Deposition and postdeposition mechanisms as possible drivers of microbial population variability in glacier ice

Abstract

Glaciers accumulate airborne microorganisms year by year and thus are good archives of microbial communities and their relationship to climatic and environmental changes. Hypotheses have focused on two possible drivers of microbial community composition in glacier systems. One is aeolian deposition, in which the microbial load by aerosol, dust, and precipitation events directly determines the amount and composition of microbial species in glacier ice. The other is postdepositional selection, in which the metabolic activity in surface snow causes microbial community shifts in glacier ice. An additional possibility is that both processes occur simultaneously. Aeolian deposition initially establishes a microbial community in the ice, whereas postdeposition selection strengthens the deposition patterns of microorganisms with the development of tolerant species in surface snow, resulting in varying structures of microbial communities with depth. In this minireview, we examine these postulations through an analysis of physical-chemical and biological parameters from the Malan and Vostok ice cores, and the Kuytun 51 Glacial surface and deep snow. We discuss these and other recent results in the context of the hypothesized mechanisms driving microbial community succession in glaciers. We explore our current gaps in knowledge and point out future directions for research on microorganisms in glacial ecosystems.

Abundance, viability and diversity of the indigenous microbial populations at different depths of the NEEM Greenland ice core

“…The conclusive finding of the present study was the existence of abundant, viable and diverse indigenous microbial populations in the NEEM Greenland ice core, at depths ranging from 100 to 2051 m. Our study also provided new evidence confirming the previously found direct correlation between microbial cell abundance in polar and non-polar ice cores and deposition climate, with higher numbers of cells found in ice layers deposited during colder climate periods versus lower cell abundance in ice core layers deposited during warmer climates (Abyzov 1993;Xiang et al 2005;Yao et al 2006;Miteva et al 2009;Xiang et al 2009). Furthermore, detecting viable cells in ice deposited 300Á80 000 years ago and recovering diverse isolates (albeit at a low culturability levels) showed once again that microbial cells are capable of long-term survival trapped in one of the harshest environments on Earth.…”

Abundances and potential activities of nitrogen cycling microbial communities along a chronosequence of a glacier forefield

“…This implies the presence of inactive nitrifying and denitrifying populations, attributed to the shortage of ammonium and nitrate. Obviously, at the initial stage of the glacier forefield, sufficient N and C are provided for mineralization: (i) allochthonous organic material (plant debris, insects and so on) is deposited in the forefield, as revealed by our and other measurements ( (Table 1), (ii) the cryoconite holes in the glacier ablation zone possibly inhabit small foodwebs of cyanobacteria and heterotrophs, which were relocated to the glacier forefield by the glacial stream after snowmelt (Schmalenberger and Noll, 2009;Xiang et al, 2009) and (iii) microbes might feed on ancient recalcitrant C (Hodkinson et al, 2002;Bardgett et al, 2007). In addition, significant inputs of nutrients may be due to atmospheric dry and wet deposition of N and C species (Table 1).…”

The dynamic bacterial communities of a melting High Arctic glacier snowpack

“…Although the potential for depositional and postdepositional processes to influence the structure of these microbial communities is proposed (Xiang et al, 2009b), our understanding of the scale of spatial and temporal variation of snow community composition is limited, in spite of the extensive global areal extent of snow cover. This study therefore offers the first quantitative evidence from 16S rRNA gene-based community structure profiling and amplicon pyrosequencing that different snowpack-associated habitats from a High Arctic glacier harbour distinct and diverse bacterial communities, and that these can be temporally dynamic within the melting season.…”

“…It is therefore likely that the 'surface snow' bacterial community is strongly influenced by organisms introduced by aeolian (or 'dry') deposition. In contrast, snow (and by extension, its decomposition product, slush) constitutes predominantly 'wet', or precipitation-related deposition (Xiang et al, 2009b); all analyses differentiate snow and slush with surface snow in terms of community composition. Glacial ice is likely to integrate microbes introduced by both deposition mechanisms, and thus may conserve dry-deposited microbes.…”

“…In Alpine snowpacks, bacterial abundances range between 10 3 -10 5 cells ml À 1 (Alfreider et al, 1996;Sattler et al, 2001). Bacteria in snowpacks may be inoculated by precipitation and dry deposition (Xiang et al, 2009b), however the relative importance of depositional and postdepositional processes in structuring bacterial communities in different environments is less clear. Seasonal shifts in snowpack bacterial communities have been reported in mountainous regions of Japan (Segawa et al, 2005), as increased snow temperature and meltwater content during seasonal melt promote rapid microbial growth, but also induce washout of cells by elution in meltwater, leading to seasonal shifts in snowpack bacterial communities.…”