Penetration of fluorescent microspheres into the NEEM (North Eemian) Greenland ice core to assess the probability of microbial contamination

Abstract

The validation of microbiological results from non-aseptically drilled deep ice cores is challenging because exogenous microbial cells can be transported into the core interior and compromise the existing microbial populations. The NEEM (North Eemian) ice core in Greenland provided a first-time opportunity to use fluorescent microspheres as tracers for assessing potential microbial contamination of glacial ice. We developed specific procedures to coat the surfaces of selected NEEM core samples representing bubbly (93–100 m), brittle (633–644 m) and clathrated (1,730 and 2,050 m) ice with melamine-based carboxylated fluorescent microspheres and tracked periodically their penetration into the core interior for 2.5 years using flow cytometry. Sufficient ice surface coating was achieved by immersing retrieved cores in plastic bags containing suspensions of pre-counted 1- and 10-μm microspheres or by down-hole microsphere deployment in plastic sleeves attached to the drill barrel and liberated during drilling. We examined the relationship between microspheres penetration and ice core depth, structure and time after coating. One consistent observation for all cores (except the brittle ice) was that removing a few millimeters of the outer layer drastically reduced microsphere counts, independent of timing, indicating that penetration was mostly limited to the surface layers. Any deeper penetration was found to be microsphere size dependent. The brittle ice showed significant microsphere penetration possibly due to microfractures. Overall, the use of fluorescent microspheres as tracers and microbial surrogates proved to be a sensitive approach for testing potential contamination during deep core projects.