1. Distribution of feast and famine strategies in soils
Contaminant bioavailability to microorganisms is affected by sorption/desorption hysteresis and kinetics on soils (geosorbents), and microbial substrate uptake kinetics. Whereas each of these processes had been studied separately, no information is available on how geochemical soil characteristics affect the affinity and metabolic rates of their associated microbial communities. Using a serial elution technique, we have demonstrated that tightly and loosely attached communities can be clustered using a range of ecological techniques. The convergence of results from denaturing gradient gel electrophoresis (DGGE), community level substrate utilization patterns (Biolog GN/GP) and fatty acid methyl ester (FAME) profiles used in this study indicate that the community response to acclimation substrates is distributed as a function of attached habitat, and that tightly-attached communities are apparently less responsive to external stimuli. Similarly, kinetic analysis of both communities and isolates from loosely and tightly attached fractions indicated that oligotrophic features increased with strength of attachment.
This work led to the hypothesis that by fractionating communities from soils ranging in grain size distribution and organic matter content, and analyzing them using ecological and kinetic methods, a conceptual framework could be developed to couple microbial and soil characteristics for bioavailability interpretation. Four soils ranging in organic carbon fraction (foc = 0.15 to 46%) were treated to obtain two microbial fractions (F1: gravity elution; F2: Tween 80/sonication), incubated with Biolog Ecoplates, and monitored for up to 72 hours. Principal component analysis of the substrates oxidized indicated that a greater degree in substrate clustering for tightly- than for loosely-attached fractions. Kinetic analysis of the substrate oxidation curves using the logistic form of the Monod model, and plotting the kinetic parameters in a Ks-Vmax plane, resulted in a clustering of tightly-attached fractions at low Ks values. Loosely-attached communities exhibited dissimilar behavior between low and high organic soils along the Ks axis. These results support the argument that sorptive properties of soils greatly influence the kinetic behavior of their indigenous microbial communities.
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