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. 2016 Jan 8;6:18990.
doi: 10.1038/srep18990.

Anaerobic Decomposition of Humic Substances by Clostridium From the Deep Subsurface

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Anaerobic Decomposition of Humic Substances by Clostridium From the Deep Subsurface

Akio Ueno et al. Sci Rep. .
Free PMC article

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.

Figures

Figure 1
Figure 1. 14CO2 evolution in an anaerobic culture of Clostridium sp. strain HSAI-1.
The 14CO2 in the headspace gas was measured via liquid scintillation counting of inoculated cultures (filled circles). Uninoculated control cultures (open circles) yielded little or no 14CO2. The data points are the mean values of triplicate samples ± standard deviations. *P < 0.05 was considered significant in Student’s t-test.
Figure 2
Figure 2. HSs are decomposed anaerobically by Clostridium sp. strain HSAI-1.
Left column, chromatograms of Aldrich HAs at 0 and 28 days. Right column, chromatograms of Koetoi HAs at 0 and 28 days. Each experimental group consisted of 5 culture setups (n = 5). One representative chromatogram from each experimental group is shown. For clarity, chromatograms from uninoculated control cultures are depicted in blue, and those from inoculated cultures (using strain HSAI-1) are shown in magenta. The tops of the main peaks with retention times ranging from 7.8 to 8.3 min are indicated with vertical lines: a dotted line for the uninoculated controls and a solid line for the inoculated cultures. The peak indicated by the downward-pointing arrowhead corresponds to the high-molecular mass HAs in cultures inoculated with strain HSAI-1.
Figure 3
Figure 3. FT-IR spectra of HAs.
Left column, Aldrich HAs. Right column, Koetoi HAs. Representative FT-IR spectra are shown for Aldrich HAs or Koetoi HAs (n = 3). (a,c) Uninoculated control cultures. (b,d) Cultures inoculated with Clostridium sp. strain HSAI-1.

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