CURRENT MICROBIOLOGYVol. 23 (1991), pp. 65-69 Current Microbiology © Springer-Verlag New York Inc. 1991 Simultaneous Degradation of the Herbicides 2,4-Dichlorophenoxyacetic Acid and 2-(2'Methyl-4- Chlorophenoxy)Propionic Acid by Mixed Bacterial Cultures Kevin B. Hallberg] Martin P. Kelly, and Olli H. Tuovinen Department of Microbiology, The Ohio State University, Columbus, Ohio, USA Abstract. The simultaneous degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-(2- methyl-4-chlorophenoxy)propionic acid (mecoprop) was achieved by two mixed cultures in the absence of any additional carbon or energy substrates. Mecoprop was not completely degraded by either of the two cultures, nor did addition of 2,4-D affect the degradation of mecoprop. The cultures completely degraded 2,4-D, and the degradation was uninfluenced by the addition of mecoprop. Nearly complete dechlorination of the mixture of two herbicides was achieved by both cultures, on the basis of the total amount of the two herbicides degraded. During the course of the reaction, however, the expected values of chloride were not met. Cell growth continued after the degradation of the parent substrates ceased. Although the mecoprop degradation did not continue to completion, spectral and growth data indicated that the metabolites which had accumulated during the reaction were degraded upon further incubation. The microbiological degradation of 2,4-dichloro- phenoxyacetic acid (2,4-D) has been extensively studied and is well documented. The degradative pathway for this herbicide has been elucidated, and the enzymes involved have been characterized [4, 11, 17]. The genetic systems involved in the degrada- tion of 2,4-D have also been well described [1, 3]. In contrast, little information has been published concerning the structurally related herbicide (-+)2- (2-methyl-4-chtorophenoxy)propionic acid (meco- prop). This herbicide is widely used and has been reported to have adverse effects on nitrogen fixation [5]. The degradation of mecoprop has been reported in soil [10, 16], and it has been demonstrated with mixed cultures of bacteria [9]. Kilpi [8] has described a culture that could degrade mecoprop, but only in the presence of benzoic acid. We recently established a mass balance for 2,4-D degradation by a mixed culture [6]. This mass balance was based on chloride appearance and the disappearance of the parent compound. For the present paper, we have extended this work by study- ing the simultaneous degradation of 2,4-D and meco- 1 Present address: Enheten f6r tilliimpad cell- och molekyl~bio- logi, Ume~ Universitet, S-90187 Ume~, Sweden. Address reprint requests to: Dr. O.H. Tuovinen, Department Columbus, OH 43210-1292, USA. prop by two mixed cultures. Although 2,4-D and mecoprop are sometimes used in mixture for weed control formulations, very little information has been available previously to evaluate their concur- rent biodegradability. Materials and Methods Culture conditions. The two mixed cultures used in this work were designated as OMS and SM. The cultures originated from environmental samples collected from a fertilizer manufacturing plant site and enriched with 2,4-D and mecoprop as the sole substrate. Other 2,4-D-degrading, closely related mixed cultures have been previously obtained from the same location [6, 12]. The cultures were maintained in a mineral salts medium [6] that contained one or both of the herbicides 2,4-D and mecoprop as the only source of organic carbon and energy. The mass balances were carried out in stirred tank reactors under nearly identical conditions, as previously described [6]. Initially, the growth me- dium contained approx. 4.5 mM each of 2,4-D and mecoprop per liter as substrates and was inoculated with a washed OMS or SM culture (5% vol/vol). Further experimentation included changing the substrate concentration to approx. 1.12 mM of each before inoculation. Growth was measured as absorbance at 550 nm, and inorganic chloride ions were measured with a chloridometer [6]. Mass balances based on chloride release and residual suhstrate concentration were also performed with the SM culture, which had previously been maintained with both 2,4-D and mecoprop in the mineral salts medium. of Microbiology, The Ohio State University, 484 W. 12th Avenue,