Kinetic and ecological distribution of soil microbial communities as a function of attachment: Implications for contaminant bioavailability.

Abstract

Bioavailability, a measure of the accessibility of a contaminant to microorganisms, is an important factor in bioremediation applications. This process depends on physicochemical phenomena (mass transfer) and biokinetic factors (microbial uptake). While mass transfer mechanisms have been extensively studied, the significance of the microbial component on bioavailability has received scant attention. This research addresses the metabolic potential and residual contaminant concentrations of soil microbiota as a function of attachment. Loosely- and tightly-attached microbial fractions, extracted by a serial custom elution procedure, were ecologically and kinetically characterized as a function of soil organic content, and their responses to acclimation substrates were evaluated. Community composition was analyzed based on phospholipid fatty acids (PLFA) profiles. In low organic soils, tightly-attached fractions exhibited an elevated biomass content and a very distinct community structure compared to the loosely-attached fractions. Moreover, a decrease in membrane permeability was discerned in the tightly-attached fractions. In high organic soils, both fractions exhibited similar community structure. Community-level substrate utilization patterns were fit to the Michaelis-Menten model for kinetic characterization of affinity (K_m), and rate (V_max) constants. Whereas, in low organic carbon soils, K_m decreased 3--4 fold with increasing strength of attachment, this parameter clustered for all fractions, in high organic soils. In contrast, V_max values spanned a narrow range, independent of carbon content and degree of attachment. Community structure and metabolic potential responses to toluene and glucose acclimation were assessed based on fatty acid methyl ester (FAME), Biolog, and denaturing gradient gel electrophoresis (DGGE) profiles. The ecological adaptive responses of the two microbial fractions were very distinct under toluene as well as glucose acclimation conditions, with tightly-attached communities oxidizing 38% more substrates than loosely-attached communities. The kinetic adaptive responses to toluene and toluic acid degradation indicated lower residual concentrations (C_t) achieved by the tightly-attached fractions and isolates for low substrate concentrations (1 to 10 mg/L). This study demonstrates the potential importance of tightly-attached communities in bioavailability assessments. Their high substrate affinity (low K_m) and low achievable residual concentrations, the result of an efficient uptake metabolism, may imply greater contaminant availability for bacterial uptake in substrate-limited environments.