Can We Fix It? Carbon fluxes on glacier surfaces
Changing climates | Supraglacial carbon fluxes | Where is the microbial activity on glaciers? | How much carbon is fixed on glaciers? | Impact of glacier microbes impact on the albedo | Summary | Glossary | References | Comments |
Italicised words are defined in the Glossary below.
Over the past 150 years, the climate has changed for the hotter, accelerated by human changes to the carbon cycle. Along with water vapour, nitrous oxides and ozone, carbon dioxide and methane represent the primary greenhouse gases, controlling the degree of the earth’s atmospheric insulation. Carbon dioxide is released in vast quantities by human activity and is the primary greenhouse gas whose atmospheric concentration is mediated by earth’s biota. Methane is a much more potent greenhouse gas which is stored in vast reserves in the permafrost. In the context of glacier and ice sheet surfaces (the ‘supraglacial’ environment), carbon dioxide is cycled extensively by carbon in microbial life; methane on the other hand is more strongly associated with the subglacial zone.
The amount of carbon fixed on glacier surfaces is critical to the world-wide carbon budget. Carbon fixation on glaciers may change as glaciers in the Arctic grow and shrink; understanding supraglacial microbial activity is therefore important to our wider understanding of changing atmospheric levels of carbon dioxide in the future.
Supraglacial carbon fluxes
Supraglacial carbon fluxes are complex, but can be summarised by two major opposing phenomena: fixation of atmospheric carbon into particulate carbon and metabolism of particulate carbon back into atmospheric carbon. The balance of these two microbially-mediated processes influences the concentration of carbon-containing compounds in the local atmosphere.
Carbon fixation into particulate carbon
On ice surfaces, cycling of carbon between atmospheric and terrestrial environments is driven by life; specifically the opposing processes of photosynthesis and respiration. The net balance of these processes is termed net ecosystem productivity (NEP). Organisms that produce energy without consuming pre-existing organic carbon (predominantly via photosynthesis) are known as autotrophs. When autotrophs photosynthesise, they absorb carbon dioxide from the atmosphere and use it to synthesise carbohydrates, some of which are used to build new cells and add biomass to the supraglacial environment. This is known as carbon fixation, because carbon in small atmospheric molecules is fixed to the terrestrial environment in larger molecules and complexes. Carbohydrates synthesised by autotrophs provide a source of energy for other microbes in the supraglacial ecosystem.
Metabolisim into atmospheric carbon
Heterotrophs metabolise carbon fixed by photosynthesis and return it to the atmosphere as carbon dioxide through the process of respiration. Net ecosystem productivity (NEP) is therefore a crucial determinant of local atmospheric carbon concentration, making supraglacial microbial processes an essential topic of study. It is also very important that the process of carbon fixation into biomass, as well as secondary processes associated with photo-autotrophy such as dark pigment production, darken ice surfaces and therefore enhance melt rates.
Where is the microbial activity on glaciers?
Microbial activity in the supraglacial zone is primarily concentrated in two habitats; cryoconite holes and weathering crust ice.
Cryoconite holes are cylindrical depressions on ice surfaces that form due to enhanced melt rates beneath patches of dark sediment (Cook et al, 2010). The sediment itself comprises mineral fragments encased in organic material (including filamentous cyanobacteria, EPS and humic substances) that helps to cohere aggregates of millimetre to centimetre diameter (usually < 2mm) (Langford et al, 2010).
The organic component of cryoconite reduces the albedo and increases the efficiency of solar radiation absorption. The surface of cryoconite grains are dominated by photosynthesising autotrophic algae and cyanobacteria, whereas the grain interior is mainly heterotrophic, meaning that thin sediment layers promote net autotrophy in cryoconite holes (Cook et al, 2010). Sediment and melt water in cryoconite holes also accommodates higher organisms such as rotifers and tardigrades (DeSmet and Van Rompu, 1994). These microbial assemblages differ in their composition on glacier surfaces around the world, particularly between large, stable ice sheet interiors, and small, dynamic ice sheet marginal areas. The more stable environments are generally associated autotrophic dominance and carbon fixation, whereas more dynamic environments seem to promote net heterotrophy and act as a source of carbon.
Weathering crust ice
The second major microbial habitat was reported by Yallop et al (2012), who identified a community of photoautotrophic algae inhabiting the surface of the Greenland ice sheet in 2010. These algae cling to weathering crust (low density, near-surface, weathered ice) crystals and photosynthesise. Due to their extremely high spatial coverage, they have been suggested to fix as much as eleven times as much carbon as cryoconite communities (Cook et al, 2012). This suggests that the Arctic might represent a crucial sink for atmospheric carbon. How much carbon is absorbed into that sink, however, remains an important research question.
How much carbon is fixed on glaciers?
A handful of studies in the past five years aimed to quantify Arctic carbon fixation. Anesio et al (2009) made initial forays, but used NEP measurements from just five sites to estimate the carbon flux for the entire cryosphere (excluding Antarctica). Shortly after, Hodson et al (2010) refined the model by including seasonal variations in snow cover in acknowledgement of temporal variations in rates of activity. However, spatial changes were left unconsidered and field measurements from close to the ice margin at the very end of the melt season were assumed to be representative of the entire ablation zone on the Greenland Ice Sheet. Furthermore, measurements from a single transect were assumed to represent all latitudes and longitudes for Greenland Ice Sheet ice. This was recognised to some extent by Cook et al (2012) who obtained field data from a transect spanning the entire ablation zone in mid-summer to estimate carbon fluxes for a small 1600km2 area of the Greenland Ice Sheet only, and also incorporated newly discovered surficial algal communities (field data reported by Stibal et al., 2012). Upscaling beyond the 1600km2 transect was not deemed justifiable without further costly and time consuming transect studies.
Cook et al’s (2012) estimates represent the most sophisticated to date, and suggest an annual carbon fixation of 16.4 Gg for a 1600km2 area of the Greenland Ice Sheet (compared to Hodson et al’s 2010 estimate of 5Gg C a-1 for all 22million km2 of ice sheet, or Anesio et al’s (2009) estimate of 64Gg C a-1 for the entire non-Antarctic cryosphere). This suggests that Arctic carbon fixation is of regionally significant magnitude and demands urgent further study. Cook et al’s (2012) estimates are still fundamentally limited by assumptions of temporally constant rates of activity throughout a melt season, relatively simplistic model equations and small spatial extent; however, they provide first order estimates of carbon fluxes which must now be built upon by future studies.
Prior to further modelling studies, it would be pertinent for the glacial microbiological community to gain a better understanding of metabolic processes in supraglacial ecosystems, such that rates of activity and controlling factors are better constrained. In particular, spatial and temporal variations and responses of communities to environmental stresses are currently poorly understood, yet they may significantly influence regional supraglacial carbon fluxes. To this end, recent work has introduced sophisticated analyses to inform us of supraglacial microbial processes at the molecular level (e.g. Edwards et al, 2013). There is much potential for these techniques to stimulate enhanced understanding of glacial microbial ecology and therefore carbon fluxes in the supraglacial environment.
Impact of glacier microbes impact on the albedo
Microbial processes in the supraglacial zone do not only impact upon the climate through changing atmospheric carbon concentrations. As suggested earlier, they also impart control upon glacier albedo. Primarily this is because biomass itself is darker than glacier ice. More biomass means lower albedo, in both cryoconite and surficial algal communities. Furthermore, microbial activity adds sticky EPS to cryoconite grains, promoting aggregation (Langford et al, 2010) and dark photo-protective pigments can be produced in response to intense irradiance (Yallop et al, 2012). These processes operate at the microscale but influence albedo at the plot scale and beyond. Question marks remain over glacial responses to climate change, not least because glacier dynamics vary so markedly with geography. The rate of melting of glacier surfaces is primarily controlled by albedo, which in turn is controlled to some extent by microbial activity and associated carbon fluxes could be regionally significant, so a solid appreciation of microbiological processes in the supraglacial environment is crucial information in climate science.
In summary, supraglacial microbiology mediates carbon fluxes on the surfaces of glaciers and ice sheets world wide. In the Arctic, net autotrophy dominates and provides a significant sink of carbon, although estimates of the magnitude of carbon fixation require improvement. In alpine and Antarctic locations there may be net release of carbon dioxide into the environment. The immediate future of supraglacial microbiology will probably be primarily defined by new molecular techniques. However we go about it, a better understanding of the microbially mediated carbon fluxes in the supraglacial environment is crucial for better constraining climate feedbacks and glacier melt dynamics – crucial information if we are to adapt to a warming world.
- Glacier Ecosystems
- Subglacial lakes
- Glacier hydrology
- Aberystwyth Cryoconite Research Group blog
- Probing the Dark Side of life on Arctic Glaciers
- To The Poles: blog by Joseph Cook
Ablation zone: the melting part of a glacier
Albedo: a measure of a surface’s reflectivity
Anthropogenic: related to human activity
Assemblage: A collection of organisms in a particular environment
Autotroph: (self-feeder)an organism that synthesises energy-rich molecules from inorganic raw materials, primarily through photosynthesis.
Biomass: biological material
Cryosphere: the sum of earth’s snow, ice and frozen ground
Cyanobacteria: a type of filamentous bacteria that photosynthesises
EPS: extracellular polymeric substances are produced by microbial activity and, because they are ‘sticky’ help to bind cryoconite granules together.
Heterotroph: (‘other’ feeder) an organism that metabolises pre-existing energy-rich molecules.
Microbially Mediated: processes which are driven by the activity of microbes
NEP: net ecosystem production is the balance between the total amount of autotrophy and heterotrophy within a community
Permafrost: permanently frozen ground
Photoautotrophic: refers to organisms who generate energy-rich molecules using light, i.e. organisms that photosynthesise.
Photo-protective: refers to molecules that shield organisms from intense light
Photosynthesis: the process of converting carbon dioxide into energy using sunlight
Respiration: the process of harvesting energy from organic molecules, releasing carbon dioxide
Supraglacial: associated with glacier surfaces
Terrestrial: associated with solid ground or land, as opposed to the ocean or atmosphere
Anesio, A.M., Hodson, A.J., Fritz, A., Psenner, R., Sattler, B. 2009. High microbial activity on glaciers: importance to the global carbon cycle. Global Change Biology, 15: 955-960
Cook, J.M., Hodson, A.J., Anesio, A.M., Hanna, E., Yallop, M., Stibal, M., Telling, J., Huybrechts, P. 2012. An improved estimate of microbially mediated carbon fluxes from the Greenland Ice Sheet. Journal of Glaciology, 58 (212)
Cook, J.; Hodson, A.; Telling, J.; Anesio, A.; Irvine-Fynn, T.; Bellas, C. 2010. The mass-area relationship within cryoconite holes and its implications for primary production. Annals of Glaciology, 51 (56): 106-110.
De-Smet, W. H., Van-Rompu, E.A. 1994. Rotifera and Tardigrada from some Cryoconite holes on a Spitsbergen Svalbard glacier. Belgian Journal of Zoology 124(1): 27-37
Edwards, A., Pachebat, J.A., Swain, M., Hegarty, M., Hodson, A.J., Irvine-Fynn, I., Rassner, S., Sattler, B. 2013. A metagenomic snapshot of taxonomic and functional diversity in an alpine glacier cryoconite ecosystem. Environmental Research Letters, 8 (035003)
Langford, H., Hodson, A.J., Banwart, S., Boggild, C. 2010. The microstructure and biogeochemistry of Arctic cryoconite granules. Annals of Glaciology, 51 (56): 87-94
Stibal, M, Telling, J, Cook, J., Mak, K. M., Hodson, A. & Anesio, AM. 2012. Environmental controls on microbial abundance and activity on the Greenland ice sheet: a multivariate approach. Microbial Ecology, 63: 74-84
Stibal, M., Sabacka, M., Zarsky, J. 2012. Biological processes on glacier and ice sheet surfaces. Nature Geoscience, 5: 771-774
Telling, J., Anesio, A.M., Tranter, M., Stibal, M., Hawkings, J., Irvine-Fynn, T., Hodson, A.J., Butler, C., Yallop, M.,Wadham, J. 2012. Controls on the autochthonous production of organic matter in cryoconite holes on high Arctic glaciers. Journal of Geophysical Research: Biogeosciences, 117 (G1)
Yallop, M.L., Anesio, A.J., Perkins, R.G., Cook, J., Telling, J., Fagan, D., MacFarlane, J., Stibal, M., Barker, G., Bellas, C., Hodson, A., Tranter, M., Wadham, J., Roberts, N.W. 2012. Photophysiology and albedo-changing potential of the ice-algal community on the surface of the Greenland ice sheet. ISME Journal, 6: 2302 – 2313