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Abstract
Biotransformation plays a key role in hydrophobic organic compound (HOC) fate, and
understanding kinetics as a function of (bio)availability is critical for elucidating
persistence, accumulation, and toxicity. Biotransformation mainly occurs in an aqueous
environment, posing technical challenges for producing kinetic data because of low
HOC solubilities and sorptive losses. To overcome these, a new experimental approach
based on passive dosing is presented. This avoids using cosolvent for introducing
the HOC substrate, buffers substrate depletion so biotransformation is measured within
a narrow and defined dissolved concentration range, and enables high compound turnover
even at low concentrations to simplify end point measurement. As a case study, the
biodegradation kinetics of two model HOCs by the bacterium Sphingomonas paucimobilis
EPA505 were measured at defined dissolved concentrations ranging over 4 orders of
magnitude, from 0.017 to 658 μg L(-1) for phenanthrene and from 0.006 to 90.0 μg L(-1)
for fluoranthene. Both compounds had similar mineralization fluxes, and these increased
by 2 orders of magnitude with increasing dissolved concentrations. First-order mineralization
rate constants were also similar for both PAHs, but decreased by around 2 orders of
magnitude with increasing dissolved concentrations. Dynamic passive dosing is a useful
tool for measuring biotransformation kinetics at realistically low and defined dissolved
HOC concentrations.