Stable carbon isotopes as tracers of microbial degradation

In an effort to better understand relationships between environmental carbon reservoirs, while eliminating the need for complicated carbon mass balancing for a system, the usefulness of stable carbon isotope ratio analysis of carbon sources, bacterial components, and respiration gases was investigated. To verify the applicability of stable carbon isotope analyses in identifying source carbon during various stages of microbial degradation, several analytical methods were developed and/or tested in both laboratory and field experiments. First, in order to better measure the stable carbon isotope ratio (δ 13C) of respired microbial CO₂ in both groundwater and marine waters, a new, more rapid method for the analysis of dissolved inorganic carbon (DIC) was developed. The precision and accuracy of this method was shown to be comparable to existing methods, while sample throughput was increased dramatically by utilizing the automation afforded by a standard gas chromatographic autosampler. This method was then applied as part of a study which tested the efficacy of a new bioremediation technology at a creosote contaminated site in Gainesville, FL.; To isotopically identify carbon substrates utilized by bacteria, several laboratory cultures were grown using a single strain of bacteria (Sphingomonas paucimobilis) and a variety of simple and complex carbon compounds both exclusively and in competition experiments. The δ 13C of the individual bacterial phospholipid fatty acids (PFAs) were then analyzed after extraction, confirming a correlation between the δ 13C of the original substrate and the δ13C of palmitic acid (a 16 carbon, saturated straight-chain PFA; 16:0). Further, it was also shown that the isotopic fractionation between substrate and PFA 16:0 δ13C was related to the type of substrate being degraded, with larger fractionations associated with the degradation of single carbon substrates (i.e. methane). This method was then used to determine the carbon isotopic fractionations between bulk sediment and PFA 16:0 δ 13C in samples collected from marine, estuarine, and terrestrial sites. The analytical results for the environmental samples verified the laboratory data in field samples, supporting the assertion that the isotopic fractionation is directly related to the type of carbon substrate utilized by the bacteria.