Ecology of viruses

Abstract In addition to grazing, another form of top-down control of microbes is lysis by viruses. There is probably a virus or several for every one organism in the biosphere, but the most common viruses are thought to be those that infect bacteria. Viruses come in many varieties, but the simplest is a form of nucleic acid wrapped in a protein coat. The form of nucleic acid can be virtually any type of RNA or DNA, single or double stranded. Few viruses in nature can be identified by traditional methods, because their hosts cannot be grown in the lab. Direct count methods have found that viruses are very abundant, being about 10-fold more abundant than bacteria, but the ratio of viruses to bacteria varies greatly, especially so in soils. Viruses are thought to account for about 50 per cent of bacterial mortality, but the percentage varies from zero to 100 per cent, depending on the environment and time. In addition to viruses of bacteria and cyanobacteria, studies by microbial ecologists conducted to date have examined viruses of phytoplankton and the possibility that when viral lysis ends, phytoplankton blooms. While viral lysis and grazing are both top-down controls on microbial growth, they differ in several crucial respects. Unlike grazers, which often completely oxidize prey organic material to carbon dioxide and inorganic nutrients, viral lysis releases the organic material from hosts more or less without modification. Perhaps even more important, viruses may facilitate the exchange of genetic material from one host to another.