Environmental Engineering Reference
In-Depth Information
Despite the spread of atrazine degradation genes to disparate genera of soil
bacteria, atrazine still remains largely effective in weed control. This is likely
due to suppression of the atrazine degradation phenotype by inorganic nitro-
gen sources. Several studies have convincingly shown that the addition of
exogenous nitrogen inhibits atrazine catabolism by indigenous soil popula-
tions and pure cultures of atrazine-degrading bacteria, including Pseudomonas
sp. strain ADP (Entry et al.
1993
; Alvey & Crowley
1995
; Struthers et al.
1998
;
Gebendinger & Radosevich
1999
; Abdelhafid et al.
2000
; Garc´a-Gonz´lez et al.
2003
). Thus, despite some limited reports that atrazine is losing effectiveness
for weed control in some soils (Krutz et al.
2007
), elevated soil nitrogen levels
likely repress atrazine degradation by indigenous soil bacteria in agricultural
systems, and thus this herbicide can remain efficacious in the presence of a
large number of degrading micro-organisms.
Enhanced atrazine degradation at the field and molecular levels
Over the past several years, there have been numerous reports of the apparent
reduced half-life of atrazine in soils (Pussemier et al.
1997
; Vanderheyden et al.
1997
; Shaner & Henry
2007
). It is widely accepted that microbial populations
adapt to and are selected for by various C and N sources, and this also appears
to be the case for atrazine-degrading micro-organisms since accelerated atra-
zine degradation in soils has been widely reported (Barriuso & Houot
1996
;
Ostrofsky et al.
1997
; Vanderheyden et al.
1997
; Houot et al.
2000
). While some
variation in reported half-lives may be attributable to abiotic soil characteris-
tics at the sites under study, it is more likely due to the rapid spread of atrazine
degradation genes, greater numbers of atrazine-degrading bacteria and
changes in expression patterns in various soil microbial populations. Increased
substrate availability is conventionally thought to increase soil population
levels of atrazine-degrading micro-organisms (Shapir & Mandelbaum
1997
;
Rhine et al.
2003
), resulting in enhanced degradation capacity. This may occur
at the very local level, in soil particles, where atrazine concentrations are
likely much greater than the average amounts found in bulk soils following
liquid extraction. In addition, these bacteria can also respond to atrazine
availability by altering gene dosage or expression levels of atrazine degradation
genes. For example, Arthrobacter strain TC1 was shown to have six copies
of trzN located within six identical direct tandem repeats of about 16 kb
(Mongodin et al.
2006
); Nocardioides sp. strain SP12 contains at least two copies
of the trzN, atzB and atzC genes (Devers et al.
2007
); and growth of Pseudomonas
ADP for 320 generations in a liquid medium containing atrazine as the sole
N source resulted in a more rapidly degrading evolved population containing
a tandem duplication of atzB (Devers et al.
2008
). In addition, RT-qPCR analyses
indicated that expression levels of atzA and atzB significantly increased in
response to atrazine addition in Chelatobacteria heintzii
(Devers et al.
2004
).
