Study on humic substances degradation by a microbial consortium from subarctic tundra soil

Abstract

The largest constituent of soil organic matter, humic substances (HS), is naturally present as aromatic heteropolymers, with a composition similar to lignin. The microbes in a sample of subarctic tundra soil (Alaska, USA) were able to degrade humic acids (HA, a major component of HS) during microcosm experiments at a low temperature of 5℃, which is similar to natural soil temperature during the thawing period (average temperature of 5.6℃ at a depth of 20 cm). 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℃, which is in the ideal soil temperature range for planting most plants. When enriched at 15℃ 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-hydroxy benzoic 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 consortium of Alaska tundra soil, which dominantly contains phylum Proteobacteria (specifically, class Betaproteobacteria), is cold-adapted and symbiotically degrades HS, possibly via a bacterial lignin-catabolic pathway in which vanillic acid is a main metabolite. To our knowledge, this is the first report describing a HS-degradative pathway at the microbial consortium level.