Bacterial chemotaxis has the potential to increase the rate of degradation of chemoattractants, but its influence on degradation of hydrophobic attractants initially dissolved in a non-aqueous-phase liquid (NAPL) has not been examined. We studied the effect of chemotaxis by Pseudomonas putida G7 on naphthalene mass transfer and degradation in a system in which the naphthalene was dissolved in a model NAPL. Chemotaxis by wild-type P. putida G7 increased the rates of naphthalene desorption and degradation relative to rates observed with nonchemotactic and nonmotile mutant strains. While biodegradation alone influenced the rate of substrate desorption by increasing the concentration gradient against which desorption occurred, chemotaxis created an even steeper gradient as the cells accumulated near the NAPL source. The extent to which chemotaxis affected naphthalene desorption and degradation depended on the initial bacterial and naphthalene concentrations, reflecting the influences of these variables on concentration gradients and on the relative rates of mass transfer and biodegradation. The results of this study suggest that chemotaxis can substantially increase the rates of mass transfer and degradation of NAPL-associated hydrophobic pollutants.Nonaqueous-phase liquids (NAPLs) pose great challenges in the remediation of contaminated soil and sediment (21, 23). Residual NAPLs are often trapped in pores, leading to smallscale heterogeneity in contaminant distribution and slow rates of contaminant transfer into the surrounding aqueous phase (5, 12, 13). In situ biodegradation by indigenous microorganisms may be a low-cost means of remediating contaminated sites, but its reliability depends on an improved understanding of governing mechanisms (22). One concern is that biodegradation of hydrophobic substrates is often limited by the rate of mass transfer from a nonaqueous phase (1,3,34). Removal of a contaminant from the aqueous phase through biodegradation can, however, improve dissolution of a pure substance or desorption from a nonaqueous phase by increasing the concentration gradient against which mass transfer occurs (3,10,33,37). The rate of mass transfer, and hence the rate of biodegradation, is predicted to increase as the degrading organisms move closer to the contaminant source (3).Chemotaxis by Pseudomonas putida strain G7 was shown recently to enhance the degradation of naphthalene diffusing from a naphthalene-saturated aqueous buffer contained in a capillary (17), consistent with theoretical predictions (2, 15) that bacterial chemotaxis towards nutrient sources can increase the rate of nutrient consumption. The effect of chemotaxis on naphthalene degradation was equivalent to increasing the concentration of nonchemotactic or nonmotile mutant strains by at least two orders of magnitude (27). Unlike the situation in a strictly aqueous system, however, bacteria do not have direct access to the substrate in a nonaqueous source. We therefore examined the effect of chemotaxis on the desorption and biodegradat...

Bacterial chemotaxis may have a significant impact on the structure and function of bacterial communities. Quantification of chemotactic motion is necessary to identify chemoeffectors and to determine the bacterial transport parameters used in predictive models of chemotaxis. When the chemotactic bacteria consume the chemoeffector, the chemoeffector gradient to which the bacteria respond may be significantly perturbed by the consumption. Therefore, consumption of the chemoeffector can confound chemotaxis measurements if it is not accounted for. Current methods of quantifying chemotaxis use bacterial concentrations that are too high to preclude chemoeffector consumption or involve ill-defined conditions that make quantifying chemotaxis difficult. We developed a method of quantifying bacterial chemotaxis at low cell concentrations (ϳ10 5 CFU/ml), so metabolism of the chemoeffector is minimized. The method facilitates quantification of bacterial-transport parameters by providing well-defined boundary conditions and can be used with volatile and semivolatile chemoeffectors.Chemotaxis, the self-directed movement of an organism or cell toward or away from a chemical (chemoeffector) along a concentration gradient, is a well-known bacterial behavior (1,5,6,12). The interest in bacterial chemotaxis is broad. Knowledge is sought of how the bacteria sense chemicals (4, 21-23), how chemotaxis affects pathogenesis and symbiosis (10,29,57), the role of chemotaxis in the formation and structure of bacterial communities (70), the influence of chemotaxis on oceanic nutrient cycling (8,9,17,75), and the effect of chemotaxis on pollutant biodegradation (43,52,(54)(55)(56).The chemoeffectors to which bacteria are attracted (chemoattractants) are often nutrients that the bacteria consume. Many methods for quantifying chemotactic transport use bacterial concentrations that are high enough that significant consumption of the chemoattractant occurs, affecting the chemoattractant gradient that forms. For example, Marx and Aitken (49) reported diminished chemotaxis at bacterial concentrations of Ն10 6 CFU per ml, which was believed to be a result of significant consumption of the chemoattractant (naphthalene). Therefore, complexity is introduced when interpreting measurements of bacterial migration by chemotaxis because consumption of the chemoeffector must be accounted for. Furthermore, bacterial accumulation by chemotaxis and consumption of a chemoattractant may create a secondary gradient that the bacteria also sense. For example, aerobic biodegradation of a chemoattractant by bacteria that are also aerotactic (responsive to oxygen) could create an oxygen gradient that influences bacterial migration, confounding measurements of movement toward the primary chemoattractant. Methods that are used to quantify chemotaxis, therefore, should minimize metabolism of a biodegradable chemoeffector so that the observed bacterial motion would be in response only to diffusion of the primary chemoeffector.Methods in which metabolism can be minimized...

Chemotactic bacteria can be attracted to electron donors they consume. In systems where donor is heterogeneously distributed, chemotaxis can lead to enhanced removal of donor relative to that achieved in the absence of chemotaxis. However, simultaneous consumption of an electron acceptor may result in the formation of an acceptor gradient to which the bacteria also respond, thus diminishing the positive effect of chemotaxis. Depletion of an electron acceptor can also reduce the rate of electron donor consumption in addition to its effect on chemotaxis. In this study, we examined the effect of oxygen on chemotaxis to naphthalene and on naphthalene consumption by Pseudomonas putida G7. The organism was able to move up an oxygen gradient when there was a naphthalene gradient in the opposite direction. In the absence of an oxygen gradient, low levels of oxygen attenuated chemotaxis to naphthalene but did not affect random motility. The rate of naphthalene consumption decreased at dissolved oxygen concentrations similar to those at which chemotaxis was attenuated. These results suggest that low dissolved oxygen concentrations can reduce naphthalene removal by P. putida G7 in systems where naphthalene is heterogeneously distributed by simultaneously attenuating chemotactic motion toward naphthalene and decreasing the rate of naphthalene degradation.