Australia: The Land Where Time Began

A biography of the Australian continent 

Glacial Microbes Their Power

A key control on biogeochemical cycles in the polar regions is organic carbon fluxes from glaciers. The importance of microbial communities on the surface of glaciers in the setting of these inputs is shown by 2 analyses of carbon cycling in glaciers.

In the past it was believed that glaciers were inhospitable environments for life. Over the past 15-20 years, however, it has been documented that vibrant, diverse microbial communities exist in all areas of ice (Boetius, 2015). Generally, these communities are associated with liquid water, either between the bedrock and the base of the glacier, or in pores ranging from micrometre-sized structures within the ice lattice to larger holes and streams on top of the glacier. The balance between the amount of fixed carbon that has been produced and consumed by these microbes is not well constrained. These microbial communities have been found to produce more carbon than expected and therefore play important roles in the glacial carbon cycle in Antarctica and Greenland ice sheets, respectively, writing in Nature Geoscience, Smith et al (Smith et al., 2017) and Musilova et al. (Musilova et al., 2017), respectively.  

The amount of dissolved organic matter (DOM) can approximate carbon fixed on glaciers, though it has proved complicated to pinpoint its sources. It is not clear, in particular, what the relative contribution to the consumption of organic matter that is sourced externally is on the one hand and , and organic matter that has been produced by photosynthesis on the other hand. It is suggested by the radiocarbon values of this organic material that the carbon is from an ancient source (Hood et al., 2009), which is potentially at odds with the microbes being implicated as the primary source of DOM (Bhatia et al., 2010; Pautler et al., 2012). If the organic matter is relict from microbial processing in the past or microbial breakdown and recycling of ancient carbon, the age of the carbon could be explained. An alternative explanation could be that aerosols derived from combustion that were deposited on the ice carry an ancient radiocarbon signature, and on glacial surfaces in the Northern Hemisphere may be a source of carbon (Stubbins et al., 2012).

Organic carbon and nutrients are carried from the glaciers to the sea by run-off from glaciers to eventually feed and stimulate the marine communities at the height of the melt season. As such, a clearer understanding may be obtained of the way in which glacial microbial communities affect carbon fluxes, and this will elucidate the links between glacial and marine biogeochemical cycles.

The balance of carbon fluxes that are microbially mediated in the supraglacial stream system in Antarctica have been addressed by Smith et al. (Smith et al., 2017) with an elegant combination of incubation experiments, carbon isotope analyses and spectroscopy.

1)    Spectroscopy was used to confirm that in Antarctic streams DOM was probably derived from microbes.

2)    They showed that the carbon demand was exceeded by total microbial production of carbon that exceeded carbon demand by approximately a factor of 4, which left the excess to be exported to downstream environments.

3)    Organic carbon was isotopically labelled to monitor uptake of organic carbon by glacial microbes, observing that DOM is utilised rapidly by supraglacial microbes.

Previous studies have reported microbial carbon delivery to Alaskan and alpine watersheds that have a high glacial cover, and this is consistent with a high fraction of carbon export (Hood et al., 2009; Singer et al., 2012). An experimental confirmation of inferences from previous studies is this molecular level demonstration of the power of microbes to assimilate and recycle existing microbial organic matter.

The question of microbial carbon cycling has been approached somewhat differently (Musilova et al., 2017), as they explored small holes that were water-filled containing microbial communities in the supraglacial system of the Greenland ice sheet. They observed overall net organic carbon production, that was the result of higher production rates via photosynthesis than consumption of via respiration in all supraglacial habitats during a single melt season, using complementary methods to Smith et al. (Smith et al., 2017). It is therefore consistent with the conclusion of Smith et al. (Smith et al., 2017), based on the Antarctic observations that they found a net supraglacial carbon surplus in Greenland.

Also, the concentrations of bulk supra glacial DOM in Greenland varied over time, as did the compounds that are more easily broken down biologically, such as amino acids and carbohydrates. The dynamic nature of microbial production and consumption of growth substrates in glacial environments is underscored by this variability. The only supraglacial of organic matter that is produced externally was found in waters that contained significant amounts of wind-blown dust, in spite of the northern Hemisphere setting.

An active microbial community is described by both papers, though at opposite poles, that has a substantial impact on carbon cycling on the surface of glaciers. The idea that the surfaces of glaciers are poor hosts for microbial metabolism is deflated by this. Instead, photosynthetic microbes support extensive microbial communities that feed off the carbon they produce, as well as being able to generate enough carbon to export reactive DOM to downstream environments. Some of these latter non-photosynthetic microbial communities in the Northern Hemisphere may also degrade material from atmospheric deposition; though Smith et al. found no evidence of this metabolism in the regions of Antarctica that were more remote.

Radiocarbon values of supraglacial DOM were not measured by either study, so it remains uncertain why the carbon that was found previously to be so old. Evidence is emerging, however, of unexpected metabolisms in glacial systems that are driven by light, such as the oxidation of carbon monoxide (Franzetti et al., 2016). Kujawinski suggests older sources of inorganic and organic carbon may be introduced in the pool of organic material, which has been actively cycled, on the surface of glaciers. Kujawinski suggests future investigations that are capable of linking metabolic processes with analyses of carbon isotopes are needed to establish the relative flux and radiocarbon age of carbon through these pathways.

Smith et al. and Musilova et al. have established that the microbial carbon cycling on the surfaces of glaciers cannot be ignored, regardless of the mechanism. Labile DOM is contributed by microbial activity to supraglacial and downstream environments where it is combined with organic matter from within or below the glacier. These organic matter pools are, of course, also probably derived from microbes (Musilova et al., 2017; Bhatia et al., 2010), which underscores that these microscopic organisms are the heart of the glacial carbon cycle. The next step will be the exploration of the molecular and metabolic details of the rates and extents of microbial processing in these habitats.

Sources & Further reading

  1. Kujawinski, E. B. (2017). "Cryospheric science: The power of glacial microbes." Nature Geosci 10(5): 329-330.



Author: M. H. Monroe
Last updated: 30/07/2017
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