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Mountain Permafrost Pages Gruber, Stephan et al. Fungi in Permafrost Pages Ozerskaya, Svetlana et al. Show next xx. However, the control mechanisms of methane production, oxidation and emission from tundra environments are still not completely understood. Permafrost relates to permanently frozen ground with a shallow surface layer of several centimetres the active layer that thaws only during the short summer period. During the short arctic summer, permafrost soils also show a large temperature gradient along their depth profiles, and this is one of the main environmental factors that influence the microbial communities in these extreme habitats Kotsyurbenko et al.
Water is another important factor for microbial life in these environments. The seasonal thawing of the upper permafrost promotes water saturation of the soils, leading to anaerobic degradation of complex organic matter to simple compounds, such as acetate, H 2 , CO 2 , formate and methanol, by fermentative bacteria. These compounds serve as substrates for methanogenic archaea, which are responsible for the production of CH 4 Garcia et al. Methanogenic archaea, which belong to the kingdom Euryarchaeota , are ubiquitous in anoxic environments.
They can be found both in moderate habitats such as rice paddies Grosskopf et al. Several studies have revealed the presence of methanogens in high-latitude peatlands by finding sequences of 16S rRNA gene and methyl coenzyme M reductase mcrA genes affiliated with Methanosarcinaceae , Methanosaetaceae , Methanobacteriaceae and Methanomicrobiales Galand et al. It has recently been shown using FISH and phospholipid analyses that the active layer of Siberian permafrost is colonized by high numbers of bacteria and archaea with a total biomass comparable to that of temperate soil ecosystems Kobabe et al.
The present investigation is part of a long-term study on carbon dynamics and microbial communities in permafrost-affected environments in the Lena Delta, Siberia Hubberten et al. The overall purpose of this study was a basic characterization of the methanogenic communities in different extreme habitats of the Laptev Sea coast using both physiological and molecular ecological methods. In addition, the potential methane production was analysed under various temperature and substrate conditions.
Soil samples were collected at various sites on the Laptev Sea coast, northeast Siberia during two Russian—German expeditions in and Further details of the study sites can be found in Schwamborn et al. Soil and vegetation characteristics show great variation over small distances owing to the geomorphological situation of the study sites Fiedler et al.
The floodplain was characterized by recent fluvial sedimentation, whereas the polygon centre was characterized by peat accumulation with interspersed sand layers. The vegetation at the floodplain site was dominated by Arctophila fulva. In the polygon centre, typical plants were Sphagnum mosses, Carex aquatilis and lichens. The vegetation here differed from that of the polygon centre on Samoylov Island and was dominated by Eriophorum spp.
For soil sampling, vertical profiles were arranged and samples were taken from defined soil horizons for physicochemical e. CH 4 concentration, dissolved organic carbon and total organic carbon contents and microbiological e. For detailed investigations, horizons with a thickness of more than 10 cm were divided and subsamples were taken.
The depth of the permafrost table was measured by driving a steel rod into the unfrozen soil until frozen ground was encountered. The water table was measured in perforated plastic pipes that were installed in the active layer. Vertical profiles of soil CH 4 concentrations were obtained by extracting CH 4 from fresh soil samples by adding 10 g of soil to saturated NaCl solution, shaking the solution, and subsequently analysing the CH 4 headspace concentration with gas chromatography.
Dissolved organic carbon DOC was extracted from various horizons of the soil profiles. Fresh soil material 9 g was taken from each horizon, weighed into glass jars 50 mL and mixed with 45 mL of distilled water. The bottles were closed and shaken for 1 h in the dark. Afterwards, the suspension was filtered 0. The substrates were chosen according to previous results obtained for the same study site, which showed that hydrogen is more important than acetate for methanogenesis in permafrost soils Wagner et al.
Under anoxic conditions, mL glass bottles were filled with 10 g of soil material, and 3 mL of sterile water was added.
All bottles were sealed with sterile butyl rubber stoppers. In the case of methanol as additional substrate, 0. Three replicates were used for the different experiments. CH 4 production was measured daily over a period of one week by sampling the headspace using a Hamilton gastight syringe. Helium was used as carrier gas. CH 4 production rates were calculated from the linear increase in CH 4 concentration. DNA was extracted directly from 0.
The quality and quantity of DNA were controlled on 0. In some cases, extracted DNA from permafrost soils was diluted fold. DNA bands that appeared sharp and clear in the gel were cut out with a sterile scalpel and were transferred to sterile 0. The physicochemical soil properties of the investigated sites showed a large vertical gradient and high small-scale variability in dependence of microrelief of the different permafrost soils Table 1.
Horizon nomenclature and soil classification according to Soil Survey Staff The soil of the polygon centre on Mamontovy Klyk was classified as a Typic Aquiturbel. The water level reached about 1 cm below the soil surface and the perennially frozen ground started at 44 cm. The comparable centre soil on Samoylov Island was a Typic Historthel , with a water table near the soil surface and the permafrost beginning at 33 cm depth.
The floodplain soil on Samoylov Island was classified as a Typic Aquorthel.
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The water table was near the soil surface and the permafrost started at 54 cm depth. In contrast to the other two permafrost soils, the floodplain soil on Samoylov Island was characterized by a silty soil texture with organic carbon contents between 0. The DOC concentration varied between 4. The CH 4 concentration increased with increasing soil depth and showed values from 0. The CH 4 production of the three different soils showed significant differences in the rate of activity and vertical distribution Fig.
In general, the activity in each profile was higher with hydrogen or methanol as additional substrate than it was without any substrate. Dashed lines indicate the permafrost tables. The highest CH 4 production rate within the centre soil Typic Aquiturbel on Mamontovy Klyk was found with hydrogen as substrate, followed by methanol as substrate.
Global warming and carbon dynamics in permafrost soils: methane production and oxidation
Without any substrate addition, only a limited activity was detectable Fig. The activity was highest in the two upper horizons, and decreased with increasing soil depth. The activity pattern of the other two studied sites on Samoylov Island was different from that for Mamontovy Klyk. The floodplain soil Typic Aquorthel showed two maxima of CH 4 production, one in the upper soil horizon and a second in the zone with the highest root density at a depth between 20 and 35 cm Fig.
Here, the highest activity was measured in the upper soil horizon with methanol, while the CH 4 production rates in all other horizons were higher with hydrogen as substrate.
New global study reveals rising soil temperatures in permafrost regions around the world
The soil Typic Historthel of the polygon centre on Samoylov Island was characterized by the highest CH 4 production taking place in the upper soil horizons. This was also observed for the comparable soil on Mamontovy Klyk Fig.
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However, in contrast to the latter soil, high activity also occurred in the polygon centre on Samoylov Island in the bottom zone of the active layer close to the permafrost table with a temperature near to the freezing point of water. With the exception of the bottom horizon, where the highest CH 4 production occurred with methanol, the preferred substrate in all other horizons was hydrogen. The effect of increasing temperature was different for the three sites, as well as in the vertical profile of each soil.
In general, the methane production activity in the upper part of the active layer of all soils rose after the increase of temperature more strongly than it did within the bottom part of the profiles near the permafrost table. Three permafrost sites on the Laptev Sea coast were compared with regard to variation in the community structure of methanogenic archaea from the top to the bottom of the investigated soil profiles. DGGE profiles showed up to nine well-defined bands per depth, and a shift within the vertical profiles of Samoylov Island polygon centre Typic Historthel and Mamontovy Klyk polygon centre Typic Aquiturbel.
In the polygon centre on Samoylov Island Typic Historthel , the number of DNA bands increased to a depth of 23 cm zone of highest root density and then decreased again Fig. The number of bands in the polygon-centre soil on Mamontovy Klyk was constant to a depth of 22 cm, with about four DNA bands in each lane Fig. Most DNA bands were observed in the middle of the profile 22—29 cm soil depth and this number decreased with increasing soil depth, as was also observed for the soil of the polygon centre on Samoylov Island Fig. Interestingly, the floodplain on Samoylov Island showed a completely different pattern.
Here, the number of bands did not decrease with increasing depth Fig. Even the soil horizon close to the permafrost table showed a diversity of methanogens comparable with the highest diversity in the middle of the two other profiles. Selected bands marked with arrows and sample IDs were used for sequence analyses. Besides the number of bands, the distribution pattern showed distinct differences, particularly within the vertical profiles of the polygon-centre soils on Samoylov Island soil depth 20—23 cm compared with the bottom of the active layer and Mamontovy Klyk the first horizon in comparison with the bottom of the active layer.
Beside these unique bands, some other bands that did not occur throughout the whole soil profile could also be seen.
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For example, DGGE bands corresponding to MAK a Methanomicrobiaceae were found only in the middle of the soil profile at a depth of 6—29 cm, and bands corresponding to MAK b Methanosarcinaceae were found only in the deeper regions of the soil profile. A total of 36 DGGE bands from three soil profiles were sequenced.
All sequences can be differentiated at the genus level. The sequences recovered from permafrost belong to Methanosarcinaceae a , and Methanomicrobiales together with Rice cluster II b. The scale bar represents 0. Identification of the bands is shown in Fig. Clone name, accession number, environment and length of each sequence are indicated. Sequences affiliated to Methanosarcinaceae and Methanomicrobiaceae were found in all studied soil profiles, whereas sequences associated with Methanosaeta were found only in the floodplain and in the polygon centre on Samoylov Island, but not in the polygon centre of Cape Mamontovy Klyk.
Our results showed differences in the CH 4 production activities and the biodiversity patterns of methanogenic archaea in the investigated permafrost soils. Activities of methanogenic archaea differed significantly in their rates and distributions among the different soils. While the CH 4 production rates in the active layer on Mamontovy Klyk decreased with increasing soil depth, the two other sites on Samoylov Island showed at least two activity optima.
The highest activity occurred in the upper soil horizons, which are characterized by in situ temperatures of up to The second optimum of methane production was found in the middle or bottom part of the active layer in both soils on Samoylov Island. Here, the second activity optimum correlates with the zone of the highest root density and amount of DOC. It is well known that plants can supply root exudates consisting of low-molecular-weight organic compounds, which can serve as a substrate for methanogens Chanton et al.
However, the extraordinarily high CH 4 production rates in the upper layer of Mamontovy Klyk correlates with the high amount of organic carbon in these horizons. The addition of different substrates led to an increase in the potential CH 4 production in all horizons of all sites. This effect was not confined to horizons with a low content of organic carbon, but could also be observed in horizons with a high amount of organic carbon. Wagner et al. This was shown to be reciprocally correlated with the amount of bioavailable organic carbon.
A reduced quantity and quality of organic matter in permafrost soils could lead to a substrate-limited methanogenesis. A higher incubation temperature resulted in a marked increase of the methanogenic activity in almost all investigated soil horizons. It is noteworthy that the effect of higher temperature on the activity was larger in the upper soil horizons with higher in situ temperatures than in the bottom of the active layer with lower in situ temperatures.
Hence, taking into consideration the physiological studies, we can conclude that the activity of methanogenic archaea in permafrost soils depends on the quality of soil organic carbon, and our results show that methanogens in deep active-layer zones might be better adapted to low temperatures. Only a few psychrophilic strains of methanogenic archaea have been described so far Simankova et al. However, our results indicate a shift in the methanogenic community from mesophilic to psychrotolerant or psychrophilic methanogens with increasing soil depth.
An important requirement for microorganisms to adapt to cold environmental conditions is constantly low in situ temperatures over a long period of time Morita, This is the case in the bottom zone of the active layer close to the perennially frozen ground. A prerequisite for prokaryotes to adapt to low temperatures is that their cell membranes should maintain fluidity. This effect was shown in a related study, carried out for the centre profile on Samoylov Island, which revealed an increase of branched-chain fatty acids in relation to the amount of straight-chain fatty acids with increasing active-layer depth Wagner et al.
The DGGE pattern of the investigated permafrost soils showed differences within the depth profile and between the different sites. The number of DNA bands at the floodplain site on Samoylov Island remained fairly constant through the whole profile. While the temperature drastically decreased with soil depth, the carbon DOC and TOC and nitrogen concentrations in the profile remained relatively constant.
These geochemical profiles can be explained by the fact that the floodplain is periodically flooded by the Lena River. Thus the vegetation is regularly buried by the accumulation of new sediments, which causes the even distribution of organic matter in the profile. Galand et al. The similarity of the community pattern for the whole soil profile of the floodplain can probably be attributed to the regular sedimentation at this site, but a significant relationship between this pattern and the methane production as reported by Galand et al.
In contrast to the profile for the floodplain site, the polygon-centre profiles for Mamontovy Klyk and on Samoylov Island showed a variety of diversity patterns.
These soils were characterized by humus accumulation in the upper part of the active layer, with decreasing organic matter content in the underlying mineral soils. However, the number of bands increased until the zone with the highest root density, but started to decrease in the deeper zones of the active layer.
The presence of root exudates Chanton et al. Among the differences in the number of detected DNA bands within the various horizons of the vertical profiles, different band patterns indicated differences in the community structure of methanogens, particularly in the polygon-centre profiles on Mamontovy Klyk and on Samoylov Island. These differences refer to the bottom zone of the active layer compared with horizons, which lie further above in the respective profiles. A depth-related change of the methanogenic community was also observed in northern peatlands Galand et al.
The results of the DGGE analysis indicate changes of the methanogenic community within the vertical soil profiles. Some DGGE bands appeared throughout the whole profile, while others were specific for distinct active-layer depths. Moreover, the band pattern showed distinct differences between specific horizons. On one hand this indicates the presence of methanogenic archaea that can exist under different environmental conditions temperature, substrate, geochemistry , which are changing within the depth of the active layer. On the other hand, it indicates the presence of methanogens that can exist only under defined environmental conditions.