Bacterial dehalogenases: biochemistry, genetics, and biotechnological applications.

Landscapes are complex ecological systems that operate over broad spatiotemporal scales. Hierarchy theory conceptualizes such systems as composed of relatively isolated levels, each operating at a distinct time and space scale. This paper explores some basic properties of scaled systems with a view toward taking advantage of the scaled structure in predicting system dynamics. Three basic properties are explored: (1) hierarchical structuring, (2) disequilibrium, and (3) metastability. These three properties lead to three conclusions about complex ecological systems. First, predictions about landscape dynamics can often be based on constraints that directly result from scaled structure. Biotic potential and environmental limits form a constraint envelope, analogous to a niche hypervolume, within which the landscape system must operate. Second, within the constraint envelope, thermodynamic and other limiting factors may produce attractors toward which individual landscapes will tend to move. Third, because of changes in biotic potential and environmental conditions, both the constraint envelope and the local attractors change through time. Changes in the constraint structure may involve critical thresholds that result in radical changes in the state of the system. An attempt is made to define measurements to predict whether a specific landscape is approaching a critical threshold.

A hierarchical framework for the analysis of scale

The kinetics of the degradation of trichloroethylene (TCE) and seven other chlorinated aliphatic hydrocarbons by Methylosinus trichosporium OB3b were studied. All experiments were performed with cells grown under copper stress and thus expressing soluble methane monooxygenase. Compounds that were readily degraded included chloroform, trans-1,2-dichloroethylene, and TCE, with Vmax values of 550, 330, and 290 nmol min-1 mg of cells-1, respectively. 1,1-Dichloroethylene was a very poor substrate. TCE was found to be toxic for the cells, and this phenomenon was studied in detail. Addition of activated carbon decreased the acute toxicity of high levels of TCE by adsorption, and slow desorption enabled the cells to partially degrade TCE. TCE was also toxic by inactivating the cells during its conversion. The degree of inactivation was proportional to the amount of TCE degraded; maximum degradation occurred at a concentration of 2 mumol of TCE mg of cells-1. During conversion of [14C]TCE, various proteins became radiolabeled, including the alpha-subunit of the hydroxylase component of soluble methane monooxygenase. This indicated that TCE-mediated inactivation of cells was caused by nonspecific covalent binding of degradation products to cellular proteins.

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Kinetics of Chlorinated Hydrocarbon Degradation by Methylosinus trichosporium OB3b and Toxicity of Trichloroethylene

The rate and capacity for chloroform (CF) and trichloroethylene (TCE) transformation by a mixed methanotrophic culture of resting cells (no exogenous energy source) and formate-fed cells were measured. As reported previously for TCE, formate addition resulted in an increased CF transformation rate (0.35 day-1 for resting cells and 1.5 day-1 for formate-fed cells) and transformation capacity (0.0065 mg of CF per mg of cells for resting cells and 0.015 mg of CF per mg of cells for formate-fed cells), suggesting that depletion of energy stores affects transformation behavior. The observed finite transformation capacity, even with an exogenous energy source, suggests that toxicity was also a factor. CF transformation capacity was significantly lower than that for TCE, suggesting a greater toxicity from CF transformation. The toxicity of CF, TCE, and their transformation products to whole cells was evaluated by comparing the formate oxidation activity of acetylene-treated cells to that of non-acetylene-treated cells with and without prior exposure to CF or TCE. Acetylene arrests the activity of methane monooxygenase in CF and TCE oxidation without halting cell activity toward formate. Significantly diminished formate oxidation by cells exposed to either CR or TCE without acetylene compared with that with acetylene suggests that the solvents themselves were not toxic under the experimental conditions but their transformation products were. The concurrent transformation of CF and TCE by resting cells was measured, and results were compared with predictions from a competitive-inhibition cometabolic transformation model. The reasonable fit between model predictions and experimental observations was supportive of model assumptions.

Product toxicity and cometabolic competitive inhibition modeling of chloroform and trichloroethylene transformation by methanotrophic resting cells.

Reduction of U(VI) to U(IV) by indigenous bacteria in contaminated ground water

Experimental and modeling studies were conducted to analyze the dynamic response behavior of a phenol-oxidizing fixed film using a differential, fluidized-bed bioreactor in a recycle loop with a well-mixed reservoir. With the bioreactor at steady state, a pulse of phenol was added to perturb the system, and the phenol concentration was monitored continuously until steady state was again achieved.

The experimental dynamics were compared with a dynamic mathematical model based on diffusion and reaction within the biofilm, liquid mixing, and biofilm growth. Constant-pH experiments could be adequately described using an unstructured, double-Monod kinetic expression with substrate inhibition by phenol.

However, in dynamic experiments without pH control, the pH of the liquid phase dropped, and damped oscillations were observed in the phenol concentration and reaction rate trajectories. Oscillatory solutions could not be induced in the model, even when product inhibition was included, and a linear stability analysis did not reveal tendencies toward instability. The cause of the experimental oscillations remains unknown.

Dynamics of a biological fixed film for phenol degradation in a fluidized‐bed bioreactor

We have constructed and operated a fixed-film packed-bed bioreactor to evaluate the technical feasibility of bioremediation of TCE-contaminated groundwater using methanotrophic microorganisms. The performance of our bench-scale bioreactor system and observations on the kinetics of degradation of TCE and DCE in the bioreactor are described in this report

Degradation of trichloroethylene and trans-1,2-dichloroethylene by a methanotrophic consortium in a fixed-film, packed-bed bioreactor

Fixed-film, fluidized-bed bioreactors have been used successfully to treat dilute synthetic and actual wastewaters for long periods of time.

Biooxidation of coal gasification wastewaters

Process condensate wastewaters from two fixed-bed gasifiers have been treated successfully using a laboratory-scale, fixed-film, fluidized-bed bioreactor. Both dilute (1 to 2% raw wastewater) and more concentrated (50% stripped wastewater) were treated continuously for periods of one to three months. Biological activity was stable, recovery from upsets was rapid, and reaction rates were high due to the high concentration of microorganisms retained on the support particles. Removal of phenolics exceeded 99%, and removal of chemical oxygen demand was typically 75 to 85% for the 50% stripped wastewater with a hydraulic residence time in the bed of ˜5 to 7 h. Sludge production rates were comparable to rates in suspended growth systems.

Biooxidation of coal gasification wastewaters using fluidized-bed bioreactors

Design verification studies and process development work were conducted at Oak Ridge National Laboratory in support of the new fluidized-bed biodenitrification system under construction at the Feed Materials Production Center (FMPC) in Fernald, Ohio. Two bioreactors in series, each 6-m tall, and one 12-m-tall bioreactor were operated for approximately 9 months using synthetic and actual wastewaters from the FMPC. The results indicate that satisfactory reaction rates can be achieved and maintained; however, some degree of water softening, pH adjustment within the bioreactors, and temperature control may be necessary.

T.L. Donaldson

Papers

Dynamics of a biological fixed film for phenol degradation in a fluidized‐bed bioreactor

Degradation of trichloroethylene and trans-1,2-dichloroethylene by a methanotrophic consortium in a fixed-film, packed-bed bioreactor

Biooxidation of coal gasification wastewaters

Biooxidation of coal gasification wastewaters using fluidized-bed bioreactors

Biodenitrification of uranium refinery wastewaters