Environmental Engineering Reference
In-Depth Information
10.5.3.3 Algae
Algae are single-celled and multicellular microorganisms and are considered to be the
abundant photosynthetic microorganisms in soils. According to Martin and Focht (1977),
(a) the availability of inorganic nutrients such as C, N, P, K, Fe, Mg, and Ca are said to be
responsible for their (soil algae) abundance, and (b) their principal functions in soil are
nitrogen ixation, colonization of new rock and barren surfaces, supplying organic matter
and nitrogen for humus formation, weathering of rocks and minerals, and binding of soil
particles through surface bonding.
10.5.3.4 Viruses
Viruses are the smallest type of microbe and can be 10,000 times smaller than bacteria.
They require a living cell to reproduce. It is said that their primary function is to repro-
duce, and they do it well by taking over a host cell. They have a direct inluence
on bacte-
rial abundance, and through lysis (cell destruction) and transduction—i.e., transfer of viral
DNA from one cell to another through viruses that attack bacteria (bacteriophages)—they
can alter bacterial genetic diversity. Beyond their direct attack on the various microbial
cells and their inluence on community composition, their other functions in soil are not
too well known or established.
10.5.3.5 Bacteria
Bacteria are single-celled microorganisms that vary in size and shape from very small
spheres to rods that can vary from 1 μm to a few micrometers in length and width. There
are literally many thousands of different bacterial species coexisting in the soil. With
favorable conditions of temperature and nutrient availability, it is reported that bacterial
population in soil can be in the order of 10
8
to 10
10
/g of soil. They are both autotrophic and
heterotrophic. Most bacteria used for bioremediation-treatment of organic chemicals are
chemoorganotrophs and heterotrophs. Those requiring organic substrates for energy are
called chemoorganotrophs and those using organics as a carbon source are called het-
erotrophs. Those that use inorganic compounds as an energy source are named chemo-
lithotrophs. Nitrifying bacteria (
Nitrosomonas
and
Nitrobacter
) that use carbon dioxide as
a carbon source instead of organic compounds are called autotrophs. Nitrifying bacteria
produce nitrite from ammonium ion, which is then followed by conversion to nitrate.
10.6 Natural Attenuation Capability of Soils
By deinition, the reduction of toxicity and concentration of contaminants in a contami-
nant plume during transport in the subsurface soil is called
contaminant attenuation
. We
use the general term contaminants to include pollutants and all other kinds of hazardous
substances in the luid phase of the soil-water system. If the various processes responsible
for contaminant attenuation are naturally occurring, the attenuation process is said be
the result of the
natural attenuation capability
(i.e., assimilative capacity) of the subsurface
soil. What are these naturally occurring attenuation processes? These are the physical,
chemical, and biological properties discussed in the previous section. They all contribute
to the assimilative capacity of soil, i.e., the capacity of the soil to “cleanse itself” through
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