Abstract
The active layer of soil overlaying permafrost in the Arctic is subjected to dramatic annual changes in temperature and soil chemistry, which likely affect bacterial activity and community structure. We studied seasonal variations in the bacterial community of active layer soil from Svalbard (78°N) by co-extracting DNA and RNA from 12 soil cores collected monthly over a year. PCR amplicons of 16S rRNA genes (DNA) and reverse transcribed transcripts (cDNA) were quantified and sequenced to test for the effect of low winter temperature and seasonal variation in concentration of easily degradable organic matter on the bacterial communities. The copy number of 16S rRNA genes and transcripts revealed no distinct seasonal changes indicating potential bacterial activity during winter despite soil temperatures well below −10°C. Multivariate statistical analysis of the bacterial diversity data (DNA and cDNA libraries) revealed a season-based clustering of the samples, and, e.g., the relative abundance of potentially active Cyanobacteria peaked in June and Alphaproteobacteria increased over the summer and then declined from October to November. The structure of the bulk (DNA-based) community was significantly correlated with pH and dissolved organic carbon, while the potentially active (RNA-based) community structure was not significantly correlated with any of the measured soil parameters. A large fraction of the 16S rRNA transcripts was assigned to nitrogen-fixing bacteria (up to 24% in June) and phototrophic organisms (up to 48% in June) illustrating the potential importance of nitrogen fixation in otherwise nitrogen poor Arctic ecosystems and of phototrophic bacterial activity on the soil surface.
Highlights
Permafrost covers approximately 25% of the land area in the northern hemisphere not covered by ice sheets (Zhang et al, 2008), and the surface of an even larger fraction of the land area undergoesSeasonal variation of active layer bacteria seasonal freezing
Even though the Arctic is currently greening due to expansion of, e.g., scrub (Elmendorf et al, 2012), plant cover on the tundra often allows enough photosynthetically active radiation to reach the soil surface for surface-dwelling Cyanobacteria and other phototrophic microorganisms to proliferate
At RNA level, the analysis revealed no significant effect of any of the measured soil parameters, while at DNA level 44.6% of the variation in bacterial community structure was explained by the analysis. pH and dissolved organic carbon correlated significantly (P < 0.05) with the data on bacterial community structure at DNA level explaining 24.6 and 20.0% of the variation, respectively
Summary
Permafrost covers approximately 25% of the land area in the northern hemisphere not covered by ice sheets (Zhang et al, 2008), and the surface of an even larger fraction of the land area undergoesSeasonal variation of active layer bacteria seasonal freezing. Permafrost is estimated to contain 1024 Pg of organic carbon in the uppermost 3 m corresponding to ∼30% of the global soil organic carbon pool (Tarnocai et al, 2009). When permafrost thaws, it is transformed into an active layer of soil where the microbial activity is likely to increase (Elberling et al, 2013; Knoblauch et al, 2013). Input of organic matter produced by phototrophic microorganisms may be of ecological importance in Arctic soils. Arctic soils receive low rates of atmospheric nitrogen deposition (Dentener et al, 2006), which makes fixation of atmospheric nitrogen by free-living and plant-associated microorganisms the primary input of nitrogen in terrestrial Arctic ecosystems (Reed et al, 2011)
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