Environmental Engineering Reference
In-Depth Information
1.3.1
Degradation by Fungi
1.3.2
Degradation by Bacteria
The process of natural bioremediation of persis-
tent compounds involves a range of microorgan-
ism. Most fungi are robust organisms and are
generally more tolerant to a high concentration
of polluting chemicals than bacteria. A variety of
fungi have been used for degradation of pollut-
ants in the environment. The contaminants pres-
ent in water and soil from industrial and agricul-
ture activities are degraded and utilized by fungi.
But use of fungi for degradation of industrial
pollutants such as chlorophenols, nitrophenols,
and polyaromatic hydrocarbons are limited. In
spite of the toxicity of the effluent and presence
of chlorophenols, the microbial flora of tannery
liquid wastes is relatively rich, with the Asper-
gillus niger group predominant. The extracel-
lular enzymes and cell mass from the pregrown
Phanerochaete chrysosporium cultures were
used by researchers for the degradation of penta-
chlorophenol (PCP). The lignin degrading fungi
P. chrysosporium, Phanerochaete sordida, Tram-
etes hirusta, and Ceriporiopsis subvermispora
were evaluated for their ability to decrease the
concentration of pentachlorophenol.
Fungi are especially well suited to polycyclic
aromatic hydrocarbon (PAH) degradation rela-
tive to other bacterial decomposers for a few rea-
sons. They can degrade high molecular weight
PAHs, whereas bacteria are best at degrading
smaller molecules. They also function well in
nonaqueous environments where hydrophobic
PAHs accumulate; a majority of other microbial
degradation occurs in aqueous phase. Also, they
can function in the very low oxygen conditions
that occur in heavily PAH-contaminated zones.
Fungi possess these decomposing abilities to deal
with an array of naturally-occurring compounds
that serve as potential carbon sources. Hydrocar-
bon pollutants have similar or analogous molec-
ular structures which enable the fungi to act on
them as well. When an area is contaminated, the
ability to deal with the contamination and turn it
into an energy source is selected for the fungal
population and leads to a population that is better
able to metabolize the contaminant.
Bacteria can be separated into aerobic types,
which require oxygen to live, and anaerobic,
which can live without oxygen. Aerobic bio-
remediation is usually preferred because it de-
grades pollutants 10-100 times faster than an-
aerobic bioremediation. Facultative types can
thrive under both aerobic and anaerobic condi-
tions. Certain bacteria belonging to Bacillus and
Pseudomonas species have these desirable char-
acteristics. They consume organic waste thou-
sands of times faster than the types of bacteria
that are naturally present in the waste. Bacteria,
Arthobacteria , Flavobacterium , Pseudomonas ,
and Sphingomonas , have been isolated and ap-
plied for the degradation of chlorinated phenol
and other toxic organic compounds. A number
of bacteria viz., Pseudomonas , Flavobacterium ,
Xanthomonas , Nocardia , Aeromonas , and Ar-
throbarterium are known to utilize lignocellu-
losic components of the bleached plant effluent
containing lignosulphonics and chlorinated phe-
nols. One particularly promising mechanism for
the detoxification of polychlorinated dibenzodi-
oxins (PCDDs) and polychlorinated dibenzofu-
rans (PCDFs) is microbial reductive dechlorina-
tion. In current scenario research data suggested
that, only a limited number of phylogenetically
diverse anaerobic bacteria have been found that
couple the reductive dehalogenation of chlori-
nated compounds the substitution of chlorine
for a hydrogen atom to energy conservation and
growth in a process called dehalorespiration. Mi-
crobial dechlorination of PCDDs occurs in sedi-
ments and anaerobic mixed cultures from sedi-
ments, but the responsible organisms have not
yet been identified or isolated. Various microbial
cultures capable of aerobic polychlorinated bi-
phenyl (PCB) biodegradation have been isolated
by researchers (Fetzner and Lingens 1994 ). Up
to 85 % degradation of Arochlors 1248 and 1242
has been shown. The more highly chlorinated
1254 and 1260 Arochlors have not shown signifi-
cant aerobic biodegradation in the laboratory or
in the field. Anaerobic degradation by dechlori-
nation reactions is widespread even for the 1254
and 1260 Arochlors.
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