Scientists Find Abundance of Viruses in Water
By WILLIAM K. STEVENS
Published: August 15, 1989
Sea water and natural fresh water contain up to 10 million times as many viruses as had previously been thought, scientists reported last week. Other experts say that the discovery raises important questions about the marine environment and its effect on human health.
The viruses' presence represents no new threat, if any, since they have presumably pervaded oceans, lakes and streams for ages. Nor are they harmful to humans in themselves.
But they infect bacteria, and scientists say that their unsuspected abundance might foster much more exchange of genetic material between bacteria living in the water than marine biologists have believed.
If that is true, natural bacteria that are harmless to humans might adopt traits of harmful bacteria from sewage and other contaminants in lakes and coastal waters. This raises the possibility that infections could be spread to humans by altered bacteria as well as by the original pathogens. Rapid Adaptation Seen
Moreover, scientists say, the heightened level of genetic transfer might enable natural bacteria to adapt more rapidly to new situations. For instance, they might more easily develop resistance to antibiotics used in aquaculture. Or they might be transformed in a beneficial way to break down chemicals released into the environment, like petroleum in an oil spill, more effectively.
All of these possibilities are just that so far. Whether they are real will be determined only by further investigation. But in any case, the latest findings are expected to stimulate research, now just under way, on the genetic composition of aquatic bacteria and the critical role of viruses.
Four Norwegian scientists reported in last week's issue of the British journal Nature that they had made direct counts of minute viruses called femtoplankton in natural waters from various parts of the world. They found up to 250 million virus particles per milliliter, a count 1,000 to 10 million times higher than in previous studies. The scientists are Oivind Bergh, Gunnar Bratbak and Mikal Heldal of the University of Bergen, and Yngve Borsheim, now of the University of Trondheim. Food Chain 'Out the Window'
The viruses, believed to be the smallest biological particles found in natural waters, are part of an ecological web that appears far more complicated than was believed until recently.
The original concept of a linear food chain from plankton to crustaceans to fish ''is completely out the window,'' Evelyn B. Sherr, a senior research scientist at the University of Georgia Marine Institute at Sapelo Island, Ga., who is an authority on aquatic microbial ecology, said in an interview.
It now appears that most of the activity in the aquatic food chain takes place in a complex network of pathways among algae, bacteria and protozoa, Dr. Sherr wrote in a Nature commentary on the Norwegians' finding. Only a small fraction of those microbes' activity supports all the higher animals. In the years ahead, Dr. Sherr wrote, the role of viruses in this web may emerge as a major focus of research in aquatic microbial ecology.
The most fundamental implication of the Norwegian research, she said, is the possibility that by routinely infecting marine bacteria, the ubiquitous viruses enhance the transfer of genetic material. Bacteria ''have a way of becoming more genetically diverse than they would be without viral attack,'' she said. 'Natural Genetic Engineering'
Until now, viral attack of waterborne bacteria was thought to be rare because not that many viruses were believed to be present in water.
''Now all bets are off,'' Dr. Sherr wrote in Nature. ''Natural genetic engineering experiments may have been occurring in bacterial populations for eons.''
The Norwegian scientists calculated that as much as a third of the bacterial population in natural waters is attacked by viruses each day.
Such a high rate ''implies the possibility of active transfer of genetic information'' between bacteria infected by the same strain of virus. Consequently, they wrote, it is possible that genes may spread to natural bacteria from bacteria introduced from outside.
A virus can pick up deoxyribonucleic acid, the substance that transmits genetic information, from a bacterial cell it has infected and carry it to the next cell it infects.
It is possible, Dr. Sherr said, that natural, harmless bacteria converted into pathogens by genetic transfer could infect humans. Among potential problems in this regard, she speculated, is that the dumping of raw sewage in many places around the world introduces harmful bacteria into the natural environment. Their characteristics might then be passed on to natural bacteria, which are far more plentiful.
Genetic transfer caused by waterborne viruses could be ''extremely important'' to aquaculture, she said. For instance, she said, salmon raised for food are treated with antibiotics to ward off disease, and the waterborne genetic transfer might enable the disease-causing bacteria to become more easily resistant to the antibiotics.
On the other hand, Dr. Sherr said, the increased genetic transfer ''could act in our favor.'' Hydrocarbons from spilled oil, as well as other chemicals introduced into the environment by humans, might be more effectively degraded as the genetic transfer enables bacteria to produce new enzymes.