Roles of viruses in the environmentemi

Viruses are important microbial predators that influence global biogeochemical cycles and drive microbial evolution, although their impact is often under appreciated. Viruses reproduce after attaching and transferring their genetic material into a host cell. The host’s cellular machinery is then redirected to the making of more viruses and results in the death of the host cell in the vast majority of cases. Viruses have developed intriguing mechanisms to utilize host proteins for their own defence and for shifting metabolism from host to virus, a topic that is reviewed for bacterial viruses in this special issue of Environmental Microbiology (Roucourt and Lavigne, 2009). Globally, there are an estimated 1e31 virus-like particles. Currently, it is thought that most of the viruses are phages that infect bacteria, but archaeal and eukaryotic viruses are certainly important components of most ecosystems. Since the average half-life of free viruses in most ecosystems is ~48 h, an estimated 1e27 viruses are produced every minute. This means that roughly 1e25 microbes, or about 100 million metric tons, die every 60 s due to viruses. Viral predation, in combination with protist grazing, is able to maintain microbial numbers at values less then the carrying capacity of the system and as such plays an important role in controlling microbial communities. In this special issue Sandaa and colleagues (2009) report on the simultaneous control of microbial diversity by both viral predation (top-down control) and substrate availability (bottom-up control). Much of our knowledge about the roles of viruses in natural environments comes from studies of marine microbial communities. In the world’s oceans, about half of the organic matter produced by photosynthesis supports the production of new heterotrophic microbes (both Bacteria and Archaea). Viruses and protists then kill roughly equal proportions of these (Fuhrman and Noble, 1995). The lysed cells become dissolved organic matter which can be used by other heterotrophic bacteria. This means that viral-mediated mortality increases net respiration, the release of CO2 and nutrient recycling in the world’s oceans. Viruses and their microbial prey are also extremely diverse, abundant and active in marine sediments (Danovaro et al., 2002; Breitbart et al., 2004a). Moreover, viruses affect primary productivity by killing diatoms, dinoflagellates and cyanobacteria, as well as by releasing nutrients (Suttle et al., 1990). Thus, viruses can account for a very significant part of the ocean’s carbon cycling. Viruses seem to be ubiquitous and have been reported from any environment where life is present, from freshwaters to the sands of the Sahara desert. However, very little is known of the ecological roles of viruses in most ecosystems. In soils, potentially the biggest biosphere on the planet, most viral studies have concentrated on estimating their abundance and taxonomy. Viruses are associated with the rhizosphere of plants (Ackermann, 1997) and are also common in some of the harshest environments in the planet, ranging from hot springs (Rice et al., 2001; Rachel et al., 2002; Breitbart et al., 2004b; Redder et al., 2009) to hypersaline waters (Nuttall and Dyall-Smith, 1993). In many of these extreme ecosystems, viruses are the only known microbial predators. Metagenomic analyses of natural communities and man-created niches have shown that viruses are extremely diverse and novel. For example, 1 kg of marine sediment may contain over a million different viral types and 200 l of seawater may contain about 5000 viral types (Breitbart et al., 2002; 2004a). And there may be at least 1000 different viruses living in the human gut (Breitbart et al., 2003). The vast majority (> 70%) of genetic material carried by these viruses is completely uncharacterized and natural viral communities probably represent the largest unexplored area of genetic information space left on the planet. Metagenomics analyses of blood have also shown that healthy humans carry a number of novel, unknown viruses (Breitbart and Rohwer, 2005), including phage to known human pathogens (Gaidelytë et al., 2007). The impact of these viruses on human health is Environmental Microbiology (2009) 11(11), 2771–2774 doi:10.1111/j.1462-2920.2009. 02101.x