Active microbial ecosystem in glacier basal ice fuelled by iron and silicate comminution-derived hydrogen

Abstract

The basal zone of glaciers is characterized by physicochemical properties that are distinct from firnified ice due to strong interactions with underlying substrate and bedrock. Basal ice (BI) ecology and the roles that the microbiota play in biogeochemical cycling, weathering, and proglacial soil formation remain poorly described. We report on basal ice geochemistry, bacterial diversity (16S rRNA gene phylogeny), and inferred ecological roles at three temperate Icelandic glaciers. We sampled three physically distinct basal ice facies (stratified, dispersed, and debris bands) and found facies dependent on biological similarities and differences; basal ice character is therefore an important sampling consideration in future studies. Based on a high abundance of silicates and Fe-containing minerals and, compared to earlier BI literature, total C was detected that could sustain the basal ice ecosystem. It was hypothesized that C-fixing chemolithotrophic bacteria, especially Fe-oxidisers and hydrogenotrophs, mutualistically support associated heterotrophic communities. Basal ice-derived rRNA gene sequences corresponding to genera known to harbor hydrogenotrophic methanogens suggest that silicate comminution-derived hydrogen can also be utilized for methanogenesis. PICRUSt-predicted metabolism suggests that methane metabolism and C-fixation pathways could be highly relevant in BI, indicating the importance of these metabolic routes. The nutrients and microbial communities release from melting basal ice may play an important role in promoting pioneering communities establishment and soil development in deglaciating forelands.

Keywords: cryosphere; environmental microbiology; extremophiles; glaciers; microbial ecology.

FIGURE 1

Examples of basal ice (BI) facies collected from Svínafellsjökull: a) stratified facies (S), rich in fine (clay/silt) sediment, with sediment arranged in angular aggregates; at a centimeter to decimetre scale, stratified facies appears layered, as the name suggests; b), dispersed facies (D) comprising dispersed aggregates of polymodal sediment; c), debris band (B), composed of sub‐vertically layered alternations between clear, bubble‐free ice and polymodal sediment—note also the white and bubble‐rich englacial ice to the right.

FIGURE 2

Map of the sampling points. Three glaciers were analyzed in this study: Svínafellsjökull (A), Kvíárjökull (B), and Skaftafellsjökull (C). Different shapes indicate different ice facies sampled.

FIGURE 3

Box plot showing the concentrations (in µg g⁻¹) of the different elements analyzed from sediment entrained in basal ice (BI). C and N were analyzed by Leco TruSpec, whereas Fe and S were analyzed by ICP‐OES. Lines represent the median and the upper and lower limit of the box represent the 75 and 25 percentiles, respectively. Dots represent the outliers.

FIGURE 4

Mineralogical analysis of the sediment entrapped in basal ice (BI) using automated Single Particle Analysis (SPA) by Scanning Electron Microscopy coupled with Energy Dispersive X‐ray (SEM‐EDX). Particles were classified using a combination of the cluster chemical boundaries defined by Kandler et al., (2011) and (Anaf et al., 2012).

FIGURE 5

Relative abundance of 16S rRNA gene sequences classified to phylum level in the three basal ice (BI) types over the 2015 and 2016 sampling campaigns at Svínafellsjökull. Based on 16S rRNA gene sequence‐derived OTU table analysis in Parallel Meta 3.4.1. Phyla below 1% abundance were reported with genera below this threshold and designated as a single group termed “Others.”

FIGURE 6

Bacterial (16S rRNA gene) network analysis and inferred metabolic groupings associated with sediment entrained within BI facies. The size of the circles is proportional to the mean abundance of the bacterial taxonomic group. Color‐coding highlights inferred functional categories based on the literature.

FIGURE 7

Potential functionality predicted by PICRUSt. A) General functions, B) Close‐up to potential energy metabolic pathways, representing ~6% of the total of the KOs predicted.