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.

本研究以阿拉斯加北部北极海岸平原湿苔原的冰楔多边形（ice wedge polygons）生境为研究对象，探究了微生物群落结构与多样性随深度和微地形（microtopography）的变化规律，以及哪些土壤变量可阐释此类空间分布格局。本研究观测发现，群落结构与多样性随深度发生显著变化，而高低微地形区域间的变化则相对平缓，其中最显著的差异出现在土壤表层附近。上述格局主要由氧化还原梯度（redox gradients）主导形成：以酸提取物（acid extracts）中还原态铁（reduced Fe）与总铁（total Fe）的比值作为替代指标（proxy），随深度增加环境还原性逐渐增强，而冰楔多边形边缘的浅表层区域氧化程度最高。有机质含量与pH值同样随深度和微地形发生变化，但二者对微生物群落结构及特定类群（taxa）相对丰度的预测能力相对较弱。在所有其余测定变量中，乳酸浓度与氧化还原指标结合时，对微生物群落的解释效果最佳。本研究得出结论：氧化还原条件是调控该景观中微生物群落构建的主导因素。氧气与其他电子受体（electron acceptors）能够支持最丰富的微生物类群繁衍；在缺乏此类物质的环境中，微生物群落会简化为以发酵菌（fermenters）为主的厌氧菌（anaerobes）核心类群。