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-13-labeled DIC with C-13 atom fractions (x(C-13)(DIC)) higher than natural abundance (typically 2-50%). The produced CO2 (x(C-13) approximate to 1.11%) gradually dilutes the initial x(C-13)(DIC) allowing to quantify microbial mineralization using mass-balance calculations. For C-13-enriched CO2 samples, a newly developed isotope ratio mid-infrared spectrometer was introduced with a precision of x(C-13) < 0.006%. As an example for extremely difficult and slowly degradable compounds, CO2 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-13. Thus, the RIL approach can be applied to sensitively monitor biodegradation of various organic compounds under anoxic or oxic conditions.