Geoscience Reference
In-Depth Information
4.1
Viruses
The smallest organisms living in soils are viruses. Being strictly parasites, they live
and reproduce mainly in bacteria and generally in phyto- and zooedaphon. The most
important kinds of viruses in soils are those living in bacteria cells. They are called
bacteriophages or simply, phages. Their name carries the Greek
phagein
meaning to
devour or to eat. These phages are able to eliminate some populations of bacteria
because they “eat” the bacterial genome and liquidate the hereditary information
required for bacteria to reproduce.
The negative impact of phages attacking nitrogen-fi xing bacteria living symbi-
otically with roots of legume crops is well known by farmers. They know that alfal-
fas growing in their fi elds in the absence of symbiotic cooperation with bacteria do
not absorb enough nitrogen to produce economic yields.
4.2
Bacteria
The next smallest soil microorganisms are bacteria - single-cell creatures having
diameters or lengths between 0.2 and 2
m. They belong to prokaryotes, cells with-
out a nucleus, with a name derived from the Greek words
pro
meaning before and
karyon
meaning kernel or nut. Without a nucleus, bacteria lack mitochondria to
serve as an internal cellular power or energy source. Mitochondria could be com-
pared to the power plant of the cell, but they have other more important functions
for the life of the cell. For millions of years prokaryotes were the only form of life
on Earth until eukaryotes evolved. A eukaryote is an organism containing mito-
chondria and other complex structures. The Greek prefi x
eu
means good. Eukaryotes
include animals, plants, and fungi.
Bacteria are present in soils mainly in the form of a thin biological fi lm covering
the solid particles. Without any chlorophyll within their bodies, they cannot use
solar radiation as a source of energy needed during their lives. For an alternative
energy source, they take over the energy forming the bonds between atoms in the
organic molecules within dead organisms and excrements. When these bonds are
broken, energy is released and further used for their life processes. This decomposi-
tion of the organic molecules sometimes produces nitrogen in the chemical forms of
nitrite and nitrate. The process is called
nitrifi cation
. Completing the two-step pro-
cess to nitrate is important to avoid the toxic infl uence of nitrites upon plants.
Another decomposition product of organic molecules is ammonia and the process is
denoted as
ammonifi cation
. It typically occurs with inadequate levels of oxygen,
i.e., under anaerobic conditions. However, ammonifi cation also happens during ini-
tial stages of decomposition and even simultaneously with nitrifi cation. Whenever
the two processes occur together, nitrifi cation usually dominates provided there are
suffi cient amounts of oxygen in the soil pores (Fig.
4.1
).
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