Microbial Ecology of Subglacial Environments: Difference between revisions

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By: Tim Coston
==Overview==
Subglacial environments, including those beneath both ice sheets and outlet glaciers, host diverse microbial communities<ref name = "Campen et al. 2019"> [https://sfamjournals.onlinelibrary.wiley.com/doi/full/10.1111/1462-2920.14607?casa_token=wtNc5bx2xl0AAAAA%3AbEfwA3aw2Mx8swLXjNugiwned-CNhFyQZwSeaE7sCd8RBReUBCaS6BjNpIVh8YiabpcgIJp-Se_vifBC 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.]</ref><ref name= "Christner et al. 2014"/><ref name= "Hamilton et al. 2013"/><ref name= "Skidmore et al. 2005"/>, despite complete darkness and sub-zero temperatures<ref name= "Christner et al. 2014"/>. These communities, largely composed of bacteria and archaea<ref name= "Campen et al. 2019"/><ref name= "Christner et al. 2014"/><ref name= "Skidmore et al. 2005"/>, appear distinct between some subglacial environments5 and possibly stable at the OTU-level over decade-long timescales<ref name= "Campen et al. 2019"/>. A diverse array of metabolisms appear present<ref name= "Christner et al. 2014"/><ref name = "Mikucki et al. 2016"> [https://royalsocietypublishing.org/doi/full/10.1098/rsta.2014.0290 Mikucki et al.: Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge. Philosophical Transactions of the Royal Society A 2016, v. 374, p. 2-22.]</ref>, including those reliant on reduced N, S, Fe, and methane for energy<ref name= "Mikucki et al. 2016"/>4. Multiple previous studies have shown the importance of subglacial microbial communities in global biogeochemical cycling <ref name= "Hamilton et al. 2013"/><ref name= "Mikucki et al. 2016"/>, making their continued characterization important as future warming alters the attributes and prevalence of these environments.
==Detailed Environmental Description==
==Detailed Environmental Description==


Subglacial environments exist at the bed below ice sheets and glaciers. ~10% of land on Earth is covered by glacial ice<ref name = NSIDC>[https://nsidc.org/cryosphere/glaciers/quickfacts.html National Snow and Ice Data Center, 2020, Facts about glaciers. Accessed June 3, 2020.]</ref> 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<ref name= NSIDC/> (Fig. 1), further signifying the expanse of subglacial environments on Earth.<br>
Subglacial environments exist at the bed below ice sheets and glaciers. ~10% of land on Earth is covered by glacial ice<ref name = NSIDC>[https://nsidc.org/cryosphere/glaciers/quickfacts.html National Snow and Ice Data Center, 2020, Facts about glaciers. Accessed June 3, 2020.]</ref> 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<ref name= NSIDC/> (Fig. 1), further signifying the expanse of subglacial environments on Earth.<br>
[[Image:RGI2.png|thumb|300px|top|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<ref name = RGI>[http://www.glims.org/maps/glims Randolph Glacier Inventory, 2017, GLIMS Viewer. Accessed June 3, 2020.]</ref>.]][[Image:Boeitus_et_al._2015_Diagram.png|thumb|300px|left|Fig. 2 Structure of subglacial environments, within the greater glacial system, is shown. Modified from Boetius et al.<ref name= "Boetius et al. 2015"/>.]]
[[Image:RGI2.png|thumb|300px|left|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 [https://www.glims.org/maps/glims/ RGI Consortium]<ref name = RGI>[http://www.glims.org/maps/glims Randolph Glacier Inventory, 2017, GLIMS Viewer. Accessed June 3, 2020.]</ref>.]][[Image:Boeitus_et_al._2015_Diagram.png|thumb|300px|right|Fig. 2 Structure of subglacial environments, within the greater glacial system, is shown. Modified from [https://www.nature.com/articles/nrmicro3522/ Boetius et al. (2015)]<ref name= "Boetius et al. 2015"/>.]]




The defining characteristics of subglacial environments include the complete lack of light<ref name = "Boetius et al. 2015"> [https://www.nature.com/articles/nrmicro3522 Boetius et al.: Microbial ecology of the cryosphere:sea ice and glacial habitats. Nature Reviews 2015, v. 13, p. 677-690.]</ref>, largely anoxic conditions<ref name = Wadham et al. 2004>[https://www.sciencedirect.com/science/article/pii/S0012821X03006836?casa_token=WfsXOx4oSqQAAAAA:WHqhHaedE1sq3ip6lTWcxdI-u-qBva_FgVPAXiX_U3ajjmgxg104iJ4g9gaCjmUS9xxnW2A1Fm4 Wadhma et al.: Stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high Arctic glacier. Earth and Planetary Science Letters 2004, v. 219, issues 3-4, p. 341-355.]</ref><ref name = Wynn et al. 2005>[https://link.springer.com/article/10.1007/s10533-005-3832-0 Wynn et al.: Chemical and isotopic switching within the subglacial environment of a High Arctic glacier. Biogeochemistry 2006, v. 78, p. 173-193.]</ref>, and low temperatures (around 0⁰C )<ref name = "Hamilton et al. 2013">[https://www.nature.com/articles/ismej201331 Hamilton et al.: Molecular evidence for an active endogenous microbiome beneath glacial ice. The ISMEJ 2013, v.7, p. 1402-1412.]</ref>. 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.
The defining characteristics of subglacial environments include the complete lack of light<ref name = "Boetius et al. 2015"> [https://www.nature.com/articles/nrmicro3522 Boetius et al.: Microbial ecology of the cryosphere:sea ice and glacial habitats. Nature Reviews 2015, v. 13, p. 677-690.]</ref>, largely anoxic conditions<ref name = Wadham et al. 2004>[https://www.sciencedirect.com/science/article/pii/S0012821X03006836?casa_token=WfsXOx4oSqQAAAAA:WHqhHaedE1sq3ip6lTWcxdI-u-qBva_FgVPAXiX_U3ajjmgxg104iJ4g9gaCjmUS9xxnW2A1Fm4 Wadham et al.: Stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high Arctic glacier. Earth and Planetary Science Letters 2004, v. 219, issues 3-4, p. 341-355.]</ref><ref name = Wynn et al. 2005>[https://link.springer.com/article/10.1007/s10533-005-3832-0 Wynn et al.: Chemical and isotopic switching within the subglacial environment of a High Arctic glacier. Biogeochemistry 2006, v. 78, p. 173-193.]</ref>, and low temperatures (around 0⁰C )<ref name = "Hamilton et al. 2013">[https://www.nature.com/articles/ismej201331 Hamilton et al.: Molecular evidence for an active endogenous microbiome beneath glacial ice. The ISMEJ 2013, v.7, p. 1402-1412.]</ref>. 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 microbes<ref name = Christner et al. 2008>[https://link.springer.com/chapter/10.1007/978-3-540-74335-4_4 Christner et al.: Bacteria in subglacial environments in R Margesin et al., eds., Pyschorphiles: from Biodiversity to Biotechnology 2008: Berlin, Springer, p. 51-71.]</ref>. Below the ice of warm and polythermal glaciers high pressures result in liquid water<ref name = Alley et al. 1997>[https://www.sciencedirect.com/science/article/pii/S0277379197000346?casa_token=w-FL-mXo9k0AAAAA:mxTm9WscBZrAVn4q5Rlauyf8ldg0wvSjS57QhrfsOZSUqzQ6xucWdmiUQT8SLwUa-q-i4MnYwMY Alley et al.: How glaciers entrain and transport basal sediment: physical constraints. Quaternary Science Reviews 1997, v. 16, p. 1017-1038.]</ref> (see Fig 2.). The amount and distribution of this water can vary from saturated sediments, to localized channels, to subglacial lakes<ref name = Priscu et al. 2008>[https://books.google.com/books?hl=en&lr=&id=ppUSDAAAQBAJ&oi=fnd&pg=PA119&dq=priscu+2008+antarctic+subglacial+water&ots=jnhL3plxwD&sig=BUPt9sRbfKz1niGDCursIrZSqCc#v=onepage&q=priscu%202008%20antarctic%20subglacial%20water&f=false Priscu et al.: Antarctic subglacial water: origin, evolution, and ecology in Vincent, W.F.., and Laybourn-Parry, J., eds., Polar Lakes and Rivers: Limnology of Arctic and Antarctic Aquatic Ecosystems 2008: Oxford, Oxford University Press.]</ref>.
Although defined by the presence of solid water (ice), many subglacial environments also contain liquid water – a required component for all life, including microbes<ref name = Christner et al. 2008>[https://link.springer.com/chapter/10.1007/978-3-540-74335-4_4 Christner et al.: Bacteria in subglacial environments in R Margesin et al., eds., Pyschorphiles: from Biodiversity to Biotechnology 2008: Berlin, Springer, p. 51-71.]</ref>. Below the ice of warm and polythermal glaciers high pressures result in liquid water<ref name = Alley et al. 1997>[https://www.sciencedirect.com/science/article/pii/S0277379197000346?casa_token=w-FL-mXo9k0AAAAA:mxTm9WscBZrAVn4q5Rlauyf8ldg0wvSjS57QhrfsOZSUqzQ6xucWdmiUQT8SLwUa-q-i4MnYwMY Alley et al.: How glaciers entrain and transport basal sediment: physical constraints. Quaternary Science Reviews 1997, v. 16, p. 1017-1038.]</ref> (see Fig 2.). The amount and distribution of this water can vary from saturated sediments, to localized channels, to subglacial lakes<ref name = Priscu et al. 2008>[https://books.google.com/books?hl=en&lr=&id=ppUSDAAAQBAJ&oi=fnd&pg=PA119&dq=priscu+2008+antarctic+subglacial+water&ots=jnhL3plxwD&sig=BUPt9sRbfKz1niGDCursIrZSqCc#v=onepage&q=priscu%202008%20antarctic%20subglacial%20water&f=false Priscu et al.: Antarctic subglacial water: origin, evolution, and ecology in Vincent, W.F.., and Laybourn-Parry, J., eds., Polar Lakes and Rivers: Limnology of Arctic and Antarctic Aquatic Ecosystems 2008: Oxford, Oxford University Press.]</ref>.
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==Microbial Diversity==
==Microbial Diversity==
[[Image:boetius2.png|thumb|550px|right|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 <i>Betaproteobacteria</I>, <i>Deltaproteobacteria</I>, <i>Gammaproteobacteria</I>, and <i>Bacteroidetes</I>. Figure modified from Boetius et al.<ref name= "Boetius et al. 2015"/>. Data from Roberson Glacier, Canada is from Hamilton et al. (2013)<ref name= "Hamilton et al. 2013"/>. Data from Lake Whillans, Antarctica is from Christner et al. (2014)<ref name= "Christner et al. 2014"/>. Data from East Skafta Lake, Iceland is from Marteinsson et al. (2013)<ref name = "Marteinsson et al. 2013"> [https://www.nature.com/articles/ismej201297 Marteinsson et al.: Microbial communities in the subglacial waters of the Vatnajokull ice cap, Iceland. The ISME Journal 2013, v. 7, p. 427-437.]</ref> ]]
[[Image:boetius2.png|thumb|550px|right|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 [https://www.nature.com/articles/nrmicro3522/ Boetius et al. (20150]<ref name= "Boetius et al. 2015"/>. Data from Roberson Glacier, Canada is from Hamilton et al. (2013)<ref name= "Hamilton et al. 2013"/>. Data from Lake Whillans, Antarctica is from Christner et al. (2014)<ref name= "Christner et al. 2014"/>. Data from East Skafta Lake, Iceland is from Marteinsson et al. (2013)<ref name = "Marteinsson et al. 2013"> [https://www.nature.com/articles/ismej201297 Marteinsson et al.: Microbial communities in the subglacial waters of the Vatnajokull ice cap, Iceland. The ISME Journal 2013, v. 7, p. 427-437.]</ref> ]]
Despite the extreme conditions of subglacial environments, current research indicates the presence of diverse microbial communities<ref name = "Christner et al. 2014"> [https://www.nature.com/articles/nature13667 Christner et al.: A microbial ecosystem beneath the West Antarctic ice sheet. Nature 2014, v. 512, p. 310-313.]</ref><ref name= "Hamilton et al. 2013"/><ref name= "Boetius et al. 2015"/>. These communities can consist of bacteria, archaea, and eukarya<ref name= "Hamilton et al. 2013"/>. The diversity within and between subglacial environments is largely driven by bedrock and sediment mineralogy<ref name = "Mitchell et al. 2013"> [https://www.nature.com/articles/nature13667 Mitchell et al.: Influence of bedrock mineral composition on microbial diversity in a subglacial environment. Geology 2013, v. 41, no. 8, p. 855-858.]</ref><ref name= "Skidmore et al. 2005"/>, which drives chemolithoautotrophy within the system, the main source of primary productivity given the complete lack of sunlight<ref name= "Boetius et al. 2015"/>.
Despite the extreme conditions of subglacial environments, current research indicates the presence of diverse microbial communities<ref name = "Christner et al. 2014"> [https://www.nature.com/articles/nature13667 Christner et al.: A microbial ecosystem beneath the West Antarctic ice sheet. Nature 2014, v. 512, p. 310-313.]</ref><ref name= "Hamilton et al. 2013"/><ref name= "Boetius et al. 2015"/>. These communities can consist of bacteria, archaea, and eukarya<ref name= "Hamilton et al. 2013"/>. The diversity within and between subglacial environments is largely driven by bedrock and sediment mineralogy<ref name = "Mitchell et al. 2013"> [https://pubs.geoscienceworld.org/gsa/geology/article/41/8/855/131299/Influence-of-bedrock-mineral-composition-on?casa_token=P_NYfSlomS8AAAAA:RoYTH1UKqrY0ohv-2ixTEFQuV6W952T5qBuDCPn3Iu2sVF0f_FSQ-q4SMtMJUlqFA1jf-HZv Mitchell et al.: Influence of bedrock mineral composition on microbial diversity in a subglacial environment. Geology 2013, v. 41, no. 8, p. 855-858.]</ref><ref name= "Skidmore et al. 2005"/>, which drives chemolithoautotrophy within the system, the main source of primary productivity given the complete lack of sunlight<ref name= "Boetius et al. 2015"/>.




This great microbial diversity of subglacial environments appears greater than other cryo-environments, such as supraglacial environments and snow<ref name= "Hamilton et al. 2013"/><ref name= "Boetius et al. 2015"/>. Hamilton et al.<ref name= "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.
This great microbial diversity of subglacial environments appears greater than other cryo-environments, such as supraglacial environments and snow<ref name= "Hamilton et al. 2013"/><ref name= "Boetius et al. 2015"/>. Hamilton et al.<ref name= "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<ref name= "Boetius et al. 2015"/>, the composition of subglacial communities appears to be characterized by relatively high bacterial abundance, specifically <i>Betaproteobacteria</I>, <i>Deltaproteobacteria</I>, <i>Gammaproteobacteria</I>, and <i>Bacteroidetes</I><ref name= "Hamilton et al. 2013"/><ref name= "Boetius et al. 2015"/><ref name= "Christner et al. 2014"/> (see fig 3). Archaea appear present in most subglacial communities as well<ref name= "Christner et al. 2014"/><ref name= "Hamilton et al. 2013"/><ref name= "Boetius et al. 2015"/>, albeit with lower relative abundance than bacteria in at least some environments<ref name= "Hamilton et al. 2013"/>. Eukaryotes, while displaying high diversity when present<ref name= "Hamilton et al. 2013"/>, are not detectable in all subglacial environments<ref name= "Christner et al. 2014"/>.
While data is limited<ref name= "Boetius et al. 2015"/>, the composition of subglacial communities appears to be characterized by relatively high bacterial abundance, specifically Betaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, and Bacteroidetes<ref name= "Hamilton et al. 2013"/><ref name= "Boetius et al. 2015"/><ref name= "Christner et al. 2014"/> (see fig 3). Archaea appear present in most subglacial communities as well<ref name= "Christner et al. 2014"/><ref name= "Hamilton et al. 2013"/><ref name= "Boetius et al. 2015"/>, albeit with lower relative abundance than bacteria in at least some environments<ref name= "Hamilton et al. 2013"/>. Eukaryotes, while displaying high diversity when present<ref name= "Hamilton et al. 2013"/>, are not detectable in all subglacial environments<ref name= "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<ref name= "Hamilton et al. 2013"/><ref name= "Mitchell et al. 2013"/><ref name= "Hamilton et al. 2013"/><ref name = "Nixon et al. 2017"> [https://core.ac.uk/download/pdf/96781708.pdf Nixon et al.: Viable cold-tolerant iron-reducing microorganisms in geographically diverse subglacial environments. Biogeosciences 2017, v. 14, v. 6, p. 1445-1455.]</ref><ref name= "Skidmore et al. 2005"/>. Much of this diversity appears to be driven specifically by the pyrite and iron composition of bedrocks and sediments<ref name= "Mitchell et al. 2013"/><ref name= "Nixon et al. 2017"/>.
 
==Key Microbial Players==
 
 
As noted above, mineralogical controls (specifically pyrite and iron compositions) help define microbial community composition<ref name= "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.
 
<i>Desulfosporosinus</I> and <i>Geobacter</I> both appear to play significant roles in the reduction of iron in subglacial environments<ref name= "Nixon et al. 2017"/>. <i>Desulfosporosinus</I> is a genus within the Firmicutes phylum, while <i>Geobacter</I> is a genus within the Deltaproteobacteria. While seemingly not abundant together at the same locations, Nixon et al. (2017)<ref name= "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. <i>Desulfosporosinus</I> may also play an important role in sulfate-reduction in subglacial environments<ref name= "Nixon et al. 2017"/>.
 
<i>Thiobacillus</I> 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)<ref name = "Harrold et al. 2016"> [https://aem.asm.org/content/82/5/1486.short 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.]</ref>. <i>Thiobacillus</I> is a genus within the Betaproteobacteria. <i>Thiobacillus</I> spp. oxidizes thiosulfate, an intermediate in the complete oxidation of pyrite. The key role of <i>Thiobacillus</I> spp. in complete pyrite oxidation, in addition to the presence of <i>Thiobacillus</I> spp. in a variety of other subglacial environments<ref name= "Christner et al. 2014"/><ref name= "Skidmore et al. 2005"/>, emphasizes the important role it likely plays in subglacial microbial communities<ref name= "Harrold et al. 2016"/>.
 
==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<ref name = "Perovich et al. 2007"> [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2007GL031480 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.]</ref>), sea-level change (i.e., Chen et al. 2013<ref name = "Chen et al. 2013"> [https://www.nature.com/articles/ngeo1829 Chen et al.: Contribution of ice sheet and mountain glacier melt to recent sea level rise. Nature Geoscience 2013, v. 6, p. 549-552.]</ref>), and ocean circulation (i.e., Cope and Winguth 2007<ref name = "Cope and Winguth 2007"> [https://www.sciencedirect.com/science/article/pii/S0031018211001726?casa_token=kXG6UbisjFcAAAAA:xjQDJwkOYqe4pJcFtG18obi0B9ygFBhZAsF3BjvPmAK9aWpC_-zXlZTcg8esevumo9URgTtUilc 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.]</ref>), among many other impacts. Subglacial microbial communities have likely played a key role as well, by possibly helping sustain life during global glaciations<ref name= "Mitchell et al. 2013"/>. These cold-adapted<ref name= "Harrold et al. 2016"/> communities are uniquely-adapted to survival during global glaciations. As such, subglacial microbial communities were possibly widespread during such cold events<ref name= "Mitchell et al. 2013"/>. 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<ref name = "Hotaling et al. 2017"> [https://sfamjournals.onlinelibrary.wiley.com/doi/full/10.1111/1462-2920.13766 Hotaling et al.: Microbial ecology of mountain glacier ecosystems: biodiversity, ecological connections and implications of a warming climate. Environmental Microbiology 2017, v. 19, no. 8, p. 2935-2948.]</ref>). Given the importance of subglacial microbial communities in global biogeochemical cycling<ref name= "Hamilton et al. 2013"/><ref name = "Wadham et al. 2008"> [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2007GB002951 Wadham et al.: Subglacial methanogenesis: A potential climatic amplifier?. Global Biogeochemical Cycles 2008, v. 22, no. 2.]</ref>), 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<ref name = "Stibal et al. 2012"> [https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2486.2012.02763.x?casa_token=yFSblUPpJegAAAAA%3ARposMUf9jEKxrdAgu6OVEaRNvaeHVw0McD-SGI4xQaaiYbAOri_VoFUqBt1QZBttZJ4iAAgc0RBrwTwj 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.]</ref>), which can eventually be exported to the atmosphere via subglacial drainage, making the magnitude of this subglacial production and export climatically relevant<ref name = "Lamarche-Gagnon et al. 2019"> [https://www.nature.com/articles/s41586-018-0800-0 Lamarche-Gagnon et al.: Greenland melt drives continuous export of methane from the ice-sheet bed. Nature 2019, v. 565, p. 73-77.]</ref>. For the 2015 melt-season alone at a single proglacial river, Lamarche-Gagnon et al. (2019)<ref name= "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<ref name= "Lamarche-Gagnon et al. 2019"/>.


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<ref name= "Hamilton et al. 2013"/><ref name= "Mitchell et al. 2013"/><ref name= "Hamilton et al. 2013"/><ref name = "Nixon et al. 2017"> [https://core.ac.uk/download/pdf/96781708.pdf Nixon et al.: Viable cold-tolerant iron-reducing microorganisms in geographically diverse subglacial environments. Biogeosciences, v. 14, v. 6, p. 1445-1455.]</ref><ref name= "Skidmore et al. 2005"/>. Much of this diversity appears to be driven specifically by the pyrite and Fe composition of bedrocks and sediments<ref name= "Mitchell et al. 2013"/><ref name= "Nixon et al. 2017"/>.
==References==

Latest revision as of 23:14, 23 July 2021

This student page has not been curated.

By: Tim Coston

Overview

Subglacial environments, including those beneath both ice sheets and outlet glaciers, host diverse microbial communities[1][2][3][4], despite complete darkness and sub-zero temperatures[2]. These communities, largely composed of bacteria and archaea[1][2][4], appear distinct between some subglacial environments5 and possibly stable at the OTU-level over decade-long timescales[1]. A diverse array of metabolisms appear present[2][5], including those reliant on reduced N, S, Fe, and methane for energy[5]4. Multiple previous studies have shown the importance of subglacial microbial communities in global biogeochemical cycling [3][5], 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[6] 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[6] (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[7].
Fig. 2 Structure of subglacial environments, within the greater glacial system, is shown. Modified from Boetius et al. (2015)[8].


The defining characteristics of subglacial environments include the complete lack of light[8], 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 )[3]. 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[4].

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[8]. Data from Roberson Glacier, Canada is from Hamilton et al. (2013)[3]. Data from Lake Whillans, Antarctica is from Christner et al. (2014)[2]. Data from East Skafta Lake, Iceland is from Marteinsson et al. (2013)[9]

Despite the extreme conditions of subglacial environments, current research indicates the presence of diverse microbial communities[2][3][8]. These communities can consist of bacteria, archaea, and eukarya[3]. The diversity within and between subglacial environments is largely driven by bedrock and sediment mineralogy[10][4], which drives chemolithoautotrophy within the system, the main source of primary productivity given the complete lack of sunlight[8].


This great microbial diversity of subglacial environments appears greater than other cryo-environments, such as supraglacial environments and snow[3][8]. Hamilton et al.[3] 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[8], the composition of subglacial communities appears to be characterized by relatively high bacterial abundance, specifically Betaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, and Bacteroidetes[3][8][2] (see fig 3). Archaea appear present in most subglacial communities as well[2][3][8], albeit with lower relative abundance than bacteria in at least some environments[3]. Eukaryotes, while displaying high diversity when present[3], are not detectable in all subglacial environments[2].

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[3][10][3][11][4]. Much of this diversity appears to be driven specifically by the pyrite and iron composition of bedrocks and sediments[10][11].

Key Microbial Players

As noted above, mineralogical controls (specifically pyrite and iron compositions) help define microbial community composition[10]. 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[11]. 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)[11] 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[11].

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)[12]. 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[2][4], emphasizes the important role it likely plays in subglacial microbial communities[12].

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[13]), sea-level change (i.e., Chen et al. 2013[14]), and ocean circulation (i.e., Cope and Winguth 2007[15]), among many other impacts. Subglacial microbial communities have likely played a key role as well, by possibly helping sustain life during global glaciations[10]. These cold-adapted[12] communities are uniquely-adapted to survival during global glaciations. As such, subglacial microbial communities were possibly widespread during such cold events[10]. 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[16]). Given the importance of subglacial microbial communities in global biogeochemical cycling[3][17]), 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[18]), which can eventually be exported to the atmosphere via subglacial drainage, making the magnitude of this subglacial production and export climatically relevant[19]. For the 2015 melt-season alone at a single proglacial river, Lamarche-Gagnon et al. (2019)[19] 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[19].

References

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