlantic seawater taken from 10 meters below the surface contained 75 million individual viruses. More than 1 billion viruses appeared in a teaspoon of water from a nutrient-rich lake, they report in the Aug. 10 NATURE.

"This is very exciting and important work," says biologist Mary E. Silver of the University of California, Santa Cruz. "With so many [viruses] there, it raises the question of what they are all doing."

Most seem busy infecting aquatic bacteria, possibly accounting for the immense and unexplained bacterial turnover rates in water, Bratbak says. Every minute, grazing protozoans gobble huge numbers of aquatic bacteria, yet studies indicate bacterial reproduction far exceeds these grazing rates. The new findings suggest that viruses, which can multiply in bacterial cells before killing them, may account for a third or more of aquatic bacterial mortality.

The implications of this covert infection frenzy are many, says Evelyn B. Sherr of the University of Georgia Marine Institute on Sapolo Island. For ecologists, it suggests that a surprisingly vast majority of the energy exchange in the aquatic food web occurs among organisms small enough to pass right through the sieves of the smallest filter-feeding animals. This could radically alter current models of aquatic nutrient cycles, which have focused on larger plankton as the food chain's first significant link (SN: 7/30/88, p.68).

Sherr adds that high rates of virus-induced bacterial rupture might account for much of the free DNA found in seawater -- scraps previously attributed to "sloppy feeding" by protozoan grazers.

Moreover, high viral concentrations might result in unusually high rates of bacterial evolution, since viruses can carry bits of bacterial DNA from one bacterium to another. On a positive note, this could result in the rapid emergence of bacteria capable of digesting toxic wastes after a spill. "On the other hand," Sherr says, some bacteria "might develop enzymes that degrade things like boat bottoms."

More worrisome, she says, is the possibility that genes for antibiotic resistance or increased bacterial virulence -- common in the raw sewage flushed into waterways -- may rapidly spread via viruses to benign bacterial strains.

And Bratbak warns that if laboratory-engineered bacteria make their way into waters teeming with viruses, they may be more likely to pass their altered genes to native bacteria. So far, scientists have looked only on land for such DNA donations and have used the negative findings to justify further releases.