No abstract

We analyzed the impact of surfactant addition on hydrocarbon mineralization kinetics and the associated population shifts of hydrocarbon-degrading microorganisms in soil. A mixture of radiolabeled hexadecane and phenanthrene was added to batch soil vessels. Witconol SN70 (a nonionic, alcohol ethoxylate) was added in concentrations that bracketed the critical micelle concentration (CMC) in soil (CMC) (determined to be 13 mg g ؊1 ). Addition of the surfactant at a concentration below the CMC (2 mg g ؊1 ) did not affect the mineralization rates of either hydrocarbon. However, when surfactant was added at a concentration approaching the CMC (10 mg g ؊1 ), hexadecane mineralization was delayed and phenanthrene mineralization was completely inhibited. Addition of surfactant at concentrations above the CMC (40 mg g ؊1 ) completely inhibited mineralization of both phenanthrene and hexadecane. Denaturing gradient gel electrophoresis of 16S rRNA gene segments showed that hydrocarbon amendment stimulated Rhodococcus and Nocardia populations that were displaced by Pseudomonas and Alcaligenes populations at elevated surfactant levels. Parallel cultivation studies revealed that the Rhodococcus population can utilize hexadecane and that the Pseudomonas and Alcaligenes populations can utilize both Witconol SN70 and hexadecane for growth. The results suggest that surfactant applications necessary to achieve the CMC alter the microbial populations responsible for hydrocarbon mineralization.Surfactants have been successfully used to enhance the apparent solubility of nonpolar organic contaminants (NOC) as well as their subsequent removal from soil. There is, however, a great deal of conflicting information regarding the ability of surfactants to enhance the bioavailability and biodegradation of NOC (10, 21, 28). For most surfactants, an increase in NOC solubility is achieved only at surfactant concentrations greater than the critical micelle concentration (CMC), where the majority of surfactant molecules are aggregated into micelles. Surfactant micelles contain a hydrophobic core with a high affinity for NOC, resulting in increases in the apparent solubility of NOC. However, surfactant application at concentrations approaching and exceeding the CMC often result in significant decreases in rates of microbial NOC degradation. Potential mechanisms of inhibition include reduction in NOC bioavailability when bound in surfactant micelles (9, 13, 27), inhibition of microbial attachment at mineral and organic surfaces (4, 7, 23), and surfactant toxicity (3,25,26,33). With a few exceptions, the majority of studies investigating the influence of surfactants on NOC biodegradation involve pure cultures. Furthermore, those using mixed cultures primarily address the impact of surfactant dose on the kinetics of NOC degradation. To our knowledge, there has been no attempt to evaluate changes in microbial community structure across a range of surfactant applications. Consequently, the goal of this study was to examine shifts of NOC-degrading popul...

A Sphingomonas species that mineralizes high concentrations of pentachlorophenol (PCP) was isolated from a PCP-contaminated EPA Superfund site. This bacterium, identified as Sphingomonas sp. strain RA2, is able to degrade PCP at concentrations of up to 300 micrograms/mL in liquid culture. This organism was tested for its ability to degrade high concentrations of PCP in a soil that did not contain organisms capable of degrading high concentrations of PCP. When inoculated into contaminated soil, Sphingomonas sp. RA2 mineralized PCP at concentrations of 300, 600, 900, and 1200 micrograms PCP/g of soil, but was unable to mineralize 1500 micrograms PCP/g of soil. Only very minimal loss of PCP was seen in uninoculated soils. The results of this study demonstrate that Sphingomonas sp. RA2 may be a useful organism for remediation of sites contaminated with high concentrations of PCP.

Six previously undescribed microorganisms capable of atrazine degradation were isolated from an agricultural soil that received repeated exposures of the commonly used herbicides atrazine and acetochlor. These isolates are all Gram-positive and group with microorganisms in the genera Nocardioides and Arthrobacter, both of which contain previously described atrazine degraders. All six isolates were capable of utilizing atrazine as a sole nitrogen source when provided with glucose as a separate carbon source. Under the culture conditions used, none of the isolates could utilize atrazine as the sole carbon and nitrogen source. We used several polymerase-chain-reaction-based assays to screen for the presence of a number of atrazine-degrading genes and verified their identity through sequencing. All six isolates contain trzN and atzC, two well-characterized genes involved in the conversion of atrazine to cyanuric acid. An additional atrazine-degrading gene, atzB, was detected in one of the isolates as well, yet none appeared to contain atzA, a commonly encountered gene in atrazine impacted soils and atrazine-degrading isolates. Interestingly, the deoxyribonucleic acid sequences of trzN and atzC were all identical, implying that their presence may be the result of horizontal gene transfer among these isolates.

No abstract