Ecology of viruses in soils: Past, present and future perspectives

Viruses play important roles in biogeochemical nutrient cycles and act as genomic reservoirs in marine and freshwater environments, the understanding of which brought about the so-called 'third age' of virus ecology in aquatic environments. Unfortunately, the third age is in oceanography and limnology and outside the soil world. The main reason why virus ecology in soils has shown less progress is that agronomical and epidemiological interests were the primary motivation of viral studies by soil microbiologists. In this review, past research on viruses in soils is summarized after the introduction of the ecological traits of viruses, which are the effects of viruses on beneficial bacteria and soil-borne plant pathogens, adsorption of viruses to soils, and soil factors affecting viral inactivation and survival in soils. Horizontal gene transfer (transduction) in soils is also reviewed. Second, the abundance of viruses and their roles in biogeochemical nutrient cycles are summarized in aquatic environments. Five to 25% of the carbon fixed by primary producers is estimated to enter into the microbial loop via virus-induced lysis at different trophic levels in aquatic environments. The diversity of virus communities in aquatic environments estimated from analyses of the frequency distribution of capsid sizes and the morphology of virus populations are reviewed, and recent findings on the genomic diversity of viruses and their roles as the greatest genomic reservoirs in aquatic environments follow in the subsequent section. Viral genomics is elucidating the viral diversity and phylogenetic relationships among viruses in different environments. As the soil environment is a more diverse habitat for viruses than aquatic environments, viruses in soils have great potential to play roles comparable in quantity, which are unique in quality, to those in aquatic environments. Therefore, the potentiality and characteristics of viruses in soils are discussed in the final section for future research on virus ecology in soils from the viewpoints of biogeochemistry and genomic diversity. Synecological approaches to viruses in soils may open up a new era of soil virus ecology.