Agriculture Reference
In-Depth Information
III.2 MICROBIAL COMMUNITIES
2.1
Taxonomic composition
The soil microflora comprises five main groups: Viruses, Bacteria, Actinobacteria, Fungi
and Algae (Table III.2).
2.1.1 VIRUSES
Viruses are the smallest micro-organisms, arguably living or inanimate (Margulis and
Schwartz, 1998); they are acellular and comprise a central nucleic acid core (DNA or
RNA) with a protein covering. They are specific internal parasites of plants, animals,
bacteria and occasionally fungi. Their size generally ranges from 0.01 to 1 and their
role in controlling other members of the soil microflora is still poorly understood.
Some viruses are specifically associated with soils. Many viruses released from
decomposing infested debris do not survive a dry period in the soil. Nonetheless, a few
species such as the tobacco mosaic virus retain their infectivities for long periods and
survive heated-steam sterilisation treatments. These may be considered as true soil-
borne viruses.
They are usually transmitted in soil, by either nematodes or fungi. When transmitted
by nematodes, viruses occur in patches and their spread is slow. Dissemination by fungi
is much faster, probably due to the aerial dispersion of infected spores by wind or
animal vectors.
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