Global climate change is predicted to impact the Arctic more strongly than other ecosystems/biomes, resulting in proportionally greater warming. With this warming will come longer growing seasons, but possibly more importantly, milder winters. The Arctic will never be truly free of winter because of sunset, but milder periods at the beginning and end of winter may be prolonged. During these shoulder periods, when soil is moderately frozen (> -10°C), soil microorganisms remain active.

Microbial communities that are active in frozen soil appear functionally different from growing season communities, as substrate use shifts as temperatures fall below freezing. However, it is unknown whether winter-active communities are merely a subset of the summer-active communities. It is also unknown how members of the microbial community use different carbon substrates under frozen conditions. The answers to these questions have important implications for Arctic ecosystem function under a changing climate. Since winter already accounts for 1/3 of annual ecosystem CO₂ flux, lengthening early and late winter may increase the amount of CO₂ produced at a time when photosynthesis cannot balance it. The balance of tundra types is also predicted to shift in favor of shrub tundra over tussock tundra.

To examine microbial community structure and function in winter vs. growing season, we collected tundra soils from Toolik Lake, AK before and after thaw and freeze in 2005. We looked at community composition and substrate use using ¹³C-substrates and bromo-deoxyuridine (BrdU), a thymidine analog that allows isolation of newly synthesized DNA.

Results/Conclusions

We measured ¹³C enrichment of biomarker phospholipid fatty acids and respired CO₂ as indicators of activity. Using immunocapture, we isolated DNA of actively growing bacteria, which we analyzed using T-RFLP. Active shrub bacteria differ between summer and winter whereas active tussock bacteria are relatively stable between seasons. Not all taxonomic groups were equally active in all soils. Generally, fungi were more active in winter and bacteria in summer, though in tussock tundra both fungi and Gram(-) bacteria were consistently active throughout the year. Microbes also used substrates differently depending on season.

In conclusion, Arctic soil microbes were actively growing in frozen soil. Shrub tundra microbial communities were more dynamic seasonally relative to tussock tundra, suggesting that whereas tussock tundra may be relatively insensitive to climate change, prolonged mild winter conditions may greatly affect shrub tundra communities, changing the balance of microbial processes in ways we cannot yet predict.