The fermentation does not require external electron acceptors because the organic contaminant serves as both an electron donor and an electron acceptor. Through a series of internal microorganism-mediated electron transfers, the organic contaminant is converted into harmless compounds known as fermentation products. Examples of fermentation products are acetate, propionate, ethanol, hydrogen and carbon dioxide. Fermentation products can be biodegraded by other species of bacteria, eventually turning them into carbon dioxide, methane and water. In some cases, microorganisms can transform contaminants even if the transformation reaction produces little or no benefit to the cell. The general term for this non-beneficial biotransformation is secondary use, and an …show more content…
Methane is the primary electron donor because it is the main food source of organisms, while the chlorinated solvent is a secondary substrate because it does not support the growth of bacteria. In addition to methane, toluene and phenol were used as primary substrates to stimulate the co-metabolism of chlorinated solvents. Another variation of microbial metabolism is reductive dehalogenation. Reductive dehalogenation is potentially important in the detoxification of halogenated organic contaminants, such as chlorinated solvents. In reductive dehalogenation, microbes catalyze a reaction in which a halogen atom on the contaminating molecule is replaced by a hydrogen atom. The reaction adds two electrons to the contaminating molecule, thus reducing the contaminant. In order for the reductive dehalogenation to proceed, a substance other than the halogenated contaminant must be present to serve as an electron donor. Possible electron donors are hydrogen and organic compounds of low molecular weight (lactate, acetate, methanol or