Effects of Toxic Chemicals and Other Pollutants on Aquatic Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 289

One common way to clean up spills of organic materials in the envi-
ronment is bioremediation (Anderson and Lovley, 1997). This entails
employing organisms that can break down or inactivate the pollutants. In
some cases, organisms are introduced to do the job, and in other cases, na-
tive bacteria have the ability to degrade the organic pollutant.
Some bacteria are able to metabolize novel organic compounds. This
probably occurs because evolution has favored microbes able to utilize
unique carbon sources that are released into the environment by other or-
ganisms. The number of individual bacteria is high, and their generation
times are short. These features of bacteria lead to the rapid establishment
of new genotypes capable of using pollutants.
The ability to metabolize or inactivate toxins is often coded upon plas-
mids (small circular pieces of DNA that are free in the cytoplast), which can
move within and among microbial species and allow transfer of genetic in-
formation. Movement of plasmids among natural populations of bacteria is
well established. Bacteria resistant to human-synthesized antibiotics have
been isolated from many rivers and billabongs
in remote rural areas of Australia (Boon,
1992); both are environments with low hu-
man densities. Bacteria resistant to multiple
antibiotics are common in aquatic environ-
ments (Leff et al., 1993). The lateral transfer
of plasmids among bacterial species likely al-
lowed for the movement of their genetic ma-
terial. Lateral transfer is of concern in rela-
tion to genetically engineered microbes but
also may be helpful in bioremediation efforts.
Bioremediation is probably most impor-
tant in cases of contaminated groundwater
because spills of any size are extremely diffi-
cult to remove from underground, particu-
larly if the compounds are not water soluble
and are associated with sediments. Several
strategies for bioremediation can be used, in-
cluding pumping the water and treating it at
the surface, addition of engineered microbes
to the aquifer to consume the pollutants, and
use of in situ microbial activity to eradicate
pollution. In most cases, surfactants (com-
pounds that decrease the ability of organic
compounds to associate with solid surfaces)
are used to dissociate the compounds from
the sediments. Nutrients and oxygen are of-
ten added to groundwaters to stimulate mi-
crobial activity. Microbes that have the abil-
ity to degrade the pollutant may be released
into the groundwater. An understanding of
the ecology of groundwaters is useful in opti-
mizing rates of bioremediation. For example,
it has been demonstrated that protozoan pop-
ulations can decrease rates of bioremediation
(Kota et al., 1999).

Sidebar 14.3.
"Lake Erie Is Dying"

Until the 1960s, most sewage and industrial
wastes were being released directly into Lake
Erie without treatment. The lake seemed so large
as to be unaffected by such releases. As the
population grew, the problems associated with
the releases, such as organic chemical contam-
ination, pathogenic bacteria, and eutrophication,
worsened. Such problems led to public pressure
to clean up the lake (hence the slogan “Lake
Erie is dying”) and confrontation between citi-
zens, state and federal government officials, and
entities causing the pollution (Kehoe, 1997).
Total loads of phosphorus increased fivefold
from 1900 to 1970, leading to eutrophication
problems. In this sense, the lake was not dying
but actually becoming more productive as the
phosphorus and nitrogen inputs stimulated al-
gae. This stimulation of algae led to undesir-
able accumulations of the filamentous green
alga Cladophora that fouled beaches (Burns,
1985). Some areas of the lake became anoxic
and taste and odor problems developed be-
cause of algal biomass.
Loading of mercury, lead, cadmium, copper,
and zinc increased greatly, with sediment con-
tents 12.4, 4.4, 3.6, 2.5, and 3 times greater, re-
spectively, than in presettlement times (Burns,
1985). Mercury contents of fish became so high
that they were not healthy for human con-
sumption. Inputs of toxic metals from industry