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Identifying and Overcoming Bottlenecks of Micropollutant Degradation at Low Concentrations

Mid-Term Report Summary - MICRODEGRADE (Identifying and Overcoming Bottlenecks of Micropollutant Degradation at Low Concentrations)

To reveal whether (i) mass transfer (bioavailability, cell uptake) or (ii) physiological limitations (enzyme down-regulation) limit degradation of chemical micropollutants at small (sub-microg/L) concentrations, the following results have been achieved in the first half of the ERC project “MicroDegrade”:
Mass-transfer limitations
- Cultivation of the atrazine-degrading strain Arthrobacter aurescens TC1 in chemostat made it possible to directly observe a decrease in compound-specific isotope fractionation of atrazine when concentrations dropped to low microgram-per-liter concentrations. This is the first direct observation of bioavailability limitations (i.e. rate-limiting mass transfer into cells) in strains adapted to low concentrations.
- Further evidence on the role of mass transfer depending on cell wall properties was provided by smaller isotope fractionation of atrazine degradation by gram negative cells (Polaromonas sp. NEA-C) compared to gram positive cells (Arthrobacter aurescens TC1). This indicates that the outer membrane of Polaromonas sp. NEA-C potentially posed an additional barrier.
Physiological response
- Evaluation of chemostat data allowed deriving a first estimate of maintenance demand in Arthrobacter aurescens TC1 making it possible to evaluate at what concentrations a physiological response can be expected.
Sediment tank experiments
- First results from 2D-sediment tank experiments suggest that isotope effects of dispersion are too small to be measured with organic compounds of natural isotopic abundance. This facilitates the interpretation of vertical isotope profiles in field data in order to look for cases of “observed, but not recognized”: field studies in which isotope profiles indicative of bioavailability limitations may have been observed, but not yet interpreted with respect to their significance.
Building on these advances, it is our aim in the next period (i) to unravel the physiological response of our model strain to even lower substrate conditions in (partial) retentostat experiments and by proteomics; (ii) to investigate whether this response will change be evolutionary adaptation, and/or can in addition be “tuned” by changing environmental parameters (e.g. co-substrate addition); (iii) to trace the occurrence of bioavailability in natural environments such as sediments.