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Source: Rand and Petrucelli (1985).

Source: Rand and Petrucelli (1985).

t(days)

Figure 21.3 Effect of thiophosphorus pesticides on ammonia oxidation by estuarine nitrifiers. (From Rand and Petrucelli, 1985; original source: Jones and Hood, 1980, Can. J. Microbiol., Vol. 26, pp. 1296-1299.)

t(days)

Figure 21.3 Effect of thiophosphorus pesticides on ammonia oxidation by estuarine nitrifiers. (From Rand and Petrucelli, 1985; original source: Jones and Hood, 1980, Can. J. Microbiol., Vol. 26, pp. 1296-1299.)

shows the reduction in respiration measured with several herbicides. It is interesting that Bacillus spp. were completely inhibited by diquat at levels that had no effect on P. fluorescens respiration. Note that inhibition of respiration does not necessarily mean that the organisms were killed.

Toxic effects on bacteria can be studied by determination of effects on specific functions, such as saprophytic activity, methanogenesis, nitrification, or other biogeochemical cycle activities. Figure 21.3 shows an example of this. The amount of inhibition was observed to vary with incubation time. For example, methylparathion's inhibition decreases up to 9 days, then increases again. This may be due to the degradation of the pesticide, followed by an accumulation of an even more toxic by-product, aminophenol.

Nitrogen fixation can be studied by measuring the conversion of acetylene to ethylene in the absence of nitrogen. This is because the enzyme nitrogenase catalyzes both reactions. Stimulation is often found with pesticides, as well as inhibition. For example, malathion causes an initial period of inhibition, followed by more than double the activity after 8 days. Other combinations of pesticides and species produced up to five times the activity.

Soil fungi and actinomycetes were found to bioaccumulate organochlorine pesticides. Bioconcentration factors ranged from 10 for lindane to 59,000 for chlordane. Bacteria are also involved in biotransformation reactions that can enhance or reduce toxicity of substances to other organisms. One of the most important is the conversion of inorganic mercury to methylmercury. Other examples include the conversion of crude oil components to genotoxic agents, the conversion of P=S groups in organophosphate pesticides to the more active P=O groups, and the formation of nitrosamines from secondary amines and nitrites.

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