Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 582

average growth rate of bacteria is in the order of two divisions per year resulting in
maximum 4 cells from one.
Aerobic bacteria are most active in moist soils, but not in water-saturated soils,
and at neutral soil pH, and where there is enough food available.
An important group of soil bacteria is formed by the Actinomycetes. They are
fungi-like bacteria. They are smaller than fungi, and are also noted for a filamentous
and branching growth pattern that results, in most forms, in an extensive colony,
or mycelium, that look like spider webs. Mobility, when present, is conferred by
flagella. Actinomycetes are perhaps best known for production of antibiotics. The
number of actinomycetes might vary between 10 and 100 million per gram of soil.
Actinomycetes are responsible for the sweet, earthy smell of soil that some peo-
ple dislike and others love. Some actinomycetes are important pathogens (i.e., a
biological agent that causes a disease).
13.3 Organisms in the Groundwater
In the water-saturated groundwater zone, up to several hundreds of meters deep,
organisms are present, mainly living under anaerobic conditions. Although the lay-
man may be surprised to learn of the numbers of organisms in the groundwater
zone, their abundance is not as numerous as in the upper soil. Most of these organ-
isms are anaerobic bacteria (mainly using nitrate or sulphate as an electron acceptor)
or Archaea, that is, methane producing organisms. In the aerobic groundwater, you
may find also eukaryotic organisms including so-called stygobionts (from the Greek
word Styx , the mythological river of the underworld) that are solely found in the sub-
soil aquatic environment. They are adapted to these specific conditions, for example,
they are depigmented, have vermiform bodies and lack eyes. Their metabolism is
reduced and the reproductive output is relatively small.
Compared to the organisms in the upper soil, not much is known about their
habits, their interactions with abiotic factors, let alone their functions. It is generally
accepted, however, that they contribute to many essential functions of the soil, for
example, to the decomposition of organic contaminants an important process in the
self purifying capacity of the soil, and important for the protection of groundwater
quality.
13.4 Significance of the Soil Ecosystem
13.4.1 The Value of Soil Biology
With an abundance of more tan 10,000 different species and 1,000,000,000 (one
billion) different organisms per gram of soil (Dykhuizen 1998 ; Torsvik et al. 1990 ),
the abundance of bacteria is very high. This has led to discussions in the interna-
tional research community on the necessity to protect soil Biodiversity. Redundancy