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Impact of cell density on microbially induced stable isotope fractionation


Quantification of microbial contaminant biodegradation based on stable isotope fractionation analysis (SIFA) relies on known, invariable isotope fractionation factors. The microbially induced isotope fractionation is caused by the preferential cleavage of bonds containing light rather than heavy isotopes. However, a number of non-isotopically sensitive steps preceding the isotopically sensitive bond cleavage may affect the reaction kinetics of a degradation process and reduce the observed (i.e., the macroscopically detectable) isotope fractionation. This introduces uncertainty to the use of isotope fractionation for the quantification of microbial degradation processes. Here, we report on the influence of bacterial cell density on observed stable isotope fractionation. Batch biodegradation experiments were performed under non-growth conditions to quantify the toluene hydrogen isotope fractionation by exposing Pseudomonas putida mt-2(pWWO) at varying cell densities to different concentrations of toluene. Observed isotope fractionation depended significantly on the cell density. When the cell density rose from 5 × 105 to 5 × 108cells/mL, the observed isotope fractionation declined by 70% and went along with a 55% decrease of the degradation rates of individual cells. Theoretical estimates showed that uptake-driven diffusion to individual cells depended on cell density via the overlap of the cells’ diffusion-controlled boundary layers. Our data suggest that biomass effects on SIFA have to be considered even in well-mixed systems such as the cell suspensions used in this study.

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This study was supported by the European MEST-CT-2004-8332 grant (AXIOM) and the Helmholtz Association grant VG-NG-338 (Great MoDE). Skilled technical help by Birgit Würz, Jana Reichenbach and Rita Remer is greatly acknowledged. This work contributed to the SAFIRA II Research Programme.

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Correspondence to Lukas Y. Wick.

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Kampara, M., Thullner, M., Harms, H. et al. Impact of cell density on microbially induced stable isotope fractionation. Appl Microbiol Biotechnol 81, 977–985 (2009).

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  • Bioavailability
  • Cell density
  • Biodegradation
  • Fractionation factor
  • Stable isotope fractionation analysis
  • Toluene