Difference between revisions of "Microbial Ecology of Subglacial Environments"
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By: Tim Coston
By: Tim Coston
Latest revision as of 23:14, 23 July 2021
By: Tim Coston
Subglacial environments, including those beneath both ice sheets and outlet glaciers, host diverse microbial communities, despite complete darkness and sub-zero temperatures. These communities, largely composed of bacteria and archaea, appear distinct between some subglacial environments5 and possibly stable at the OTU-level over decade-long timescales. A diverse array of metabolisms appear present, including those reliant on reduced N, S, Fe, and methane for energy4. Multiple previous studies have shown the importance of subglacial microbial communities in global biogeochemical cycling , making their continued characterization important as future warming alters the attributes and prevalence of these environments.
Detailed Environmental Description
Subglacial environments exist at the bed below ice sheets and glaciers. ~10% of land on Earth is covered by glacial ice 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 (Fig. 1), further signifying the expanse of subglacial environments on Earth.
The defining characteristics of subglacial environments include the complete lack of light, largely anoxic conditionsCite error: Invalid
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<ref> tag; invalid names, e.g. too many, and low temperatures (around 0⁰C ). 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
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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.
Despite the extreme conditions of subglacial environments, current research indicates the presence of diverse microbial communities. These communities can consist of bacteria, archaea, and eukarya. The diversity within and between subglacial environments is largely driven by bedrock and sediment mineralogy, which drives chemolithoautotrophy within the system, the main source of primary productivity given the complete lack of sunlight.
This great microbial diversity of subglacial environments appears greater than other cryo-environments, such as supraglacial environments and snow. Hamilton et al. 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, the composition of subglacial communities appears to be characterized by relatively high bacterial abundance, specifically Betaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, and Bacteroidetes (see fig 3). Archaea appear present in most subglacial communities as well, albeit with lower relative abundance than bacteria in at least some environments. Eukaryotes, while displaying high diversity when present, are not detectable in all subglacial environments.
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. Much of this diversity appears to be driven specifically by the pyrite and iron composition of bedrocks and sediments.
Key Microbial Players
As noted above, mineralogical controls (specifically pyrite and iron compositions) help define microbial community composition. 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. 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.
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, emphasizes the important role it likely plays in subglacial microbial communities.
Relationship with Climate - Past and Future
Over the course of Earth’s history, glaciers and ice sheets have played a significant role in climate dynamics, notably through ice-albedo feedbacks (i.e., Perovich et al. 2007), sea-level change (i.e., Chen et al. 2013), and ocean circulation (i.e., Cope and Winguth 2007), among many other impacts. Subglacial microbial communities have likely played a key role as well, by possibly helping sustain life during global glaciations. These cold-adapted communities are uniquely-adapted to survival during global glaciations. As such, subglacial microbial communities were possibly widespread during such cold events. The same is not likely during expected future warming.
As climate change threatens the existence of glaciers, ice sheets, and ice caps, the total area of subglacial environments will undoubtedly decrease. This will drastically alter the structure and dynamics of these microbial communities). Given the importance of subglacial microbial communities in global biogeochemical cycling), this eventual destruction of subglacial microbial communities will likely have global impacts.
A portion of subglacial microbes produce methane (i.e., Stibal et al. 2012), which can eventually be exported to the atmosphere via subglacial drainage, making the magnitude of this subglacial production and export climatically relevant. For the 2015 melt-season alone at a single proglacial river, Lamarche-Gagnon et al. (2019) estimated that ~6.3 tons of methane were transported from the inner portions of the Greenland Ice Sheet. As arctic regions continue to warm, the microbial production and subsequent export of methane may increase.
- Campen et al.: Microbial diversity of an Antarctic subglacial community and high-resolution replicate sampling inform hydrological connectivity in a polar desert. Environmental Microbiology 2019, v. 21, no. 7, p. 2290-2306.
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- Hamilton et al.: Molecular evidence for an active endogenous microbiome beneath glacial ice. The ISMEJ 2013, v.7, p. 1402-1412.
- Skidmore et al.: Comparison of microbial community compositions of two subglacial environments reveals a possible role for microbes in chemical weathering processes. Applied Environmental Microbiology 2005, v. 71, p. 6986-6997.
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- National Snow and Ice Data Center, 2020, Facts about glaciers. Accessed June 3, 2020.
- Randolph Glacier Inventory, 2017, GLIMS Viewer. Accessed June 3, 2020.
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- Nixon et al.: Viable cold-tolerant iron-reducing microorganisms in geographically diverse subglacial environments. Biogeosciences 2017, v. 14, v. 6, p. 1445-1455.
- Harrold et al.: Aerobic and anaerobic thiosulfate oxidation by a cold-adapted, subglacial chemoautotroph. Applied and Environmental Microbiology 2016, v. 82, no. 5, p. 1486-1495.
- Perovich et al.: Increasing solar heating of the Arctic Ocean and adjacent seas, 1979-2005: Attribution and role in the ice-albedo feedback. Geophysical Research Letters 2007, v. 34, no. 19.
- Chen et al.: Contribution of ice sheet and mountain glacier melt to recent sea level rise. Nature Geoscience 2013, v. 6, p. 549-552.
- Cope and Winguth.: On the sensitivity of ocean circulation to arctic freshwater input during the Paleocene/Eocene Thermal Maximum. Palaeogeography, Palaeoclimatology, Palaeoecology 2011, v. 306, no. 1-2, p. 82-94.
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- Stibal et al.: Methanogenic potential of Arctic and Antarctic subglacial environments with contrasting organic carbon sources. Global Change Biology 2012, v. 18, no. 11, p. 3332-3345.
- Lamarche-Gagnon et al.: Greenland melt drives continuous export of methane from the ice-sheet bed. Nature 2019, v. 565, p. 73-77.