Dissolved hydrocarbon metabolism: The concentration-dependent kinetics of toluene oxidation in some North American estuaries’

The metabolism of toluene by natural populations of marine bacteria, stored to enhance their activity, gave hyperbolic kinetics with saturation at only K, = 0.6 to 3.4 pg liter-‘. Similarly Kl in fresh seawater was 0.26 pg toluene liter-‘. Freshly collected populations could be moderately active toward toluene, affinity azA = 1.1-37.5 liters g-cells-l h-l. These moderate affinities taken together with the small K, values give an explanation for the failure of most marine bacteria to grow at the expense of a single hydrocarbon. In two experiments there was a significant first-order region in toluene in the 10-50 pg liter-l range. When cultures of Pseudomonas sp. strain T2 were cast in suspended agar blocks to impede diffusion, toluene oxidation still followed hyperbolic kinetics although CO, production and organic product formation were characterized by larger Michaelis constants. A large widespread population of bacteria with inducible capacity to metabolize hydrocarbons is indicated. Observed turnover times for toluene were 0.2-40 years with growth rates of lO-‘j h-l from toluene at 1 pg literI. When radioactive hydrocarbons such as dodecane are incubated with seawater, the label appears in cell material and carbon dioxide (Robertson et al. 1973). The presence of metabolically labile dissolved hydrocarbons in seawater is suggested by the fact that both toluene (Button et al. 198 1 a) and terpene mixtures (Button 1984) are immediately attacked at measurable rates by estuarine bacteria. Induction kinetics (in prep.) indicate that recent exposure of the organisms to hydrocarbons is necessary for significant immediate metabolism of them because the affinity (initial slope of specific rate vs. hydrocarbon concentration: Button 1983) of both the indigenous microflora of seawater and pure cultures of a marine bacterium is greatly enhanced by exposure to pg liter-l quantities of hydrocarbons, and the regained ability of pure cultures is lost upon hydrocarbon removal. Turnover time calculations indicate that continuous sources are required to maintain inducing concentrations. Activity of the indigenous population toward hydrocarbons should therefore reflect the hydrocarbon concentration and the proximity to the source. ’ Financial support was provided by grant 808 17601 from the US. Environmental Protection Agency Office of Exploratory Research and by the State of Alaska. That the microbial metabolism of hydrocarbons follows saturation kinetics has been suggested (Lee and Ryan 1976; Pfaender and Bartholomew 1982; Button et al. 198 1 b), but dissolved hydrocarbon metabolism kinetics have not been examined in detail. Several factors are relevant to the description of the metabolism kinetics of dissolved substrates. First is the affinity of the population for substrate (Button 1983) because it defines the rate at which a substrate is processed at subsaturating concentrations (the dominant marine situation) and also relates rate to the responsible population as well as (in some cases) to the functionality of the transporter components of a population. Second is the concentration where saturation kinetics come to fore, which can reflect transport mechanisms. Third is the transient state situation, in which the metabolic capability of the organisms changes in response to the concentration of nutrient that they have experienced. Important elements of the transient process are repression and induction, both of which reflect long term culture history, and metabolic regulation. Fourth is the efficiency of nutrient processing with the related release of metabolically labile products because these products may affect neighboring organisms that share the same microenvironment. Organisms seem particularly prone to liberate