Title: Measurements of Bacterial Concentrations on a Millimeter Scale in Ice Cores With a Scanning Laser Fluorescence Spectrometer
Author: Price, P., Rohde, R.A., Bramall, N.E. and Bay, R.C.
Periodical: American Geophysical Union, Fall Meeting 2006, abstract #U43B-0868
Abstract: We report non-destructive detection of variability on a mm depth scale in the organic content of ice cores at NICL, as determined by the fluorescence spectrum measured by a Targeted Ultraviolet Chemical Sensor (TUCS). Many of the spectra we obtained are consistent with the amino acid tryptophan, a strongly fluorescing constituent in microbes. Identification with native fluorescence of microbes is supported by previous measurements of varying microbial concentration in samples from selected regions of the GISP2 core (Tung et al., 2005; 2006) that are consistent with our observations at the same depths. Sub-mm depth resolution was achieved and structure at this scale was observed. At each depth the fluorescence emission spectrum was measured at 5 wavelengths using 20-nm narrow band filters plus a long pass channel. The spectrum of microbes was calibrated by making lab measurements of fluorescence of various species and is distinguishable from mineral dust and metals due to differences in spectral shape. In bulk ice samples from 3 depths in the GISP2 core, where a table of methane concentrations (Ed Brook, unpublished) had shown several excesses above the atmospheric contribution, Tung et al. (2005) found 10-fold excesses of microbial concentrations at 2954 m and 3036 m and a 3-fold excess at 3018 m. In the present work we found strong, rapidly varying organic signals at all three depths. At 3018 m the peak value was much stronger than that obtained by Brook and occurred in the core section below the one he studied. Since he measured methane at several-meter depth intervals, and since we found the microbial excesses to be concentrated in 0.3 m intervals, we conclude that of order 30 microbe-rich regions may be present in GISP2. The 3 microbe-rich depths found by Tung et al. (2005) were less than 90 m above the basal ice at 3041-3053 m. The large fluctuations in apparent tryptophan concentrations that we found at 2954, 3018, and 3036 m are consistent with microbe-rich wetland material from the base of GISP2 being repeatedly entrained into the ice. In the basal ice, which contains very high concentrations of both silt and microbes (Tung et al., 2006), the tryptophan signal seems to have been obscured by mineral grains. In many sections of ice cores we found very high spikes of tryptophan-like fluorescence in sub-mm depth regions. Our interpretation, which we will follow up in future work, is that the TUCS laser-excitation beam occasionally intersected veins at triple junctions in the ice, where organic material had been concentrated via rejection from the growing ice grains. Micron-size microbes in veins are known to metabolize at a very low rate via redox reactions on impurities (Price, 2000). An additional virtue of the TUCS technique is its ability to serve as a very sensitive probe of drill fluid contamination. We identified residual drill fluid on interior surfaces of a number of Vostok core sections, with the amount of contamination varying by orders of magnitude from sample to sample. In support of the WAIS Divide drilling project we also measured the rate of evaporation of drill fluid at -24 C after it was deliberately coated onto ice samples for our inspection at NICL. Though a valuable probe of drill fluid contamination, even low levels of drill fluid can prevent the TUCS from registering an accurate reading of native fluorescence. The success of this initial survey of ice cores indicates that UV fluorimetry can be a powerful new non-destructive tool with numerous applications to microbiology and climatology. This research was supported in part by NSF grant ANT-0440609.
Year: 2006