3,483 research outputs found
How do the environmental extremes of Siberian permafrost soils shape the composition of the bacterial soil community?
Microbial communities in permafrost soils of the Siberian Arctic are exposed to extreme environmental conditions. The soils are frozen throughout the entire year except for the short summer period, when thawing of the uppermost 20 to 50 cm of the permafrost sediment allows for the formation of a so-called active layer. Active layers show steep temperature gradients between 10 to 18 °C near the surface and 0 to 1 °C near the permafrost table. Additionally, seasonal freezing and thawing processes lead to the formation of patterns of low-centered polygons. Low-centered polygons determine a pronounced small-scale heterogeneity with regard to their physical and chemical properties between the elevated polygon rims and the depressed polygon centers.Within the active layer of a polygon rim, vertical profiles of potential methane oxidation rates in respond to different temperatures indicated a shift in the temperature optimum from 21 °C near the surface to 4 °C near the permafrost table [1]. This temperature shift could not be shown in samples of the polygon center. Based on these results we used 16S rDNA clone libraries as well as in-situ cell counting to compare the bacterial, in particular the methane oxidizing, community near the surface and near the permafrost table in samples of the polygon rim. The phylogenetic analyses show that the composition of the bacterial community near the surface is significantly different from the bacterial community near the permafrost table. The results also show that bacterial diversity and abundance in Siberian permafrost soils are comparably high as in temperate terrestrial environments.[1] Liebner. S. and Wagner, D. (in press) Abundance, distribution and potential activity of methane oxidizing bacteria in permafrost soils from the Lena Delta, Siberia. Environmental Microbiology doi: 10.1111/j.1462-2920.2006.01120.
Methane fluxes, microbial activities and community structures in a wet tundra of the Lena Delta
Wet tundra environments of the Arctic are natural sources of the climate relevant trace gas methane. The underlying biogeochemical processes are not yet well understood. The field investigations were carried out on the island Samoylov (N 72°, E 126°) located in the Lena Delta, Siberia. The study site represented an area of typical polygonal patterned grounds with ice-wedges, which were considered for analyses of methane fluxes, organic matter quality and microbial communities.The mean flux rate of the depression was 53.2 ± 8.7 mg CH4 m-2 d-1, whereas the mean flux rate of the dryer rim part of the polygon was 4.7 ± 2.5 CH4 m-2 d-1. The quantity of dissolved organic matter (DOM), which represents an important C pool for microbial communities, correlated significant with the total concentrations of phospholipid fatty acids and ether lipids (PLFA and PLEL) a measure for microbial biomass. Although permafrost soils represent a large carbon pool, it was shown, that the reduced quality of organic matter leads to a substrate limitation of the microbial metabolism. This is an important finding for modelling and calculating trace gas fluxes from permafrost environments, because the known models are consider only the total carbon amount.It can be concluded by the presented results firstly that microbial communities in permafrost environments are composed by members of all three domains of life at numbers comparable to temperate soil ecosystems and secondly that the permafrost microorganisms are well adapted to the extreme temperature gradient of their environment
Impact of energy dissipation on interface shapes and on rates for dewetting from liquid substrates
We revisit the fundamental problem of liquid-liquid dewetting and perform a
detailed comparison of theoretical predictions based on thin-film models with
experimental measurements obtained by atomic force microscopy (AFM).
Specifically, we consider the dewetting of a liquid polystyrene (PS) layer from
a liquid polymethyl methacrylate (PMMA) layer, where the thicknesses and the
viscosities of PS and PMMA layers are similar. The excellent agreement of
experiment and theory reveals that dewetting rates for such systems follow no
universal power law, in contrast to dewetting scenarios on solid substrates.
Our new energetic approach allows to assess the physical importance of
different contributions to the energy-dissipation mechanism, for which we
analyze the local flow fields and the local dissipation rates.Comment: 15 pages, 5 figure
Stationary solutions of liquid two-layer thin film models
We investigate stationary solutions of a thin-film model for liquid two-layer
flows in an energetic formulation that is motivated by its gradient flow
structure. The goal is to achieve a rigorous understanding of the contact-angle
conditions for such two-layer systems. We pursue this by investigating a
corresponding energy that favors the upper liquid to dewet from the lower
liquid substrate, leaving behind a layer of thickness . After proving
existence of stationary solutions for the resulting system of thin-film
equations we focus on the limit via matched asymptotic analysis.
This yields a corresponding sharp-interface model and a matched asymptotic
solution that includes logarithmic switch-back terms. We compare this with
results obtained using -convergence, where we establish existence and
uniqueness of energetic minimizers in that limit
Droplets on liquids and their long way into equilibrium
The morphological paths towards equilibrium droplets during the late stages
of the dewetting process of a liquid film from a liquid substrate is
investigated experimentally and theoretically. As liquids, short chained
polystyrene (PS) and polymethyl-methacrylate (PMMA) are used, which can be
considered as Newontian liquids well above their glass transition temperatures.
Careful imaging of the PS/air interface of the droplets during equilibration by
\emph{in situ} scanning force microscopy and the PS/PMMA interface after
removal of the PS droplets reveal a surprisingly deep penetration of the PS
droplets into the PMMA layer. Droplets of sufficiently small volumes develop
the typical lens shape and were used to extract the ratio of the PS/air and
PS/PMMA surface tensions and the contact angles by comparison to theoretical
exact equilibrium solutions of the liquid/liquid system. Using these results in
our dynamical thin-film model we find that before the droplets reach their
equilibrium they undergo several intermediate stages each with a well-defined
signature in shape. Moreover, the intermediate droplet shapes are independent
of the details of the initial configuration, while the time scale they are
reached depend strongly on the droplet volume. This is shown by the numerical
solutions of the thin-film model and demonstrated by quantitative comparison to
experimental results
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