Fluorescence Spectroscopy as a Rapid, High-Resolution Tool for Detecting Biomolecules in Glacial Ice

We have developed new instruments utilizing the intrinsic fluorescence of specific biomolecules as a sensitive, non-destructive tool for detecting microorganisms. Using a 224-nm excitation, we detect protein-bound tryptophan (an amino acid present in all cells) at a detection threshold of approximately 1 cell per laser excitation volume and a duty cycle of 100 ms per measurement. Tryptophan is easily distinguished from inorganic backgrounds due to its characteristic spectral shape and ~300 times higher intensity per unit volume than typical inorganic compounds. A different excitation was also used to detect coenzyme F420, a characteristic marker for viable methanogenic cells. At the National Ice Core Laboratory, systematic scans of a 1 meter core sections took about 15 minutes and generated ~5000 measurements per meter. The high-resolution of this work revealed strong variability of microbial content on a scale of cm within individual cores, which suggests that microbial deposition at polar sites is strongly influenced by meteorological events (e.g. storms) on subannual and interannual scales. In addition, high levels of microbes are found to correlate with anomalously high concentrations of metabolic gases (e.g. methane, nitrous oxide, and 18O/16O of O2), suggesting that many of the isolated "gas artifacts" identified in deep ice cores are the accumulated waste products of in situ metabolism. This means that fluorescence spectroscopy may be a useful tool for identifying regions where high microbial concentrations have contaminated gas records. The existing instrumentation is suitcase portable and could be easily deployed in a variety of environments. Future versions of these instruments may be practical for continuous, rapid scans of entire cores, as an on-site deployable technique for characterizing microbial abundances in ice, and for searching for as few as 1 microbe per cm3 in ice-bound planets. This work was supported by NSF grant ANT-0440609.