Humic substances degradation by a microbial consortium enriched from subarctic tundra soil

Abstract

The largest constituent of soil organic matter in polar cold regions, humic substances (HS), are natural aromatic heteropolymers, with a composition similar to lignin. The microbes in subarctic tundra soil from Alaska, USA, were able to degrade humic acids (HA, a major component of HS) during microcosm experiments at a low temperature of 5？C, which is similar to natural soil temperature during the thawing period (an average temperature of 5.6？C in 2011-2012). The relative abundance of HA decreased to approximately 71% compared with the non-incubated soil control (100%). The microbes, however, were unable to degrade HA at 25？C, which is in the ideal soil temperature range for planting most plants. When enriched at 15？C in liquid mineral medium provided with HA as a sole carbon source, the HA-enriched microbial consortium was metabolically activated to degrade abundant soil carbons (e.g., 4-hydroxybenzoic acid and D-cellobiose) and completely degraded 2-methoxy phenols (ferulic and vanillic acids), which are lignin-derived mono-aromatics. Our data indicate that the microbial community of Alaska tundra soil is cold-adapted and symbiotically degrades HS, possibly via a bacterial lignin-catabolic pathway in which vanillic acid is a primary metabolite. To our knowledge, this is the first report describing a HS-degradative pathway at the microbial consortium level.