In environmental sciences, compound specific stable isotope analysis (CSIA) is applied for source identification, detection and quantification of in situ biodegradation and identification of reaction mechanisms or other processes related to isotope fractionation of organic contaminants.
Nowadays online-coupling of chromatographic systems to isotope ratio mass spectrometers (GC-IRMS, LC-IRMS) is well established. This allows analyzing the compound-specific isotope composition of organic analytes with regard to carbon (13C/12C), hydrogen (2H/1H) and nitrogen (15N/14N). Recently developed approaches also enable measuring chlorine isotope ratios (37Cl/35Cl), - a breakthrough for evaluating degradation processes of chlorinated ethenes. The availability of δ37Cl- CSIA will enable us to determine chlorine isotope fractionation factors for microbial dehalogenation processes; where the database is still lacking. Most importantly it will open the enormous prospects of 2-dimensional isotope analysis for process identification for halogenated organic compounds. To evaluate a compound´s transformation processes the kinetic isotope effect(s) (KIE) accompanying (bio)chemical reactions can be used. In general, organic contaminants consist of a mixture of “heavy” and “light” molecules (isotopologues). During the breakdown, typically molecules exhibiting a heavy isotope (e.g. 13C) in the reactive position are discriminated from those exhibiting a light isotope, as a higher activation energy is needed to break the bonds with the heavy isotope. This leads to gradual enrichment of the lighter isotopologues in the product, while the heavier ones remain in the substrate.
Using cultivation-based approaches we investigate pure and mixed cultures of organohalide respiring organisms that utilizes organohalides (e.g. tetrachlorethene) as the terminal electron acceptor to gain energy. In collaboration with the Geomicrobiology - Microbial Ecology group, we use molecular techniques like qPCR, cDNA synthesis or Western blot analysis to investigate the response of the cells to defined growth conditions. These techniques enable us to monitor e.g. total cell numbers, the abundance and transcriptions of specific genes as well as the detection of processed enzymes like reductive dehalogenases (the key enzmyes in organohalide respiration where the carbon-chlorine bond is cleaved).
Combining molecular techniques with 2D isotope analysis will improve our understanding of microbe driven contaminant degradation and therefore the approach of engineered bioremediation using microorganisms to clean up contaminated sites.