Abstract
AbstractWarming climate is thawing the permafrost in arctic and subarctic regions, leading to formation of thermokarst ponds. During the formation and geomorphological succession of these ponds, carbon that has been trapped in frozen soils for thousands of years is hydrologically mobilized and returned to the active carbon cycle. We sampled 12 thermokarst ponds representing three different stages of pond succession to study the potential of microbial communities to metabolize the organic carbon in the water. We investigated the quality of the dissolved organic carbon (DOC) in the water column based on the spectrophotometric and fluorometric properties of the chromophoric dissolved organic matter combined with parallel factor analysis and the potential of the microbial community for degrading these carbon compounds based on genetic markers related to carbon degradation. Our analysis showed a clear difference in the DOC quality across the different developmental stages. In the younger ponds, organic matter quality suggested that it was originating from the degrading permafrost and in the metagenomes collected from these ponds, the normalized abundance of genes related to degradation of carbon compounds was higher. There was also a shift in the degradation potential in the water column of the ponds, with higher potential for organic matter degradation in deeper, anoxic layers. In conclusion, our results show that the DOC quality and the genetic potential of the microbial community for carbon cycling change across the pond ontogeny, suggesting a capacity of the microbial communities to adapt to changing environmental conditions.
Highlights
The thawing of the permafrost ice contributes to the creation of thermokarst ponds and lakes (Osterkamp et al 2000; Luoto and Seppälä 2003) where the ancient carbon can be reintroduced in the active carbon cycle (Roehm et al 2009)
In order to analyze dissolved organic carbon (DOC) concentrations and perform optical analyses of the chromophoric dissolved organic matter (CDOM), water samples were collected from the surface and filtered through prerinsed cellulose acetate filters (0.2 μm)
Our study shows that both the DOC quality and the microbial potential for carbon cycling shift over time as the permafrost thaws and the ponds develop from small emerging puddles to deep and stratified mature ponds
Summary
The thawing of the permafrost ice contributes to the creation of thermokarst ponds and lakes (Osterkamp et al 2000; Luoto and Seppälä 2003) where the ancient carbon can be reintroduced in the active carbon cycle (Roehm et al 2009) These waterbodies represent one of the most common and widespread aquatic ecosystem types in the arctic and subarctic. The systems are sensitive to stochastic effects such as fires that may be regarded as a key trigger for thaw/regeneration cycles in the permafrost (Zoltai 1993) The lifecycle of these ecosystems is expected to be shorter under warmer climate conditions (Vincent et al 2013) and this will in turn expose a higher number of waterbodies to strong lateral influence from the adjacent carbon-rich permafrost erosion that will enrich the lakes and ponds and stimulate bacterial activity (Vonk et al 2013). In the areas where thawing is ongoing, a shift toward increasing terrestrial dominance of the carbon in freshwaters has already been reported (Wauthy et al 2018)
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