Object of the study
The contamination of aquifers by chlorinated solvents, caused by their widespread use in industrial processes and their improper handling and disposal, is today a major problem. The pollution of water resources by organo-halogenated compounds, started during 50s and continued for several decades, and is common to all the industrialised countries, including Italy. For the Italian legislation the decontamination of the polluted site by the responsible of the pollution or, if unknown, preliminarily by the public administration, is mandatory (D.L. 471, 1998).
Differently from homologous aliphatic compounds, that floats on the water surface and are easily aerobically degraded, the halogenated hydrocarbon are denser and less viscous then water and therefore can rapidly reach the aquifer percolating through the ground and depositing on the aquifer bottom. Highly chlorinated compounds, such as trichloroethene (TCE) and tetrachloroethene (PCE), for their oxidised nature cannot be aerobically degradated by microbial actions while they can be dehalogenated in anaerobic condition by reductive dechlorination (RD). The reduced compounds, as dichloroethylene and vinyl chloride particularly, are of major concern for their suspected or ascertained carcinogenicity, leading to the setting of very low maximum admissible limits in drinking water.
However the presence of appropriate microorganisms, suitable substrates and conditions, can lead to the complete microbial degradation of these toxic chlorinated ethenes to the innocuous ethylene, a compound environmentally quite acceptable.
Microorganisms of anaerobic dechlorinating consortia involved in the ethylene production process
Complex metabolic and cometabolic, processes, catalyzed by many different bacterial populations, are involved in the reductive dehalogenation of chloroethenes by microbial consortia. The knowledge of these dehalogenating consortia composition is crucial for bioremediation of polluted ground water either by in situ (directly in the aquifer) or ex situ (in biological reactors) processess. Traditional microbiological methods for the identification of bacteria, thought their isolation, are not suitable for description of dechlorinating communities considering the particular microbial components nutritional requirements.
Detection of the dechlorinating bacterium Dehaloccoides ethenogenes in mixed microbial biomasses enriched from contaminated sites in Northern Italy
Dehaloccoides ethenogenes is, at present, the only know bacterium able to perform the complete dechlorination of PCE to ethylene. The presence of this eclectic microorganism has been detected in anaerobic dehalogenating consortia developed from samples of contaminated sites: polluted industrial channels and ground water aquifers in Venice and Milan areas.
The approach utilised, known as fluorescent in situ hybridization ( FISH analysis) is a rapid methodology (the response is obtained within three hours) based on the use of a molecular probe connected to a fluorescent compound that can be visualised by optic/fluorescent microscopy. The molecular probe (Figure 1) is a short synthetic nucleotide strand whose sequence is exactly complementary to a specific sequence present in the bacterial ribosomal
RNA, characteristic of each microorganisms. The specific binding of the fluorescent probe to the complementary site in the target microorganisms allows its detection within a complex biomass by optic/fluores