Viruses of Microorganisms in Soil Ecosystems
Kurt E. Williamson
from: Viruses of Microorganisms (Edited by: Paul Hyman and Stephen T. Abedon). Caister Academic Press, U.K. (2018) Pages: 77-94.
Soils are extremely complex ecosystems that display fine-scale spatial and temporal heterogeneity. Depending on the soil type, total viral abundance in soils can range from as few as 10,000 (104) to over 109 virus particles per gram dry weight. While very limited data are available, comparative analyses suggest that soils contain a more genetically diverse array of viruses than either aquatic or sediment habitats. Viruses of Bacteria (bacteriophages) represent the most studied and best understood group of viruses of microorganisms in soils. They also are believed to be the most abundant virus type within soil viral communities and can have important impacts on host bacteria population dynamics as well as on biogeochemical processes. Bacteriophages can also impact bacterial genetic diversity through host selection, as well as host phenotypic conversion and gene transfer events. Fungi are important soil microbes and fungal viruses (mycoviruses) appear to be ubiquitous in nature as well, as most fungal lineages show evidence of viral infections. Virus-mediated lysis of fungal hosts is exceptionally rare, however, and most mycoviruses establish persistent, asymptomatic infections of their hosts. The significance of mycovirus infections therefore may lie in subtle modulations of host gene regulation but also can affect host secretion of toxins, hypovirulence, and thermotolerance. Protozoa are key players in soil microbial food webs and viruses of the protozoa, especially viruses of amoeba, have touched off a revolution in modern virology as recently discovered "giant viruses" have been found to exceed the 0.22 μm operational size cutoff that had been historically applied to viruses. Since 2003, four novel giant virus families have been established, with two representatives isolated from soils, though the ecological impacts of these giant amoebal viruses have yet to be determined. In addition to these unknowns, almost nothing is known regarding the viruses of Archaea, cyanobacteria, algae, or diatoms with specific regard to soil habitats. These represent significant knowledge gaps and targets for future research endeavours. Soils remain an under-studied ecosystem in spite of their complexity and importance to human civilization read more ...