Philip C. Kearney

Microbial degradation of s -triazine herbicides

Biodegradation is a significant factor affecting the residual life and toxicity of many pesticides1 in soils. Soil microorganisms may act upon a pesticide in several ways. One mechanism may involve degradation with ultimate detoxication and/or metabolism of the pesticide, whereas another mechanism may involve the activation or toxication of an initially nontoxic pesticide molecule. Still another mechanism may involve the transformation of a toxic molecule into a product which exerts some beneficial influence upon higher plants, soil fauna, or microorganisms. Such reactions have been observed during the microbial degradation of a number of pesticides. Because of the public health and environmental significance of pesticides and their residues, a thorough understanding of the chemical, physical, and microbial forces acting upon these chemicals is important. The purpose of this review is to discuss the parameters involved in the microbial degradation of s-triazine herbicides.

Behavior of Pesticides in Soils

Publisher Summary The behavior of pesticides in soils has been the subject of research long before pollution became a byword. In recognition that soil is the ultimate sink for most widely used pesticides, and given the impetus of recent public awareness of the quality of our environment, the past decades have marked much progress in the understanding of the fate and behavior of pesticides in soils. A strong case for the continued use or precipitate abandonment of agricultural pesticides is inappropriate. It is reasonable, however, to note the vitally important role pesticides have assumed in increasing the quantity and quality of foodstuffs, timber, and ornamental plants; in improving animal health; and in combating certain diseases transmitted to man. This chapter reviews the behavior of pesticides in soils from the standpoint of processes affecting pesticides (physicochemical and metabolic), the effect of pesticides on the soil microbiota, and the implications of these processes on persistence, bioactivity, and plant uptake. Adsorption, the most influential process affecting pesticides in soils, depends on both soil and pesticide properties. Other significant soil factors include total surface area, water content, temperature, and pH. Pesticide properties that are relevant include overall chemical character and configuration, dissociation constant, water solubility, charge distribution, and molecular size. Movement of the pesticides occurs by leaching, volatilization, or runoff. The importance of photodecomposition as a process degrading pesticides in soils is uncertain. Generally, photolysis occurs more readily for compounds in solution, with soil inhibiting the reaction.

Metabolism of carbofuran by a pure bacterial culture

Abstract A bacterium WM111, isolated by soil enrichment and identified as an Achromobacter species, was capable of rapidly utilizing the insecticide carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) as a sole source of nitrogen. The doubling time was 4 hr, and 200 μg/ml carbofuran was over 99% degraded within 42 hr. The reaction resulted in the hydrolysis of carbofuran and concomitant formation of the 7-phenol metabolite (2,3-dihydro-2,2-dimethyl-7-benzofuranol) as measured by high-performance liquid chromatography. Resting cell suspensions of WM111 degraded carbofuran at an exceptionally rapid rate and also rapidly degraded several other N -methylcarbamate insecticides.

Persistence and leaching of atrazine, alachlor, and cyanazine under no-tillage practices

Abstract Soil residues of atrazine, alachlor, and cyanazine were measured in no-till corn plots that had received annual herbicide applications from 1981–1985. Treatments were as paired combinations of herbicides at recommended rates. Three cores were collected from 3–4 replicated plots per treatment; sampling occurred late in 1983, then periodically during the growing season in 1984 and 1985. Persistence up to 6 weeks after herbicide application was in the order: atrazine > cyanazine > alachlor. Sampling after ca. 6 weeks gave the order atrazine > alachlor > cyanazine. Atrazine and alachlor were found to the lowest sampled depths in 1983 (1–1.5 m) and 1984 (0.4–0.5 m). Cyanazine was not detected below 0.3 m. In 1985, atrazine leached to ≥0.3–0.5 m by 40 days, but no subsoil residue was detected at 139 days. Alachlor and cyanazine had virtually disappeared by 40 days.

Microbial degradation of para-chloroaniline as sole carbon and nitrogen source

Abstract A Pseudomonas sp., isolated from soil, was grown aerobically on para-chloroaniline (PCA) as an only carbon and nitrogen source with a generation time of 15 hr. Balance studies with 14C-ring-labeled PCA revealed that 64% of the carbon of PCA was released as CO2 and 14% was associated with the biomass. For the ring substituents, 60% of the nitrogen and 96% of the chlorine of PCA accumulated in the medium as ammonium and chloride, respectively. The strain was also able to grow on aniline and 3-chloroaniline rapidly and 2-chloroaniline slowly as sole carbon and nitrogen sources.

Enzyme from Soil Bacterium Hydrolyzes Phenylcarbamate Herbicides

An enzyme preparation from Pseudomonas sp., isolated from a soil culture by an enrichment technique, liberated 3-chloroaniline from the herbicide isopropyl N-(3-chlorophenyl) carbamate. Aniline, 3-chloroaniline, and 3,4-dichloroaniline were detected when the enzyme preparation was incubated with several alkyl esters of the phenylcarbamates and chlorophenylcarbamates. No chloroaniline was detected when the 3-(p-chlorophenyl)-1,1-dimethylurea (monuron) was used as a substrate. The substrate specificity of the isolated enzyme suggests that it catalyzes the initial hydrolysis of many biologically active phenylcarbamates in soils.

Physical comparison of parathion hydrolase plasmids from Pseudomonas diminuta and flavobacterium sp.

Restriction maps of two plasmids encoding parathion hydrolase have been determined. pPDL2 is a 39-kb plasmid harbored by Flavobacterium sp. (ATCC 27551), while pCMS1 is a 70-kb plasmid found in Pseudomonas diminuta (strain MG). Both plasmids previously have been shown to share homologous parathion hydrolase genes (termed opd for organophosphate degradation) as judged by DNA-DNA hybridization and restriction mapping. In the present study, we conducted DNA hybridization experiments using each of nine PstI restriction fragments from pCMS1 as probes against Flavobacterium plasmid DNA. The opd genes of both plasmids are located within a highly conserved region of approximately 5.1 kb. This region of homology extends approximately 2.6 kb upstream and 1.7 kb downstream from the opd genes. No homology between the two plasmids is evident outside of this region.

Decontamination of pesticides in soils.

The soil, as a medium for decontamination, offers a large number of processes by which organic substances can be destroyed. As such, progressive accumulation of organic pesticides would appear to be unlikely. Unfortunately, the chemical and physical properties of certain insecticides and herbicides afford them a degree of stability against the natural destructive processes in soils. The stability of these compounds is best illustrated in a recent summary of persistence data on 12 major classes of pesticides in a number of soil types (Fig. 1) (Kearney et al. 1969). Persistence values are expressed in months and each bar represents one or more classes of herbicide or insecticide. Each open space in the bar represents an individual pesticide falling within the larger chemical class of compounds. The length of each bar depicts the time for each class of pesticide to decrease 75 to 100 percent of the amount applied. These values are based on normal rates of application. As anticipated, the organochlorine insecticides are the most persistent pesticides. The organic herbicides persist for a few days or for more than 12 months depending on their respective properties. Only the major herbicides that persist for a month or longer are shown in Figure 1. The phosphate insecticides do not persist for long periods in most soils. A more detailed picture of organochlorine pesticide persistence is shown in Figure 2. Chlordane and DDT usually persist for several years while heptachlor and aldrin extend their activity through the formation of their respective metabolites, i.e., heptachlor epoxide and dieldrin.

Groundwater residues of atrazine, alachlor, and cyanazine under no-tillage practices

Abstract Groundwater from no-till corn plots treated with atrazine, alachlor, and cyanazine was analyzed for residues of these herbicides over a 3-year period. Detectable levels of atrazine, alachlor, and cyanazine were found in 75, 18, and 13% of the recovered samples, respectively. Maximum residue levels were 5.9, 3.6, and 1.0 μg L −1 for atrazine, cyanazine, and alachlor, respectively. Rapid vertical transport to the shallow unconfined groundwater (ca. 1 m depth), as well as substantial lateral subsurface flow, was indicated.

Oxidative pretreatment accelerates TNT metabolism in soils

The combined effect of ultraviolet (UV)-ozonation (O3) of aqueous 14C-TNT solutions followed by direct addition of the solutions to aerobic soils was examined as a method of disposal. The effect of TNT concentration was studied on both UV-O3 and soil metabolism. The amount of TNT degraded by either process decreased as the concentration increased. UV-O3 of a 1 ppm solution of TNT using a laboratory 450 W lamp for 10, 20, and 30 minutes resulted in substantial fragmentation of the ring and an increase in polarity of the resultant products. Soil metabolism, as measured by metabolic CO2 evolution, increased as the time of prior UV-O3 increased. A large amount of the 14C associated with 14C-TNT recovered from soil was in the non-extractable fraction. When a Pseudomonasputida, adapted to metabolize ortho-nitrophenol or picric acid as a sole source of carbon and nitrogen, was substituted for the soil phase, about 25% of the added 14C appeared as 14CO2. 1,3,5-Trinitrobenzene, 2,4,6-trinitrobenzaldehyde, 3,5-dinitrophenol, 3, 5-dinitrocatechol, 3,5-dinitrohydroquinone, and oxalic acid were identified as products of UV-O3. Rapid destruction of TNT took place in a large 66 lamp unit, and the resultant distribution of 14C was similar to the results from the laboratory studies.