Cl biphenyl pyrene benzo(a)pyrene pyrene benzo(a)pyrene

Figure 13.12 Possible cometabolism. The main (growth) substrate in each pair is located on the left, with areas of similarity indicated by a, b, or c.

cometabolism, or alternatively, it may develop new enzymes and pathways so that the dead-end product can be further utilized. Although it is not beneficial to the microorganism, cometabolism may be desirable from a human point of view because it may be possible to exploit it to degrade otherwise nondegradable compounds.

Even with energy-yielding metabolism, dead-end products may form. In other cases, intermediates may accumulate because their rate of production is greater than their rate of further transformation. Occasionally, this accumulating compound will be more toxic than the parent material, leading to at least a temporary increase in toxicity during biodegradation. The increased toxicity may have the effect of further slowing degradative activity, as well as having other undesirable effects on the ecosystem.

In aerobic systems it is common for a single species of bacteria to be able to utilize a single organic compound as its sole source of carbon and energy. However, occasionally, organisms of one species can degrade the compound only partially, and those of another species will further transform, and probably mineralize, the intermediate produced. Such a combination of organisms of different species ''working together'' to metabolize substrates is referred to as a consortium (plural, consortia).

The activities of consortia can make detection of cometabolism difficult, as a cometa-bolic product may be degraded by other organisms before it accumulates sufficiently to be noticed. Also, the fact that a compound can be completely degraded by a single organism does not necessarily mean that this is the way it will be degraded in a particular environment—the actual degradation still may be by a consortium.

Under anaerobic conditions, consortia are common. A wide variety of species may be involved in hydrolyzing, or solubilizing, complex organics, followed by fermentations of the subunits produced. Then sulfate-reducing or methanogenic organisms may utilize the fermentation products, leading to mineralization.

The organisms in a microbial consortium may be only loosely associated, or they may be so closely linked that they are difficult to separate. A classic example is the case of methane production from ethanol. For many years this was attributed to Methanobacillus omelianskii, an "organism" that could be grown in "pure" culture with ethanol as the sole carbon and energy source. However, it was later demonstrated that, in fact, the culture contained two species: a bacterium that converted ethanol to acetate, and a methanogen (Archaea) that utilized the acetate, producing methane and carbon dioxide.

Occasionally, the balance among organisms in an anaerobic consortium is disturbed and mineralization does not occur. In the absence of sulfate, anaerobic mineralization is dependent on methanogens. Compared to the wide variety of organisms that are able to hydrolyze and ferment organics anaerobically, methanogens are a relatively limited group of strictly anaerobic archaea. Although most methanogens utilize acetic acid as a substrate, as a group they are sensitive to low pH. If the acetic acid is produced too quickly, the pH of the system will drop, inhibiting methanogenic activity. In turn, this leads to a further buildup of acid, a greater drop in pH, and even lower rates of methano-genesis and acid destruction.

Some human foods, such as pickles and sauerkraut, are preserved in this way. Ensilage, the process of making silage, is a means of animal feed preservation utilizing acid anaerobic conditions that has been employed in agriculture for many years. Fresh corn, hay, or other feed crops are placed in a silo, where they quickly ferment. The acid anaerobic conditions then prevent substantial further degradation. However, if oxygen is introduced in large amounts, the organic acids are quickly mineralized, pH rises, and biodegradation resumes.

Acid anaerobic conditions are also responsible for the preservation of the large masses of spongy rich organic material in bogs. This waterlogged peat soil, composed mainly of dead sphagnum moss (an acid-loving plant), accumulates over hundreds or even thousands of years. If the water is drained from such systems, aerobic conditions develop and the organics are quickly oxidized.

During waste treatment, acid anaerobic conditions are undesirable because they slow degradation, leading to preservation of the waste. In poorly run leaf "composting" (Section 16.2.3) operations (really, leaf dumps), large piles of leaves may be formed and simply left unattended. Acid anaerobic conditions develop quickly, so that on opening such piles 10 years later, the tree species of individual leaves can still be determined. Similarly, in anaerobic digesters for sludge treatment (Section 16.2.1), overloading or toxic shocks may lead to acid anaerobic conditions that prevent methane formation. The digester under such conditions is referred to as having gone "sour."

Another problem with acid anaerobic conditions in waste treatment is the potential for odors. The organic acids themselves may be volatilized, leading to a sour smell. Often, some alcohols are also formed, and apparently react with the acids to give esters, which may have a sweet smell. Thus, such systems often have a sweet-sour smell. This is not necessarily unpleasant for silage in an agricultural setting, but it is often not appreciated near wastewater treatment, composting, and landfill sites.

If sufficient proteinaceous material is present in the waste, more severe odors may also develop. Under most conditions the amino acids released by protein hydrolysis are dea-minated; that is, the amino group is removed, releasing ammonium (Figure 13.13). This can lead to an ammonia odor at high pHs where volatilization is favored. However, under acid anaerobic conditions, amino acids may instead be decarboxylated. This leaves an amine, some of which are highly odorous, with names such as putrescene and cadavarene.

Another odor concern with anaerobic conditions is hydrogen sulfide. This can be formed through the reduction of sulfate during anaerobic respiration (Section 13.3.1) or through the release of reduced sulfur from organic compounds such as the amino acids methionine and cysteine.

In some cases, under both aerobic and anaerobic conditions, rather than being mineralized, organics are polymerized to form products with long-term stability. One example is with nitroaromatic compounds such as trinitrotoluene (TNT). Because the aromatic ring

2 CH

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