Cilia L. Fuentes

Prevalence of the gene trzN and biogeographic patterns among atrazine-degrading bacteria isolated from 13 Colombian agricultural soils

The following study evaluated the diversity and biogeography of 83 new atrazine-degrading bacteria and the composition of their atrazine degradation genes. These strains were isolated from 13 agricultural soils and grouped according to rep-PCR genomic fingerprinting into 11 major clusters, which showed biogeographic patterns. Three clusters (54 strains) belonged to the genus Arthrobacter, seven clusters (28 strains) were similar to the genus Nocardioides and only one strain was a gram-negative from the genus Ancylobacter. PCR assays for the detection of the genes atzA, B, C, D, E, F and trzN conducted with each of the 83 strains revealed that 82 strains (all gram positive) possessed trzN, 74 of them possessed the combination of trzN, atzB and atzC, while only the gram-negative strain had atzA. A similar PCR assay for the two analogous genes, atzA and trzN, responsible for the first step of atrazine degradation, was performed with DNA extracted directly from the enrichment cultures and microcosms spiked with atrazine. In these assays, the gene trzN was detected in each culture, while atzA was detected in only six out of 13 soils. These results raise an interesting hypothesis on the evolutionary ecology of the two atrazine chlorohydrolase genes (i.e. atzA and trzN) and about the biogeography of atrazine-degrading bacteria.

Microbial Degradation of Pesticides in Tropical Soils

Although their use is at least as essential in tropical as in temperate zones, pesticides remain little studied as regards their fate and microbial degradation in tropical soils. To contribute to closing this gap, this review examines to what extent results from studies on pesticide microbial degradation in temperate zones can be extrapolated to the tropics. It is concluded that geographical distances or barriers are not expected to create profound differences between tropical and temperate soil microbial communities, although fine-tuning adaptation might exist. This suggests that environmental conditions, mainly temperature and humidity, are the principal factors contributing to a difference between pesticide degradation phenomena as they occur in soils from tropical and temperate zones. According to this hypothesis, the kinetics or metabolic pathways of microbial pesticide degradation would be similar in temperate and tropical environments that would themselves be similar with respect to temperature, humidity, and other parameters. The hypothesis also predicts that in the hot humid tropics, pesticide degradation is expected to occur faster than in temperate zones, whereas in cool tropical highlands or in arid tropical zones the degradation rate would be reduced to a level comparable to or even lower than that in temperate zones. Finally, it is proposed there that pesticide degradation rate in the tropics can be predicted by models developed in temperate climates, insofar as these models have been validated as applying to tropical countries.