title: Anaerobic decomposition of humic substances by Clostridium from the deep subsurface.
abstract: Decomposition of humic substances (HSs) is a slow and cryptic but non-negligible component of carbon cycling in sediments. Aerobic decomposition of HSs by microorganisms in the surface environment has been well documented; however, the mechanism of anaerobic microbial decomposition of HSs is not completely understood. Moreover, no microorganisms capable of anaerobic decomposition of HSs have been isolated. Here, we report the anaerobic decomposition of humic acids (HAs) by the anaerobic bacterium Clostridium sp. HSAI-1 isolated from the deep terrestrial subsurface. The use of (14)C-labelled polycatechol as an HA analogue demonstrated that the bacterium decomposed this substance up to 7.4% over 14 days. The decomposition of commercial and natural HAs by the bacterium yielded lower molecular mass fractions, as determined using high-performance size-exclusion chromatography. Fourier transform infrared spectroscopy revealed the removal of carboxyl groups and polysaccharide-related substances, as well as the generation of aliphatic components, amide and aromatic groups. Therefore, our results suggest that Clostridium sp. HSAI-1 anaerobically decomposes and transforms HSs. This study improves our understanding of the anaerobic decomposition of HSs in the hidden carbon cycling in the Earth's subsurface.
authors.affiliation: Horonobe Research Institute for the Subsurface Environment, Northern Advancement Centre for Science and Technology, 5-3, Sakae-machi, Horonobe-cho, Teshio-gun, Hokkaido 098-3221, Japan.
authors.forename: Akio
authors.initials: A
authors.lastname: Ueno
authors.affiliation: Horonobe Research Institute for the Subsurface Environment, Northern Advancement Centre for Science and Technology, 5-3, Sakae-machi, Horonobe-cho, Teshio-gun, Hokkaido 098-3221, Japan.
authors.forename: Satoru
authors.initials: S
authors.lastname: Shimizu
authors.affiliation: Horonobe Research Institute for the Subsurface Environment, Northern Advancement Centre for Science and Technology, 5-3, Sakae-machi, Horonobe-cho, Teshio-gun, Hokkaido 098-3221, Japan.
authors.forename: Shuji
authors.initials: S
authors.lastname: Tamamura
authors.affiliation: Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan.
authors.forename: Hidetoshi
authors.initials: H
authors.lastname: Okuyama
authors.affiliation: Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Naganuma
authors.affiliation: Horonobe Research Institute for the Subsurface Environment, Northern Advancement Centre for Science and Technology, 5-3, Sakae-machi, Horonobe-cho, Teshio-gun, Hokkaido 098-3221, Japan.
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Kaneko
mesh_terms: D000693:Anaerobiosis; D001421:Bacteria, Anaerobic / Q000378:metabolism*; D001673:Biodegradation, Environmental; D040761:Biomimetic Materials; D002247:Carbon Isotopes; D002396:Catechols / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003013:Clostridium / Q000378:metabolism*; D019015:Geologic Sediments / Q000382:microbiology*; D006812:Humic Substances / Q000032:analysis*; D007700:Kinetics; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D002247:Carbon Isotopes; D002396:Catechols; D006812:Humic Substances