Herbicides microbial metabolism

M Shariatpanahi, AC Anderson, AA Abdelghani, AJ Englande. Microbial metabolism of an organic arsenical herbicide. In TA Oxley and S Barry, eds. Biodeterioration. 5. New York Wiley, 1983, pp 268-277. 

Several processes may play a role in the environmental dissipation of -triazine herbicides. Dissipation processes can include microbial or chemical degradation in soil metabolism or conjugation in plants photodegradation in air, water, and on soil and plant surfaces and volatilization and transport mechanisms. This chapter will address photolytic degradation and abiotic hydrolysis of the currently used triazine herbicides, the triazinone herbicides (metribuzin and metamitron), and the triazinedione herbicide hexazinone. 

In addition to plant activation of promutagens, such as dimethyInitrosamine,16 that are also activated in mammalian metabolism, the possibility of unique abilities of plants to activate some substances warrants attention. For example, interest in metabolism of promutagens by plants was stimulated by the evidence11 0 354 that maize can activate the nonmutagenic herbicide atrazine and some related compounds to substances that cure detectable as mutagens in microbial tests. Atrazine was also mutagenic in the waxy locus test in the maize plants themselves.354... 

Aniline is released in the presence of denitrifying and methanogenic microbial activity238b. The pKa value suggests that in moist soils, aniline will be protonated, and bound to soil, which inhibits degradation. Volatilization does not take place from dry soils, based on the vapor pressure. Studies have been made on the metabolism of aniline-derived products, such as herbicides and fungicides, in soil. Chloroanilines bind to organics in... 

Metabolic Studies with Microbial Isolate JE1. EPTC Is rapidly metabolized by microbial isolate JE1 which utilizes it as its sole carbon/energy source (Figure 3). The degradation products were identified either by TLC or GC/MS analysis. Hydroxylation and sulfoxidation were found to be important reactions leading to the mineralization of EPTC by soil microorganisms. The rapid evolution of UC02 from uC-l-propyl EPTC suggests that mineralization of this portion of the herbicide molecule is very rapid. 

Several interrelated factors complicate the relationship between growth and degradation multispecies interactions, cometabolic degradation, and substrate utilization. If multispecies consortia are required for degradation, then the amount of pesticide incorporated into microbial biomass of a particular species may be even lower. Multispecies interactions have been investigated Lappin et al. (21) isolated a consortium of 5 bacteria that metabolized the herbicide mecoprop in culture after a lag period of 20 hr. The growth of the consortium was in contrast to the failure of individual species to grow on mecoprop. The extent of these kinds of interactions in soils is essentially unknown. 

Reduced microbial activity and lower metabolic diversity are likely reasons for the reduced biodegradation of herbicides in subsoils and groundwater aquifers. Although the total microbial population in most aquifers range between 104 and 107 cells/g aquifer material (22). the proportion of these populations that can degrade xenobiotics appears to be low. In a pristine aquifer from Lulu, Oklahoma, degrader populations measured by 14C-MPN were less than 5 cells/g for chlorobenzene, napthalene, and toluene (4Q). 

Other bacterial CYPs also undertake the breakdown of carbon sources for microbial growth. For instance, CYP 108A1 (P450 gjjp) metabolizes ter-pineol , and CYPl 76A1 (P450 j J can metabolize cineol, while others can metabolize pollutants such as thiocarbamate herbicides and atrazine, as illustrated by CYPl 16 from a Rhodococcus sp. °. The CYP 105 family of streptomycetes especially is associated with a wide variety of xenobiotic... 

Halogenated heterocyclic aromatic con imds are widely used for the production of pesticides, pharmaceuticals, and dyes, but much less is known regarding their metabolism by microorganisms con ared with their homocyclic analogs. 3,5,6-Trichloro-2-pyridinol (TCP) is a major metabolite of the insecticide chlorpyrifos and herbicide triclopyr. It has been detected in environments where chlorpyrifos and triclopyr were previously applied (1-6). TCP can be mineralized in soil, and its half-life varies with soil type, ranging from 10 to 325 days (5, 7). The mineralization of TCP in soil is microbially mediated, but isolation of the degradative microorganisms has rarely been atten ted. 

The final two categories listed in Table 1 are xenobiotics and other factors. Xenobiotics, which include pesticides (herbicides, insecticides, fungicides), are discussed later but there are many cases of accidental spillages of chemicals (e.g. hydrocarbons) which often have disasterous effects on vegetation. Damage to plants by physical insults or pest attack may alter lipid metabolism. For example, wounding stimulates the plant to repair the cut surface, especially by suberin whose precursors are fatty acids. Insect attack allows microbial entry into... 

---