Monitoring Microbial Mineralization Using Reverse Stable Isotope Labeling Analysis by Mid-Infrared Laser Spectroscopy.

Assessing the biodegradation of organic compounds is a frequent question in environmental science. Here, we present a sensitive, inexpensive, and simple approach to monitor microbial mineralization using reverse stable isotope labeling analysis (RIL) of dissolved inorganic carbon (DIC). The medium for the biodegradation assay contains regular organic compounds and C-labeled DIC with C atom fractions (x(C)) higher than natural abundance (typically 2-50%). The produced CO (x(C) ≈ 1.11%) gradually dilutes the initial x(C) allowing to quantify microbial mineralization using mass-balance calculations. For C-enriched CO samples, a newly developed isotope ratio mid-infrared spectrometer was introduced with a precision of x(C) < 0.006%. As an example for extremely difficult and slowly degradable compounds, CO production was close to the theoretical stoichiometry for anaerobic naphthalene degradation by a sulfate-reducing enrichment culture. Furthermore, we could measure the aerobic degradation of dissolved organic carbon (DOC) adsorbed to granular activated carbon in a drinking water production plant, which cannot be labeled with C. Thus, the RIL approach can be applied to sensitively monitor biodegradation of various organic compounds under anoxic or oxic conditions.