Approaches for Measuring Stable Carbon and Nitrogen Isotopes in Bacteria

Stable isotopes have been used successfully over the past three decades to trace geochemical cycles and pathways of elemental transfer through aquatic food chains. To measure stable carbon and nitrogen ratios in situ, nucleic acids are used as a biomarker for bacteria. The method relies on the observation that bacteria grown on organic matter with a known stable carbon isotope value produce particulate biomass with a similar value. While this approach requires further testing to determine the degree of fidelity between respired CO₂, bacteria, and the substrate source, preliminary results suggest substrate sources with largely different can be differentiated. Extraction of nucleic acids from a concentration of diverse microorganisms necessitates a confirmation that the nucleic acids are, in fact, from the targeted microbial assemblage. Purging respired inorganic carbon — samples from the respirometer experiments are prepared for isotopic analysis by a modification of the in vacuo acidification and purging technique described in Grossman.