Microbial Ecology of Subglacial Environments

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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].


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.

Fig. 2 Structure of subglacial environments, within the greater glacial system, is shown. Modified from Boetius et al.[3].

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

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[7][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 (Hamilton et al. 2013; Boetius et al. 2015). Hamilton et al. (2013) 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 (Boetius et al. 2015), the composition of subglacial communities appears to be characterized by relatively high bacterial abundance, specifically Betaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, and Bacteroidetes (Hamilton et al. 2013; Boetius et al. 2015; Christner et al. 2014) (see fig x). Archaea appear present in most subglacial communities as well (Christner et al. 2014; Hamilton et al. 2013; Boetius et al. 2013), albeit with lower relative abundance than bacteria in at least some environments (Hamilton et al., 2013). Eukaryotes, while displaying high diversity when present (Hamilton et al., 2013), are not detectable in all subglacial environments (Christner et al. 2014). 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 (Hamilton et al. 2013; Mitchell et al. 2013; Nixon et al., 2017; Skidmore et al. 2005) Much of this diversity appears to be driven specifically by the pyrite and Fe composition of bedrocks and sediments (Mitchell et al. 2013; Nixon et al., 2017).