Psychrophiles from biodiversity to biotechnology (second edition) Psychrophiles from biodiversity to biotechnology (second edition)
Microbial Diversity and Activity in Cold Ecosystems
Boundary Conditions for Microbial Life in the Cold
The Climate of Snow and Ice as Boundary
Michael Kuhn and Andrew G Fountain
The microclimate and structure of snow and ice act as both boundary conditions and a matrix for microbial life in alpine and polar environments Biological activity critically depends on energy, water and nutrients, with solar radiation serving as the primary energy source whose availability varies with latitude and altitude The energy balance at the snow or ice surface sets the boundary conditions for the fluxes of energy and water, and displays significant latitudinal differences from temperate to polar regions Extreme settings—such as sunlit rocks surrounded by snow and Antarctic cryoconite holes—illustrate how ice, water, solar radiation and nutrients interact in ways that shape microbial life.
1.1 The Source of Energy: Solar Radiation 4
1.2 Distribution of Energy: The Energy Balance of Snow and Ice 7
1.3 Air Temperature: Effects of Altitude and Latitude 8
1.5 The Cryosphere: A Matrix for Life 12
1.6 Liquid Water in the Cryosphere 14
1.7 Hot Spots in the Ice 16
1.8 Ice as a Component of the Biosphere 17
Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria e-mail: michael.kuhn@uibk.ac.at
Department of Geology, Portland State University, Portland, OR 97201, USA © Springer International Publishing AG 2017
R Margesin (ed.), Psychrophiles: From Biodiversity to Biotechnology,
1.1 The Source of Energy: Solar Radiation
At the Earth's surface, the biosphere, atmosphere, hydrosphere, and lithosphere interact most extensively, with the cryosphere adding complexity in mountains and polar regions Biogeochemical cycles are driven by vertical exchanges of energy and water locally and by the horizontal movement of air, trace gases, and water within the global circulation The energy absorbed from solar radiation heats the ground, snow, or water, which in turn warms the overlying air via turbulent convection, drives evaporation or melting and sublimation of ice, and is partly re-emitted as infrared radiation Solar radiation, the primary energy source for climatic and biological processes, shows strong daily and seasonal variation in mid- and high-latitudes; this variation is illustrated by its reference value—the theoretical solar energy input if no atmosphere existed—and daily sums depicted in Fig 1.1 by latitude and time of year While the tropics receive the highest annual sums, polar regions can experience the greatest daily totals during their respective summers, with Antarctica receiving more energy than the Arctic because the Earth is closest to the Sun during the Austral summer.
Gases, aerosols, and clouds in the Earth's atmosphere reduce solar intensity through absorption, scattering, and reflection, and their effects vary spatially and with altitude Global irradiance—the total solar radiation reaching the surface—combines direct sunlight and diffuse radiation, and its value depends on atmospheric composition and weather conditions Understanding how atmospheric components affect direct and diffuse solar radiation is essential for climate studies, solar energy planning, and surface energy balance assessments.
Fig 1.1 Daily sums of extraterrestrial solar irradiance, the reference amount of energy that would be received without atmosphere Values are given in MJ m 2 day 1 computed for a solar constant of 1368 W m 2 Figure courtesy of C Fr € ohlich was compiled from records at Austrian stations in Fig.1.2, as function of cloudiness and altitude Results show an increase of global irradiance of the order of 1% per
100 m altitude at mean cloudiness and a decrease by 50% when comparing cloud- less and cloud covered sky at an altitude of 3000 m.
At 3000 m altitude, the daily average solar irradiance reaches about 400 W/m^2, with an instantaneous noon peak of roughly 1000 W/m^2 (Fig 1.2) In the Dry Valleys of Antarctica near sea level, the daily average is around 300 W/m^2 and the noon maximum about 400 W/m^2 (Hoffman et al 2008) This alpine daily average amounts to roughly 83% of extraterrestrial irradiance, a fraction similar to that observed in central Antarctica during the summer solstice Figures 1.1 and 1.2 show that this fraction decreases at lower solar elevations.
A significant portion of incident solar radiation is reflected upward toward the atmosphere, where clouds, aerosols and air molecules can scatter energy back toward the snow surface, thereby influencing the total solar radiation that interacts with snow Higher surface reflectance, also known as albedo, increases the amount of light reflected by the snow surface and further affects the global solar radiation balance.
Fig 1.2 Daily averages of global irradiance at eastern alpine stations, according to altitude and cloudiness, based on data by Dirmhirn
Snow and ice shape the boundary conditions for microbial life by increasing the diffuse portion of incoming solar radiation This effect is evident when comparing radiation fluxes at the Sonnblick Observatory (3110 m) in the Austrian Alps with the valley station of Innsbruck (580 m a.s.l.) When the surrounding landscape is snow-covered, multiple reflections significantly amplify the diffuse component of global solar radiation, resulting in greater diffuse light at the mountain site than at the valley station.
The spatial and temporal variability of snow cover and clouds drives the seasonal and short-term fluctuations in direct and diffuse solar radiation, and in their sum—the global radiation These fluctuations are not apparent in the smoothed values shown in Fig.1.2.
Dry alpine or polar snow exhibits a broad-band albedo typically above 80%, reaching around 90% in the visible and ultraviolet, while dropping to less than 20% in the near-infrared In the thermal infrared, snow is nearly a perfect absorber and emitter, with an emissivity close to 0.98 The solar incidence angle also affects albedo; at low solar angles common in polar regions, forward scattering increases albedo to over 90% Snow and ice albedo also depend on snow grain size and liquid water content, decreasing as grain size increases and as liquid water content rises Clean alpine snow, initially with albedo above 80%, drops to about 60–70% by the end of the observation period.
Figure 1.3 illustrates the surface solar radiation fluxes at Sonnblick Observatory (3150 m) and the Innsbruck valley station (580 m) using 2012–2015 average values Diffuse radiation (DIF) responds to atmospheric aerosols and clouds and to surface–atmosphere multiple scattering, with the effect most evident in spring (March–May) when snow cover at Sonnblick yields significantly higher diffuse radiation than snow-free Innsbruck Correspondingly, global radiation (GLO) represents the sum of direct and diffuse components The data are provided by the ARAD Project (Olefs et al., 2016) for the summer season In addition, the presence of dust, biological materials, or other particulates further reduces surface albedo.
Ice albedo is largely controlled by cracks and air bubbles, with clean alpine glacier ice reflecting about 40% of incoming light while dust- and dirt-covered ice may reflect as little as 15–20%, similar to surrounding rocks In Antarctica, blue ice forms at the surface after deep burial under about 1100 meters of ice that exerts roughly 100 bars of hydrostatic pressure, which dissolves air bubbles into the crystal lattice This bubble-free blue ice appears as the darkest naturally occurring ice.
1.2 Distribution of Energy: The Energy Balance of Snow and Ice
Solar radiation is the Earth's primary energy source, providing a global annual average of about 240 W/m^2 Geothermal heat, supplied by the hot interior of the Earth and by radioactive decay, amounts to roughly 60 mW/m^2—negligible in comparison with solar radiation, but of vital importance at the bases of ice sheets where it drives basal melting and influences ice dynamics.
The snow surface—the cryosphere–atmosphere interface—provides the clearest example of the energy balance, with incoming shortwave solar radiation (S#) supplemented by incoming atmospheric infrared longwave radiation (L#) emitted by greenhouse gases, clouds, and aerosols, while portions are reflected and emitted back to the atmosphere as S" and L" The sum of these four fluxes (S#, L#, S", and L") constitutes the radiation balance, and the energy interacting with the surface is distributed among these four components.
Heat flux to and from the snow surface occurs by conduction through the snowpack Internal heating of the snow may also occur due to solar radiation penetrating the snow and via air convection in the snow’s pore spaces, which may also cool the snow.
2 Turbulent transfer of sensible heatHto/from the atmosphere
3 Turbulent transfer of latent heat of evaporation, sublimation, or condensation LE
4 The latent heat of melting or refreezing LM
Water in contact with the ice of lakes and glacial ice shelves may supply additional sensible and latent heat.
All fluxes are defined positive if they deliver energy to the surface so that, at the surface, their total must be zero.
These quantities are typically expressed as energy flux densities in watts per square meter (W/m²) Because they depend on atmospheric variables that are not determined locally, it is essential to obtain information about the local climatic boundary conditions to accurately characterize them.
1 The Climate of Snow and Ice as Boundary Condition for Microbial Life 7
1.3 Air Temperature: Effects of Altitude and Latitude
The change of air temperature and other climatic conditions with altitude in mid-latitude mountains has often been compared to their change with latitude: a