On Lake Superior’s surface, in a layer no thicker than a film of oil, exists a universe teeming with microscopic, single-celled creatures and decaying organic material. In this surface microlayer – which is only a few microns thick – bacteria thrive, spending their lives breaking down the organic material and recycling the nutrients for other living organisms to use. The bacteria, in turn, are eaten by other aquatic microorganisms. In fact, these invisible bacteria help form the base of the aquatic food web. And, just as in our human universe, this microscopic universe includes viruses that attack bacteria.
Because of the difficulty of identifying and counting these microscopic particles, little is known about the abundance of these viruses within the waters of Lake Superior. Are they more or less abundant than the bacteria that they infect? What would happen if dormant viruses suddenly became active and overwhelmed a population of bacteria? What if many bacteria actually disappeared from the water – what would be the effects? Could it change the entire aquatic food web?
Sea Grant researchers Randall Hicks, associate professor and head of the University of Minnesota Duluth’s (UMD) Department of Biology, and Mark Tapper, a UMD graduate student, recently completed a study that began to address this complex question. It was the first time anyone looked at the abundance of viruses in one of the Great Lakes.
Hicks explained that each virus attacks different bacteria. These viruses, known collectively as “bacteriophage,” are somewhat similar to the viruses that attack humans. In order to work, they must first bump into and attach themselves to the bacteria. Once attached, the virus injects nucleic acid into the bacterial cell. This nucleic acid then becomes incorporated into the bacteria’s DNA. “But,” said Hicks, “there are two types of viral infections – lytic (active) and lysogenic (dormant).”
“In a lytic infection, the virus takes over the metabolic machinery of a bacterium right away, tells the cell to produce new viral particles, and eventually ruptures the bacterial cell wall to release these new viral particles, which may then infect other bacterial cells,” said Hicks. Since a bacterium has only one cell, rupture of the cell wall causes it to die.
In a lysogenic infection, however, the viral nucleic acid is incorporated into the bacterial DNA, but it remains dormant- being replicated and passed on when a bacterial cell divides. These dormant viruses don’t take control of the cell until they are activated by some sort of environmental trigger. Ultraviolet (UV) light from the sun is one of these triggers.
“There is a lot of interest right now in global climate change and the possibility of an increase in UV light at the earth’s surface as a result of thinning of the ozone layer,” said Hicks. “Some types of UV light can damage DNA and can also cause dormant viruses to become active.”
“We wanted to determine how many dormant viruses infect bacteria in the lake so we would have some idea of what the potential problems might be if UV light does increase. If there are many dormant viruses, we might see major impacts on bacterial populations and, in turn, nutrient cycling and food webs,” said Hicks.
In order to find out, they collected water samples from Lake Superior in the spring, summer, and fall of 1993, then counted the number of free viruses in the water samples. They also exposed other water samples to UV light in order to activate and count dormant viruses. As a result of their work, they concluded that less than 7.5 percent of the bacteria in the samples contained dormant viruses. Even if all these dormant viruses were triggered, this level of infection does not appear to be a significant threat to bacterial populations.
But that doesn’t mean the research is done. As Hicks explained, “We’re better able to see and recognize more viruses now that we have better technology for observing them.” So this project was just a start. To build a more complete and accurate picture of the universe of aquatic bacteria and their viruses, researchers will have to study other freshwater lakes as well as the oceans. This Sea Grant-funded research has already stimulated similar studies in coastal oceans in other parts of the world.
Minnesota Sea Grant. December 1998.