METALS - Marine Pollution: What Everyone Needs to Know - page 97

already present at the site (natural attenuation or intrinsic bio-
remediation), or can be accelerated with the addition of fertil-
izers. Particular microbe strains can be added to enhance the
resident microbe population's ability to remediate the contami-
nants. Special genetically-engineered strains can be used. Not
all contaminants, however, are easily treated by bioremediation.
For example, metals such as Cd and Pb are not readily absorbed
or captured by microbes. The assimilation of metals such as Hg
into the food chain may have more negative than positive out-
comes. Appropriate microbes should be tolerant to metals, have
high metal-binding capacity, and synthesize metal-binding pro-
teins in response to metal exposure. In some microbes, highly
specific biochemical pathways have evolved to protect the
microbial cells themselves from metal toxicity. A good example
is the microbial reduction of mercury. In other cases, microbes
can sequester toxic metals within soils or sediments or produce
materials that can bind metals in contaminated soils. The mobi-
lized metals can then be pumped out of the soil or sediment.
Metals can be extracted from contaminated environments by
two mechanisms. First, some heterotrophic microorganisms
(those that need food, like animals) mobilize metals by produc-
ing organic acids. Secondly, some highly specialized autotrophic
bacteria (those that make their own food, like plants) generate
large amounts of metal-leaching sulfuric acid from the oxida-
tion of elemental sulfur. Some multiple metal tolerant fungi and
bacteria have been identified that can be used to adsorb multi-
ple metal pollutants. Bioremediation is cost-effective, and much
less expensive than landfill disposal. New tools and techniques
for use in bioremediation (e.g., genetically engineered organ-
isms) are contributing to the rapid growth of this field.
What is phytoremediation?
Plants such as are found in salt marshes and mangroves can
reduce the amount of metal pollution entering estuaries, and
can remediate—to a degree—the contaminants in sediments.