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

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