PRISTINE SOILS MINERALIZE 3-CHLOROBENZOATE AND 2,4-DICHLOROPHENOXYACETATE VIA DIFFERENT MICROBIAL-POPULATIONS

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.