Microbial abundance in surface ice on the Greenland Ice Sheet

Abstract

Measuring microbial abundance in glacier ice and identifying its controls is essential for a better understanding and quantification of biogeochemical processes in glacial ecosystems. However, cell enumeration of glacier ice samples is challenging due to typically low cell numbers and the presence of interfering mineral particles. We quantified for the first time the abundance of microbial cells in surface ice from geographically distinct sites on the Greenland Ice Sheet (GrIS), using three enumeration methods: epifluorescence microscopy (EFM), flow cytometry (FCM), and quantitative polymerase chain reaction (qPCR). In addition, we reviewed published data on microbial abundance in glacier ice and tested the three methods on artificial ice samples of realistic cell (10(2)-10(7) cells ml(-1)) and mineral particle (0.1-100 mg ml(-1)) concentrations, simulating a range of glacial ice types, from clean subsurface ice to surface ice to sediment-laden basal ice. We then used multivariate statistical analysis to identify factors responsible for the variation in microbial abundance on the ice sheet. EFM gave the most accurate and reproducible results of the tested methodologies, and was therefore selected as the most suitable technique for cell enumeration of ice containing dust. Cell numbers in surface ice samples, determined by EFM, ranged from ~ 2 × 10(3) to ~ 2 × 10(6) cells ml(-1) while dust concentrations ranged from 0.01 to 2 mg ml(-1). The lowest abundances were found in ice sampled from the accumulation area of the ice sheet and in samples affected by fresh snow; these samples may be considered as a reference point of the cell abundance of precipitants that are deposited on the ice sheet surface. Dust content was the most significant variable to explain the variation in the abundance data, which suggests a direct association between deposited dust particles and cells and/or by their provision of limited nutrients to microbial communities on the GrIS.

Keywords: Greenland Ice Sheet; epifluorescence microscopy; flow cytometry; glacier ice; microbial abundance; multivariate analysis; quantitative PCR.

Figure 1

Map of sampling sites on the GrIS. Red dots represent sites in the ablation area and blue dots in the accumulation area of the ice sheet. The background clear sky end of melt season mosaic is by J. E. Box using MODIS data from year 2011.

Figure 2

Microbial cell abundances in surface ice samples from the GrIS determined by EFM plotted against the respective dust concentrations in the samples. Note the logarithmic scales on both axes.

Figure 3

Microbial cell abundances and dust concentrations in glacier ice samples. Blue oval represents samples collected in this study and measured by EFM (see Figure 2); the remainder of the data was compiled from the literature (see Table 4). Black frame represents the ranges used for method testing in this study. Note that due to logarithmic scales on both axes zeroes cannot be shown.

Figure 4

Cell abundance measured by EFM in the Saddle ice core. The five sections used for enumeration are depicted by black frames. Values are means ± st.devs. of three measurements (2013 winter snow top layer, 2012 melt layer) or two measurements (remaining samples).

Figure 5

Redundancy analysis biplot visualizing the effects of environmental variables on the microbial abundance in surface ice on the GrIS. Red arrows denote significant quantitative physical variables, red triangles the surface type, and black arrows the abundances determined by EFM and qPCR.