Rr. Fulthorpe et al., PRISTINE SOILS MINERALIZE 3-CHLOROBENZOATE AND 2,4-DICHLOROPHENOXYACETATE VIA DIFFERENT MICROBIAL-POPULATIONS, Applied and environmental microbiology, 62(4), 1996, pp. 1159-1166
Biodegradation of two chlorinated aromatic compounds was found to be a
common capability of the microorganisms found in the soils of undistu
rbed, pristine ecosystems. We used 2,4-dichlorophenoxyacetate (2,4-D)
and 3-chlorobenzoate (3CBA) as enrichment substrates to compare popula
tions of degrading bacteria from six different regions making up two e
cosystems. We collected soil samples from four Mediterranean (Californ
ia, central Chile, the Cape region of South Africa, and southwestern A
ustralia) and two boreal (northern Saskatchewan and northwestern Russi
a) ecosystems that had no direct exposure to pesticides or to human di
sturbance. Between 96 and 120 samples from each of the six regions wer
e incubated with 50 ppm of [U-C-14]2,4-D or [U-C-14]3CBA. Soils from a
ll regions samples mineralized both 2,4-D and 3CBA, but 3CBA was miner
alized without a lag period, while 2,4-D was generally not mineralized
until the second week. 3CBA degradative capabilities were more evenly
distributed spatially than those for 2,4-D. The degradative capabilit
ies of the soils were readily transferred to fresh liquid medium, 3CBA
degraders were easily isolated from most soils. We recovered 610 stra
ins that could release carbon dioxide from ring-labeled 3CBA. Of these
, 144 strains released chloride and degraded over 80% of 1 mM 3CBA in
3 weeks or less, In contrast, only five 2,4-D degraders could be isola
ted, although a variety of methods were used in an attempt to culture
the degraders. The differences in the distribution and culturability o
f the bacteria responsible for 3CBA and 2,4-D degradation in these eco
systems suggest that the two substrates are degraded by different popu
lations, We also describe a C-14-based microtiter plate method that al
lows efficient screening of a large number of samples for biodegradati
on activity.