Suppl Files: Redox gradients structure microbial communities in polygonized Arctic wet tundra soils

This study investigated how microbial community structure and diversity varied with depth and

microtopography in ice wedge polygons of wet tundra of the Arctic Coastal Plain in northern

Alaska, and what soil variables explain these spatial patterns. We observed strong changes in

community structure and diversity with depth, and more subtle changes between areas of high

and low microtopography, with the largest differences apparent near the soil surface. These

patterns arose primarily from redox gradients (measured using the ratio of reduced Fe to total Fe

in acid extracts as a proxy): conditions grew more reducing with depth and were most oxidized

in shallow regions of polygon rims. Organic matter and pH also changed with depth and

microtopography, but were less effective predictors of the microbial community structure and

relative abundance of specific taxa. Of all other measured variables, lactic acid concentration

was the best, in combination with redox, for describing the microbial community. We conclude

that redox conditions are the dominant force in shaping microbial communities in this landscape.

Oxygen and other electron acceptors allowed the greatest diversity of microbes to flourish: in

their absence the community was reduced to a simpler core of anaerobes, dominated by

fermenters.