Microbial Ecology of Subglacial Environments: Difference between revisions

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==Key Microbial Players==
==Key Microbial Players==
As noted above, mineralogical controls (specifically pyrite and iron compositions) help define microbial community composition (Mitchell et al. 2013). Given this, key microbes in subglacial environments are those involved in the breakdown and reduction of iron and sulfur. The below list of key players is limited and non-exhaustive given the limited amount of available data from subglacial environments.
Desulfosporosinus and Geobacter both appear to play significant roles in the reduction of iron in subglacial environments (Nixon et al. 2017). Desulfosporosinus is a genus within the Firmicutes phylum, while Geobacter is a genus within the Deltaproteobacteria. While seemingly not abundant together at the same locations, Nixon et al. (2017) successfully cultured them from varying glacial settings in Greenland, Svalbard, Norway, and Antarctica, suggesting their widespread abundance and importance, and other iron reducers, in subglacial environments. Desulfosporosinus may also play an important role in sulfate-reduction in subglacial environments (Nixon et al. 2017).
Thiobacillus spp. is  believed to play a key role in the oxidation of pyrite in subglacial environments, and was cultured and isolated from Robertson Glacier, Canada, by Harrold et al. (2016). Thiobacillus is a genus within the Betaproteobacteria. Thiobacillus spp. oxidizes thiosulfate, an intermediate in the complete oxidation of pyrite. The key role of Thiobacillus spp. in complete pyrite oxidation, in addition to the presence of Thiobacillus spp. in a variety of other subglacial environments (Christner et al. 2014; Skidmore et al. 2005), emphasize the important role it likely plays in subglacial microbial communities (Harrold et al. 2016).

Revision as of 22:34, 12 June 2020

Detailed Environmental Description

Subglacial environments exist at the bed below ice sheets and glaciers. ~10% of land on Earth is covered by glacial ice[1] making subglacial environments a vast and important environment worthy of study. Glacial ice, while often associated with Earth’s poles, are also found well outside of polar regions[1] (Fig. 1), further signifying the expanse of subglacial environments on Earth.

Fig. 1 Location of glaciers within the Randolph Glacier Inventory are shown by teal dots. Note: This map only includes glaciers and not ice sheets or caps. Modified from RGI Consortium[2].
Fig. 2 Structure of subglacial environments, within the greater glacial system, is shown. Modified from Boetius et al. (2015)[3].


The defining characteristics of subglacial environments include the complete lack of light[3], largely anoxic conditionsCite error: Invalid <ref> tag; invalid names, e.g. too manyCite error: Invalid <ref> tag; invalid names, e.g. too many, and low temperatures (around 0⁰C )[4]. Despite these common characteristics, subglacial environments are diverse in their environmental attributes. This partly derives from the diversity of Earth’s cryosphere. As suggested by the name, subglacial environments are found beneath glaciers, both alpine and outlet, but also below Earth’s massive ice sheets – the Antarctic and Greenlandic. These differing environments, while seemingly similar, are quite distinct and require their own fields of study. Glaciers only need be tens of meters thick, while ice sheets are kilometers thick.

Although defined by the presence of solid water (ice), many subglacial environments also contain liquid water – a required component for all life, including microbesCite error: Invalid <ref> tag; invalid names, e.g. too many. Below the ice of warm and polythermal glaciers high pressures result in liquid waterCite error: Invalid <ref> tag; invalid names, e.g. too many (see Fig 2.). The amount and distribution of this water can vary from saturated sediments, to localized channels, to subglacial lakesCite error: Invalid <ref> tag; invalid names, e.g. too many.


In addition to liquid water, the chemical components of subglacial minerals are required for microbial life. Due to the complete lack of light mentioned above, microbial communities rely on the presence of chemical energy within minerals at the ice-sediment interface. The flow of glaciers grinds these minerals, making them more available to the present microbial communitiesCite error: Invalid <ref> tag; invalid names, e.g. too many. As a result, the minerology below glaciers and ice sheets is an important control on nutrient availability and thus community composition[5].

Microbial Diversity

Fig. 3 Relative abundance of bacteria from various subglacial environments, and their conglomerate. Note, East Skafta Lake, Iceland is greatly influenced by its unique geothermal setting. Most abundant bacteria include Betaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, and Bacteroidetes. Figure modified from Boetius et al. (20150[3]. Data from Roberson Glacier, Canada is from Hamilton et al. (2013)[4]. Data from Lake Whillans, Antarctica is from Christner et al. (2014)[6]. Data from East Skafta Lake, Iceland is from Marteinsson et al. (2013)[7]

Despite the extreme conditions of subglacial environments, current research indicates the presence of diverse microbial communities[6][4][3]. These communities can consist of bacteria, archaea, and eukarya[4]. The diversity within and between subglacial environments is largely driven by bedrock and sediment mineralogy[8][5], which drives chemolithoautotrophy within the system, the main source of primary productivity given the complete lack of sunlight[3].


This great microbial diversity of subglacial environments appears greater than other cryo-environments, such as supraglacial environments and snow[4][3]. Hamilton et al.[4] suggested this greater level of diversity is driven by limited nutrient availability, requiring metabolic specificity, and resulting in “minimal niche overlap.” This minimal overlap allows for the proliferation of a variety of metabolically specific microbes.

While data is limited[3], the composition of subglacial communities appears to be characterized by relatively high bacterial abundance, specifically Betaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, and Bacteroidetes[4][3][6] (see fig 3). Archaea appear present in most subglacial communities as well[6][4][3], albeit with lower relative abundance than bacteria in at least some environments[4]. Eukaryotes, while displaying high diversity when present[4], are not detectable in all subglacial environments[6].

Since bedrock and sediment mineralogy drive chemolithoautotrophy and consequently microbial diversity within subglacial environments, the beta diversity of subglacial environments is partially reflective of mineralogical differences[4][8][4][9][5]. Much of this diversity appears to be driven specifically by the pyrite and iron composition of bedrocks and sediments[8][9].

Key Microbial Players

As noted above, mineralogical controls (specifically pyrite and iron compositions) help define microbial community composition (Mitchell et al. 2013). Given this, key microbes in subglacial environments are those involved in the breakdown and reduction of iron and sulfur. The below list of key players is limited and non-exhaustive given the limited amount of available data from subglacial environments.

Desulfosporosinus and Geobacter both appear to play significant roles in the reduction of iron in subglacial environments (Nixon et al. 2017). Desulfosporosinus is a genus within the Firmicutes phylum, while Geobacter is a genus within the Deltaproteobacteria. While seemingly not abundant together at the same locations, Nixon et al. (2017) successfully cultured them from varying glacial settings in Greenland, Svalbard, Norway, and Antarctica, suggesting their widespread abundance and importance, and other iron reducers, in subglacial environments. Desulfosporosinus may also play an important role in sulfate-reduction in subglacial environments (Nixon et al. 2017).

Thiobacillus spp. is believed to play a key role in the oxidation of pyrite in subglacial environments, and was cultured and isolated from Robertson Glacier, Canada, by Harrold et al. (2016). Thiobacillus is a genus within the Betaproteobacteria. Thiobacillus spp. oxidizes thiosulfate, an intermediate in the complete oxidation of pyrite. The key role of Thiobacillus spp. in complete pyrite oxidation, in addition to the presence of Thiobacillus spp. in a variety of other subglacial environments (Christner et al. 2014; Skidmore et al. 2005), emphasize the important role it likely plays in subglacial microbial communities (Harrold et al. 2016).