Walking the tightrope of bioavailability: growth dynamics of PAH degraders on vapour-phase PAH.
Bottom Line: Controlled microcosm experiments revealed that high cell densities increased growth rates close (<2 cm) to the NAPH source, whereas competition for NAPH decreased the growth rates at larger distances despite the high gas phase diffusivity of NAPH.At larger distance, less microbial biomass was likewise sustained by the vapour-phase NAPH.Such varying growth kinetics is explained by a combination of bioavailability restrictions and NAPH-based inhibition.
Affiliation: UFZ - Helmholtz Centre for Environmental Research, Department of Environmental Microbiology, 04318 Leipzig, Germany.Show MeSH
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Mentions: The theoretically predicted influence of substrate inhibition on degradation rates is shown for an arbitrary but representative example (Fig. 4A) to demonstrate the differences between the considered rate expressions (Eq. 1 in the absence of inhibition effects, and Eq. 4 in the presence of inhibition effects). Omission of inhibition effects results in a rate monotonously increasing with substrate concentration. In turn, when consideration of inhibition effects leads to predicted maximum rates at a concentration of . At concentrations of the rate is decreasing due to substrate limitation while higher concentrations of lead to a rate decrease due to substrate inhibition. Similar observations were made using alternative expressions suggested in the literature (e.g. Mulchandani and Luong, 1989) to describe rate inhibition effects (results not shown). Figure 4B relates the influence of inhibition to changes of the substrate degradation rates (and hence of bacterial growth rates) at varying substrate bioavailability conditions. In the absence of inhibition degradation rates are highest when the substrate is highly bioavailable (i.e. lower values for 1/Bn), whereas reduction of substrate bioavailability (e.g. due to increased distance or increased competition as in the LCS or HCS) leads to concomitant decrease of the degradation rates. Interestingly, the degradation rates at all bioavailability conditions (values of 1/Bn) are lower in the presence of inhibition (Fig. 4A and B). While at low bioavailability inhibition effects remain small, differences grow significantly larger at high bioavailability conditions. Figure 4B further reveals that inhibition leads to highest degradation rates at intermediate substrate bioavailability conditions representing a compromise of moderate inhibition and sufficient substrate supply to a metabolically active cell.
Affiliation: UFZ - Helmholtz Centre for Environmental Research, Department of Environmental Microbiology, 04318 Leipzig, Germany.