Viruses are dynamic and integrated components of both aquatic and terrestrial ecosystems, where they are usually present in abundances of 106-108 viruses per milliliter of water or cm3 of sediment or soil. Most of these viruses are infectious to bacteria, and they constitute a signifi - cant mortality factor for prokaryotes in all ecosystems. In the marine environment, 5%-30% of heterotrophic bacteria and cyanobacteria are infected by viruses, and approximately 5%-40% of the daily bacterial and cyanobacterial production is lost as dissolved organic matter due to viral lysis. Also, eukaryotic protists are signifi cantly infl uenced by viral infections, and viruses have been shown to cause mass lysis of blooming phytoplankton populations. In addition to the direct lethal effect of viral lysis, viral activity may signifi cantly affect the cycling of carbon and other nutrients in ecosystems. Lysis of heterotrophic and autotrophic microorganisms by viruses liberates cellular components rich in nitrogen and phosphorus, which become available for uptake by heterotrophic bacteria. This virusinduced transformation of particulate to dissolved organic matter, the so- called viral shunt, stimulates microbial respiration and reduces the transfer of carbon to higher trophic levels and the export of particulate material to the seafl oor. As agents of infectious diseases of potentially all microorganisms in the world's ecosystems, viruses therefore have a signifi cant impact on microbial mortality and biogeochemical cycling on a local, regional, and global scale. According to recent models, as much as 25% of the total photosynthetically fi xed carbon in the sea is channeled through the viral shunt, either directly by viral lysis of phytoplankton or indirectly by lysis of bacterioplankton. Since about 75% of these viral lysates are rapidly taken up and metabolized by noninfected bacteria, viral lysates obviously contribute signifi cantly to bacterial carbon and nutrient supply. In fact, viral lysis may be a key mechanism in supplying carbon for heterotrophic bacteria, especially in oligotrophic systems, with small direct inputs of organic matter from primary producers.