Successful bioremediation of contaminated soils and water involves three-way interaction among the contaminant, microorganisms, and their environment.

Distribution and activity of microorganisms in soil Effect of contaminants on microbial activity Effects of plant growth on microbial activity Chemical and biological control of the microbe-contaminant interactions Organism and conditions for the complete degradation of pollutants

Biodegradability of chemicals in soil can be divided into three groups: I. Immediate degradation; II Acclimation followed by degradation; and III. Recalcitrance. Heider, 1999

In general, bioavailability of chemicals in soil decreases with time. Heider, 1999

Fate of chemicals in soil involves: Microbiological, chemical, and physical processes that result in formation of complex three dimensional structure of soil- microbe-contaminant. As a result, the chemicals are stabilized against fast degradation.

Sadowsky amd Turco, 1999

Heider, 1999

Mode of action

Heider, 1999

Alexander, 1999

Solutions

Sadowsky amd Turco, 1999

Is microbial activity affected by addition of soil pollutants?

After reviewing about 3000 articles on side effects of pesticide on soil microbiota, Domsch (1992) concluded that most properly applied agrochemicals have little or no long lasting negative effects. Fluctuation in bioactivities within two orders of magnitude are not uncommon for microbes when coping with natural stress conditions.

Are enzyme activities affected by soil contaminants? Considerable metabolic activity was carried out by cell free enzymes that are originated predominantly from microorganisms and more seldom from plants.

Some contaminants may reversibly suppress soil enzyme activity. For example, a 10-fold dosage of the fungicides folpet (N-trichloromethylmercapto-pthalimide) and captafol [N-(1,1,2,2-tetrachloroethyl mercapto-) tetrahydrophthalimide] reduced the dehydrogenase activity significantly, but it recovered to the control level after 21 d.

Observations: A striking relationship existed between the number of Zn- and Cd-resistant organisms and the numbers of organisms that used aromatic and phenolic compounds as a substrate for growth. On average, 8.7% of the Cd-resistant organisms and 9.4% of the Zn-resistant could grow on phenols and phenol carboxylic acids as the only C source (4.3% and 2.6% for others).

Overall The effects of wastes (soil contaminants) on microbiota and their activities are not well investigated.

How to enhance indigenous microorganisms to bioremediate contaminated soils

In situ biodegradation and remediation of contaminated soils can be enhanced or stimulated by adding limiting nutrients, microbial growth factors, or surfactants to the contaminated environment.

If we introduce or promote establishment of a microorganism to an environment, will these organisms become indigenous?

Indigenous microorganisms: Definition? …. When and how an organism becomes indigenous is not known.

Research data indicated that: 1. Removing an organism from a given soil environment, for a short time, and replacing it into the same local environment does not appreciably affect its ability to reestablish itself as an indigenous member of that microbial community. 2. A nonindigenous microbe can become a member of the indigenous population by prolonged and repeated applications of the microbe in soil inoculants.

To become an indigenous microorganism: Primary or early preemptive colonization, followed by prolonged periods of stable maintenance in a soil population leads to the establishment of the indigenous state.

It seems that we can enhance biodegradation by inoculation if needed. However, bioaugumentation is not always a successful bioremediation strategy

Alternative--rhizosphere technology (phytodegradation)

Rhizosphere  The volume of soil surrounding and under the influence of the roots of all plants  The zone of altered microbial diversity, increased activity and number of organisms, and complex interactions of microorganisms and the root.  Can extend more than 5 mm from the root and, more importantly, is the area of increased microbial activity

R/S ratio— refers to the ratio of microorganisms in the rhizosphere compared to the microorganisms in the bulk soil. Every microbial species has its own R/S ratio, which varies depending on the plant and the soil environment. The R/S ratio typically ranges from 10 to 50, which means that there are 10 to 50 times as many microorganisms right around a plant root compared to the surrounding soil.

 Microorganisms cover 5% to 10% of the root surface, and most terrestrial plants are infected by fungi called endo- and ectomycorrhizae.  There is a steep decrease in microbial populations that occurs with small distances (within 5 mm) from plant roots.  The rhizosphere is known to harbor more gram-negative bacteria (e.g. Pseudomonas) and denitrifiers and fewer gram-positive and gram-variable forms (e.g. Bacillus).

 Microorganisms in the rhizosphere are characterized by saprophytic (nonparasitic nutritional mechanisms by which an organism obtains its food exclusively from the degradation of nonliving organic material) and pathogenic organisms.  Roots do not appreciably alter the total counts of fungi. Actinomycetes, protozoa, and algae are not significantly benefited by their approximate to roots, and their R/S ratios are not as high as those of bacteria

Influence of Plant Species on In Situ Rhizosphere Degradation

Shann and Boyle, 1994

Plant cell wall possess both anion- and cation- exchange properties. However, it was investigated primarily as a cation-exchanger. In this study, All soils and treatment solutions were adjusted to pH 7.4, where a majority of the 2,4-D would be in the anionic form. Addition of wheat cell wall affected bioavailability of 2,4-D, which affected degradation of 2,4-D.

Shann and Boyle, 1994

Where was the added 2,4-D? Over 80% was found in the plant. This support the general assumption that plant will, to some extent, determine how much of the xenobiotic is available for degradation by rhizosphere microorganisms.