Herbicides and Nitrification

Nowadays, in Nature, approximately the same numbers of the ammonia-oxidizing and the nitrite-oxidizing bacteria are present in most cases, as already mentioned. So it may be attributable to the presence of approximately the same numbers of both the bacteria in Nature that nitrite does not accumulate by use of paraquat in the field. Namely, the effect of paraquat on the nitrifiers when both the 

The herbicidal effect of paraquat is attributable to the formation of superoxide anion (02 ). Superoxide anion is very toxic compound and is formed by the reaction of oxygen with paraquat radical (paraquat ). Plants, algae, and cyanobacteria have ferredoxin-NADP reductase to form NADPH for the reduction of carbon dioxide (see below). The chemolithoautotrophs also have NAD(P) (NAD and NADP) reductase to form NAD(P)H for the reduction of carbon dioxide. Paraquat [mid-point redox potential at pH 7.0 (Emj 0) = -0.43 V] radical is produced when paraquat is reduced by the catalysis of ferredoxin-NAD(P) reductase or NAD(P) reductase, which catalyzes the reduction of many compounds with of around -0.4 V. Although the aerobic organisms (and even many anaerobic organisms) have superoxide dismutase (SOD) which detoxifies superoxide anion in cooperation with catalase [ascorbate peroxidase in the case of plants (Asada, 1999)], the anion accumulates in the organisms when it is over-produced beyond the capacity of SOD. 

When paraquat is sprayed on plants, superoxide anion is over-produced because paraquat produces superoxide anion, as mentioned above, and destroys plant tissues. This is the mechanism by which paraquat acts as a herbicide. 

The growth of the chemolithoautotrophic bacteria can be inhibited by paraquat, because the bacteria have NAD(P) reductase. So it is rather mysterious that N. europaea is not affected by the compound at 4 pM. It will be attributable to hydrox-ylamine which is formed as an intermediate metabolite during the oxidation of ammonia by the bacterium that the inhibitory effect of paraquat is not observed with the bacterium. Hydroxylamine is a scavenger of superoxide anion (Elstner et al., 1975). Probably, N. europaea (and other ammonia-oxidizing bacteria) are exceptionally resistant to paraquat, but other chemolithoautotrophs all will be sensitive to paraquat. Therefore, we should take care not to disturb the soil bacteria when we use paraquat as a herbicide. 

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