Viruses were originally described in the mid-1890’s and are composed of nucleic acid (either DNA or RNA), protein (capsid) and which is perhaps surrounded by an envelope (Schlegel, 1986). Viruses are much smaller than bacteria, being around 10-300 nm. A typical bacterial cell (E.coli) is around 2000 nm in length.
It is thought that there are about 100 million ‘species’ of viruses (Rohwer, 2003) and about 1031 viral particles in the world (Breitbart & Rohwer, 2005). These vast numbers have huge implications for the cycling of nutrients and in influencing microbial diversity (Kimura et al., 2008). In oceans, it is estimated that up to 25% of the C fixed by primary producers is moved through the ecosystem by virus-led lytic activity of microbes (Kimura et al., 2008).
In soil, estimates of numbers of viral particles which infect bacteria are put at around 107 to 109 per gram of soil depending on the type of soil and soil moisture (Swanson et al, 2009; Kimura et al., 2008), with forest soils having a higher number of virus like particles compared to agricultural soils (Williamson et al., 2005). The numbers of bacteriophages found generally reflect that of the bacterial population (Sharp, 2001) although non-rhizosphere soils have a higher ratio of virus to bacteria than in rhizosphere soil (Swanson et al., 2009). However all estimates of viral numbers are dependant upon the method used to enumerate them (Weinbauer, 2006).
Temperature (range from <4°C to 95°C), pH (1 to 11), ionic strength, moisture content, presence of cations such as Mg2+ and Ca2+ as well as the type of clay present (Kimura et al., 2008) all affect the survival and propagation of viruses as well as adsorption to and movement in soil, which will affect how the viral particles interact with a potential host. Many of these factors and conditions are related to the conditions preferred by their hosts. For example, bacteria such as the extreme thermophile Sulfolobus solfataricus which can grow in hot springs at 80°C (range 70-87°C) at pH 3 (range 1.5-5.5) in Yellowstone national park harbour viral particles which have a similar range in specificity (Rice et al., 2001).
Being able to infect cells directly, viruses have the ability to integrate their genetic material into their hosts that they have infected and so are able to alter the genetic make-up of the host. This may alter how the host interacts with the environment. The interaction between virus and host tends to be very specific although it has been shown that it is possible for marine viruses infect soil bacteria, suggesting that they can transfer DNA between different biomes (Sano et al., 2004).
Viruses have been found to infect all other forms of life from plants to animals and from bacteria to fungi. Viruses occupy a unique position in biology – since they are not really alive until they enter into a host cell (Villarreal, 2004) they cannot capture energy on their own but have a specific relationship with their host in that they depend on their host to allow them to reproduce and propagate.
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