Living the High Life… in the aeolian biome

At the top of the highest mountains – where air is thin, solar irradiance intense, meteorology unpredictable, temperatures low and food scarce – spiders live on snow. The same spiders that are found in much more favourable conditions at sea level around the world. With no specific adaptations and no obvious lower trophic levels to feed on, the presence of these spiders seems to oppose ecological sense. This puzzled twentieth century ecologist Lawrence Swan until he realised… there is biology in the sky!

This species of salticid spider was found on snow slopes on Everest (ph. Wikimedia Commons)
This species of salticid spider was found on snow slopes on Everest (ph. Wikimedia Commons)

Swan (1992) proposed the existence of an “aeolian biome” that transports both flora and fauna around the globe in atmospheric suspension, depositing them in a wide range of environments. Once deposited, fauna either tolerates local conditions and survives, or dies due to exposure or competition. In ecology, a species that is tolerant to a wide range of conditions and can survive many environments is described as “cosmopolitan” whereas one with very specific adaptations to a particular habitat is known as “endemic”. The spiders were simply widely cosmopolitan and able to survive in the extreme conditions on high mountain tops, feeding upon smaller organisms also delivered by wind.

Swan (1992) also suggested an aeolian origin for the microbes inhabiting cryoconite holes on ice at the poles. Later, researchers such as Sattler et al (2001) and Pearce et al (2009) realised that not only are microbes transported in the atmosphere, they are active there too. This is important for understanding microbial ecology on glacier surfaces since many of the stresses experienced by microbes in the atmosphere reflect those of glacier surfaces, e.g. low temperatures, intense irradiance, dessication and freeze-thaw cycles. This suggests communities might be shaped to some extent before they are deposited. A viable aeolian community of cryo-tolerants is therefore established to be deposited on ice surfaces. Sattler et al (2001) showed that some cryo-tolerants even exploit super-cooled water droplets in the atmosphere. Certain bacteria with specific surface proteins have been found to initiate ice crystal formation in supercooled water in the atmosphere by acting as nuclei, as demonstrated in the video below…

Bacteria has recently been found thriving at 33,000 feet in the upper atmosphere, leading to suggestions of a “bubble of bacteria” surrounding the earth that could contribute significantly to global nutrient cycling and provide condensation nuclei for cloud  and snow formation (DeLeon-Rodriguez et al, 2013; Bauer et al, 2003), thereby playing a role in snow accumulation and ultimately glacier formation. Some have conjectured their potential as climate regulators (Morris et al, 2011). Climate regulation by microbes on ice surfaces is an area of current research; however the pre-depositional selection of microbes in the aeolian biome, and the potential for climate regulation in the “bacterial bubble” suggest global-scale climate mediation by microbes across all the earth subsystems.

The aeolian biome could extend as far as the upper layers of earth's atmosphere (ph. Wiki commons)
The aeolian biome could extend as far as the upper layers of earth’s atmosphere (ph. Wiki commons)

Even within specific biomes, community structures are probably highly dynamic, responding to local environmental stresses over very short timescales, as has recently been shown on glacier surfaces by Edwards et al (2014) using combined FTIR-spectroscopy and metabolite profiling. Clearly, the microbial ecology of ice surfaces is very complex, dependent not only upon ice surface conditions but also pre-depositional history and this is likely the case in other biomes too.

 

Refs:

Bauer, H.H. GieblR. HitzenbergerA. Kasper-GieblG. ReischlF. Zibuschka, and H. Puxbaum(2003), Airborne bacteria as cloud condensation nucleiJ. Geophys. Res.108, 4658, doi:10.1029/2003JD003545D21.

DeLeon-Rodriguez, N., Lathem, T.L., Rodriguez, L.M., Barazesh, J.M., Andersond, B.E., Beyersdorf, A.J., Ziembad, L.D., Bergin, M., Nenes, A., Konstantinos, T.K. 2013. Microbiome of the upper troposphere: species composition and prevalence, effects of tropical storms, and atmospheric implications. PNAS, 110 (7): 2575-2580

Edwards, A., Mur, L. A.J., Girdwood, S. E., Anesio, A. M., Stibal, M., Rassner, S. M.E., Hell, K., Pachebat, J. A., Post, B., Bussell, J. S., Cameron, S. J.S., Griffith, G. W., Hodson, A. J. and Sattler, B. (2014), Coupled cryoconite ecosystem structure–function relationships are revealed by comparing bacterial communities in alpine and Arctic glaciers. FEMS Microbiology Ecology. doi: 10.1111/1574-6941.12283

Morris, C. E., Sands, D. C., Bardin, M., Jaenicke, R., Vogel, B., Leyronas, C., Ariya, P. A., and Psenner, R.: Microbiology and atmospheric processes: research challenges concerning the impact of airborne micro-organisms on the atmosphere and climate, Biogeosciences, 8, 17-25, doi:10.5194/bg-8-17-2011, 2011.

Pearce, D.A., Bridge, P.D., Hughes, K.A., Sattler, B., Psenner, R., Russell, N.J. 2009. Microorganisms in the atmosphere over Antarctica. FEMS Microbiology Ecology, 69 (2): 143-157

Sattler, B., Puxbaum, H., Psenner, R. 2001. Bacterial growth in supercooled cloud droplets. Geophysical Research Letters, 28: 239-242

Swan, L. 1992. The aeolian biome. Bioscience, 42 (4): 262-270

Ice Sheet Microbes and Melt: Dark Snow 2014

Here’s an article I recently wrote with professor Jason Box of Dark Snow – keep checking here for updates on our field plans for Greenland 2014!

Ice Sheet Microbes and Melt

Greenland contains the largest continuous mass of ice in the northern hemisphere; an area over 2 million km2. The frequency of Greenland surface melting has increased, likely as a result of human-induced climate warming, with the melt-area covering almost the entire ice sheet surface in 2012 (Ngheim et al. 2012; Box et al, 2013; Tedesco et al. 2013).

Ablation zone extent on the Greenland ice sheet: July 8 (left) and July 12 (right). On July 8, ~40% of the ice sheet was melting. Four days later, ~97% of the ice sheet surface had thawed.  Credit: Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory
Ablation zone extent on the Greenland ice sheet: July 8 (left) and July 12 (right). On July 8, ~40% of the ice sheet was melting. Four days later, ~97% of the ice sheet surface had thawed. Credit: Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory

Although its fair to say that higher temperatures mean more melt, the response of earth’s glaciers and ice sheets to climate warming is complex, also depending upon a range of feedbacks (e.g. Box et al, 2012). For example, when ice melts, liquid water runs over its surface, sometimes collecting in pools and lakes. Liquid water is a more effective absorber of sunlight than snow or ice, so the overall reflectivity (also called albedo, Greek for ‘whiteness’) of the ice decreases. The result is faster ice melt. Melting promotes more melting.

Box et al’s (2012) image of albedo anomaly in summer 2012. Darker blue means greater darkening compared to average albedo.
Box et al’s (2012)
image of albedo anomaly in summer 2012. Darker blue means greater darkening compared to average albedo.

It is not only melt water that reduces  Greenland ice sheet albedo. A variety of aerosol ‘impurities’ further reduce surface albedo. These include black carbon (BC) derived from incomplete combustion of fossil fuels, other industrial activity, biomass burning, and wildfire. Black carbon can be transported across the hemisphere through the atmosphere and deposited on ice, and currently the impacts remain uncertain (Hodson, 2014). Dark Snow field science is examining this question in detail based on 2013 field measurements and planned measurements for June-August, 2014.

Black Carbon - produced during the incomplete combustion of fossil fuels
Black Carbon – produced during the incomplete combustion of fossil fuels (photo, wikimedia commons)

The presence of microbes on the ice surface also alter albedo and can therefore influence melt rates (Yallop et al, 2012). They do so by growing and adding dark biomass to the ice, causing mineral fragments to aggregate and resist removal (flushing) by melt water, and by producing dark humic substances and pigments. There may even be a relationship between microbial activity and BC. Yet, it is not yet known whether microbes metabolise BC and reduce its impact, or cause it to “stick” to the ice surface and prevent its removal by flushing (Hodson, 2014).

Many microbes on the Greenland ice sheet inhabit ‘cryoconite’ holes; cylindrical tubes ‘drilled’ into the bright ice surface ice by dark cryoconite debris. The debris is a loose bonding of minerals encased in microbial biomass (e.g. Gribbon, 1979; Cook et al, 2010). Cryoconite holes on the Greenland ice sheet likely provide favourable conditions for photosynthesis by: 1.) maintaining light intensities that are high but not harmful because ultraviolet radiation is not transmitted by water; 2.) providing nutrients in melt water flowing in through the hole walls; and 3.) providing relatively long term (years) storage of microbes. This also promotes proliferation of bacteria and “grazers” that feed upon other microbes. These factors make cryoconite holes active and biodiverse ice sheet habitats (Hodson et al, 2008).

Cryoconite holes – generally considered to be the most biodiverse microbial habitats on glacier surfaces
Cryoconite holes – generally considered to be the most biodiverse microbial habitats on glacier surfaces (photo, J. Cook)

Despite being the most studied biological entity on the surface of the Greenland ice sheet, cryoconite holes remain poorly understood in terms of their biological community dynamics, thermodynamics, evolution and impact on albedo. Further, the characteristics of the holes themselves, and the microbes inhabiting them, have been shown to vary depending upon location on the ice sheet (Stibal et al, 2012), and these spatial patterns probably also evolve over time. Edwards et al (2014) recently found microbial communities to be extremely dynamic in response to environmental change, while Irvine-Fynn and Edwards (2014) showed that hydrological and glaciological processes might also influence microbial activity. Cryoconite holes will provide research foci for some members of the field team in summer 2014. For further information on cryoconite, see: herehere; and here.

by Joseph Cook and Jason Box

References:

Box, J.E., Fettweis, X., Stroeve, J.C., Tedesco, M., Hall, D.K., Steffen, K. 2012. Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers. The Cryosphere, 6, 821-839. open access.

Box, J.E., Cappelen, J., Chen, C., Decker, D., Fettweis, X., Mote, T., Tedesco, M., van de Wal, R.S.W., Wahr, J. 2013. Greenland ice sheet. Arctic Report Card. http://www.arctic.noaa.gov/reportcard/greenland_ice_sheet.html

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

Edwards, A., Mur, L. A.J., Girdwood, S. E., Anesio, A. M., Stibal, M., Rassner, S. M.E., Hell, K., Pachebat, J. A., Post, B., Bussell, J. S., Cameron, S. J.S., Griffith, G. W., Hodson, A. J. and Sattler, B. (2014), Coupled cryoconite ecosystem structure–function relationships are revealed by comparing bacterial communities in alpine and Arctic glaciers. FEMS Microbiology Ecology. doi: 10.1111/1574-6941.12283

Gribbon, P.W. 1979. Cryoconite holes on Sermikaysak, West Greenland. Journal of Glaciology, 22: 177-181

Hodson, A., Anesio, A.M., Tranter, M., Fountain, A., Osborn, M., Priscu, J., Laybourn-Parry, J., Sattler, B. 2008. Glacial Ecosystems. Ecological monographs, 78 (1): 41-67

Hodson, A. 2014. Understanding the dynamics of black carbon and associated contaminants in glacial systems.WIREs Water 2014, 1:141–149. doi: 10.1002/wat2.1016

Irvine-Fynn, T.D.L., and A, Edwards. 2014. A frozen asset: The potential of flow cytometry in constraining the glacial biome. Cytometry Part A 85 (1), 3-7

Nghiem, S. V., D. K. Hall, T. L. Mote, M. Tedesco, M. R. Albert, K. Keegan, C. A. Shuman, N. E. DiGirolamo, and G. Neumann (2012), The extreme melt across the Greenland ice sheet in 2012, Geophys. Res. Lett., 39, L20502, doi:10.1029/2012GL053611.

Stibal, M., Telling, J., Cook, J., Mak, K.M., Hodson, A., Anesio, A.M. 2012. Environmental controls on microbial abundance on the Greenland ice sheet: a multivariate analysis approach. Microbial Ecology, 63: 74-84.

Tedesco, M., X. Fettweis, T. Mote, J. Wahr, P. Alexander, J.E. Box, B. and Wouters. 2013. Evidence and analysis of 2012 Greenland records from spaceborne observations, a regional climate model and reanalysis data, The Cryosphere, 7, 615-630, doi:10.5194/tc-7-615-2013.

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

Measuring NEP

Some under- and post-grad students recently asked me to explain how to measure NEP in cryoconite holes, and this post represents a brief overview on their behalf – apologies to other readers who may find this a bit “niche” – something more accessible next time!

What is NEP?

NEP stands for Net Ecosystem Productivity and is a measure of the balance between primary production (PP) and respiration (R) occurring in all the organisms within a microbial community. Primary production is the conversion of atmospheric inorganic carbon (IC) into organic carbon (OC), primarily using energy from sunlight (photosynthesis). This is opposed by respiration, which is the process of metabolising OC back into IC for the purposes of energy harvesting. PP uses CO2 and releases O2, R uses O2 and releases CO2.

Proteobacteria are one of many microbial species that are commonly found on ice surfaces (ph. wikimedia)
Proteobacteria are common heterotrophs that influence NEP in cryoconite (ph. wikimedia)

How can it be measured?

Since NEP involves the usage and production of  O2, dissolved IC (DIC) and dissolved OC (DOC), NEP can be measured using changes in the concentrations of these nutrients after a period of activity. Most analyses have used closed-bottle incubations and measured changes in these nutrients over time periods of hours to days. Some incubations are undertaken under normal light conditions (to measure NEP) and some are undertaken wrapped in tin foil to eliminate irradiance (to measure R). These measurements are based upon several fundamental assumptions: firstly, that primary production ceases in the dark; second, that confining the community within a bottle does not significantly alter nutrient availability or hydrochemistry during the incubation; third that the temperature is not significantly lower in tin-foil wrapped incubations; fourth that the glass walls of the bottles do not attenuate harmful UV-B radiation such that photosynthesis is artificially enhanced; fifth that respiration rates are constant for both light and dark incubations; and finally that we can accurately correct for sulphide oxidation (in oxygen-based studies) and carbonate dissolution (in carbon-based studies). Whether these assumptions are justifiable is still somewhat uncertain.

Incubated cryoconite suspended in a cryoconite hole to simulate 'in situ' conditions on the Greenlanbd ice sheet
Incubated cryoconite suspended in a cryoconite hole to simulate ‘in situ’ conditions on the Greenlanbd ice sheet

Methods:

Accepting the assumptions listed above, there are three primary ways to measure NEP. One of these is to measure changes in DO2, and there are two ways to measure changes in C.

Changes in dissolved O2 concentration in the water overlying cryoconite in incubations can easily be measured by swirling a DO2 meter in the solution. A decrease in DO2 between pre- and post-incubation measurements indicates net respiration; an increase indicates net primary production. Although simple, oxygen probes can be large compared to the diameter of incubation bottles, and they require agitation to simulate flow >15cm/min, which introduces the possibility of both degassing of oxygen into the atmosphere and spillage of the solution. There is also simply a longer exposure of the solution to the atmosphere using DO2 meters than the other methods, increasing the time in which degassing can occur. DO2 meters sometimes require frequent calibration using very sensitive manual controls, which can be awkward and time consuming – especially in the cold in the field!

Changes in total dissolved inorganic carbon (TDIC) concentration has become the favoured technique for measuring NEP. This involves acidifying the incubated solution with HCl to force DIC to degas as CO2 into a headspace full of “scrubbed” air. The CO2 concentration of this air can then be measured using an infra-red gas analyser (IRGA). This is a slightly more convoluted procedure than using a DO2 meter; however it has been proven to be robust, even in the field. There is less opportunity for degassing since the solution hardly ever becomes open to atmospheric exchanges.

The final technique is radiolabel incorporation. Here, radio-isotopes  of carbon (14C) are added to incubated cryoconite. After incubation the original 14C concentration and the remaining 14C concentration in the solution are used to calculate a rate of 14C incorporation. This requires only very short incubation times (minimising bottle effects) and is very sensitive; however it is possible for radioactive 14C to be incorporated into biomass and then respired by heterotrophs, releasing it back into the solution – there is no way to distinguish this from non-incorporation. Therefore, radiolabel incorporation is only really useful for approximating net PP. Furthermore, this technique cannot be used in net heterotrophic systems since the incubations become flooded with DIC which dilutes the 14C.

Telling et al (2010) identified TDIC as the optimum method for calculating NEP in cryoconite incubations for the reasons outlined above. Standard procedures for carrying out NEP measurements using TDIC were developed by Hodson et al (2010) and Telling et al (2010; 2012). They suggested that, since sediment arrangement significantly impacts NEP (Cook et al, 2010; Telling et al, 2012), measurements should be normalised for sediment mass and incubations should last for entire days.

Measuring NEP is important because it illustrates whether a habitat is a net source or sink of carbon. Cryoconite holes could be particularly active sites of microbial activity, and understanding their NEP tells us about their influence on carbon cycling. The video linked below (shared from Youtuber “Cryoconite314”) shows a fascinating time lapse of cryoconite hole dynamics on Qaanaaq Glacier in 2012.

 

References:

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.

Hodson, A. and 6 others. 2010b. The structure, biological activity and biogeochemistry of cryoconite aggregates upon an Arctic valley glacier: Longyearbreen, Svalbard. J. Glaciol., 56(196), 349–362.

Telling, J, Anesio, A, Hawkings, J, Tranter, M, Wadham, J, Hodson, A, Irvine-Fynn, T & Yallop, M 2010, ‘Measuring rates of gross photosynthesis and net community production in cryoconite holes: a comparison of field methods’. Annals of Glaciology, vol 51(56)., pp. 153 – 162

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)