Degradation microbial biomass

The stepwise nature of nitrification during the aerobic decomposition of detrital PON is illustrated in Figure 24.6. Initially, the degradation of PON produces ammonium, which stimulates the growth of the nitrate oxidizers. These bacteria transform the ammonium into nitrite, causing ammonium concentrations to decline and nitrite concentrations to rise. The elevated nitrite levels stimulate the growth of the nitrite oxidizers. These bacteria transform the nitrite into nitrate. Eventually all of the DIN is oxidized to nitrate. The residual pool of PON includes microbial biomass and any PON too inert to be degraded by aerobic marine bacteria. 

In a shallow aquifer material, ethylene dibromide aerobically degraded to carbon dioxide, microbial biomass, and nonvolatile water-soluble compound(s) (Pignatello, 1987). 

Most often, the rates for feedstock destruction in anaerobic digestion systems are based upon biogas production or reduction of total solids (TS) or volatile solids (VS) added to the system. Available data for analyses conducted on the specific polymers in the anaerobic digester feed are summarized in Table II. The information indicates a rapid rate of hydrolysis for hemicellulose and lipids. The rates and extent of cellulose degradation vary dramatically and are different with respect to the MSW feedstock based on the source and processing of the paper and cardboard products (42). Rates for protein hydrolysis are particularly difficult to accurately determine due the biotransformation of feed protein into microbial biomass, which is representative of protein in the effluent of the anaerobic digestion system. 

For now, we focus on cases in which microbial growth is limited by the substrate of interest to us (i.e., Case 1 in Box 17.1). In order to evaluate changes in the limiting chemical s concentration, we need to relate microbial growth to changes in compound concentration. This can be done by recognizing that degrading a certain amount of chemical mass enables a proportional enhancement in microbial biomass ... 

There are several cases where o-xylene degradation did not occur under nitrate-reducing conditions (see Table 4.5). Possible reasons for these observations were considered above. However, two reports not considered in the previous subsection failed to measure the biodegradation of o-xylene (Morgan et al., 1993 Flyvbjerg et al., 1993) In both cases there was probably very little microbial biomass as only groundwater was used as an inoculum and rates of o-xylene metabolism were likely too slow to measure. 

Although the rate at which components of plant and animal residues are decomposed by the soil microbial biomass varies widely (Stout et al., 1981), none of the classes of naturally produced organic compounds persist in the soil indefinitely as there are always species or a succession of species that can degrade them. Jenkinson and Ladd (1981) pointed out that if it were not so, the completely recalcitrant SOM fractions would accumulate indefinitely in the soil and by now would cover the surface of the earth. 

In addition to the amount of pesticide present, the degradation rate could be affected by the availability of the chemical for degradation. Ogram et al. (30) have recently presented evidence suggesting that only the 2,4-D (2,4-dichloro phenoxyacetic acid) in soil solution, but not that adsorbed on soil colloids, could be degraded by soil microbes both in soil solution and sorbed on soil colloids. Other considerations should also be given to the nature and quantity of soil microbial biomass present in relation to nutrient availability (26,28,31) and the adaptability of microbes, either by natural selection or by genetic manipulation, to attack and utilize the pesticide chemical (32). 

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