Geoscience Reference
In-Depth Information
Chapter 15
Biologically Mediated Transformations
15.1 Subsurface Microbial Populations
The subsurface microbial population includes a fascinating array of organisms,
with diverse capabilities for deriving energy from the metabolism of organic and
inorganic compounds (Alexander
1980
). This population is active mainly in soils
forming the upper layer of the subsurface and decreases with depth. Contaminants
reaching the land surface, such as industrial solvents and agrochemicals, may be
incorporated into the biological body or redistributed in subsurface phases.
Microbial life is adapted to the available energy and nutrient supply under
varying environmental conditions. A contaminant reaching the subsurface may
become a source of energy for the biomass. When a source of energy is offered to a
specific microbial population, the population rapidly increases, enhancing the
activity for utilizing the available energy source (Keeney
1983
). Energy sources
and environmental conditions interact to determine the micropopulation ecology
for microbially mediated transformation of subsurface contaminants. Table
15.1
shows the main microbial reactions in relation to energy.
15.2 Processes Governing Contaminant Attenuation
The relationship between microbial physiology and biologically mediated trans-
formation of contaminants is summarized by Azadpour-Keely et al. (
1999
), who
reviewed microbial processes affecting natural attenuation of toxic organic
chemicals in the subsurface. Information encoded in the DNA (deoxyribonucleic
acid) of unicellular bacteria is transferred through RNA (ribonucleic acid) to the
ribosome to produce proteins or enzymes. These enzymes induce organocarbon
degradation, which is used by bacteria as a source of energy. The electrons or
reducing equivalents (hydrogen or electron-transferring molecules) produced are
transferred to a terminal electron acceptor (TEA), and during the transfer process,
energy is produced and utilized by the cell.
