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 13 C-labeled DIC with 13 C atom fractions (x( 13 C) DIC ) higher than natural abundance (typically 2-50%). The produced CO 2 (x( 13 C) ≈ 1.11%) gradually dilutes the initial x( 13 C) DIC allowing to quantify microbial mineralization using mass-balance calculations. For 13 C-enriched CO 2 samples, a newly developed isotope ratio mid-infrared spectrometer was introduced with a precision of x( 13 C) < 0.006%. As an example for extremely difficult and slowly degradable compounds, CO 2 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 13 C. Thus, the RIL approach can be applied to sensitively monitor biodegradation of various organic compounds under anoxic or oxic conditions.