A Possible Anaerobic Decomposition of Humic Substances

Humic substances (HSs) are organic substances than can be present in soil, sediments, and aquatic environments. HS play an important role in environment due to the biogeochemistry of organic carbon in global ecosystem and their role in the control the fate of environmental pollutants. HSs are formed during degradation of precursors from plants and microorganisms and represent a significant fraction of organic matter in earth.

Aerobic decomposition of HSs has been well described, but anaerobic microbial decomposition is not completely understood. Ueno et al (2016) reported the anaerobic decomposition of humic acids (HAs) with a new isolated bacterium Clostridium sp. HSAI-1, that was taken from deep terrestrial subsurface. They use 14C-labelled polycatechol as an HA analogue to demonstrate that anaerobic decomposition of HA is possible.

In this study they monitored the cellular growth in two different media containing commercial Has (Aldrich) or naturally Has from the Koetoi diatomite layer, and with or without glucose (0.5% glucose). The best growth was reported with the addition of glucose. In subsequent experiments the cultures were supplemented with glucose. The media were inoculated or uninoculated to verify if the decomposition has been carried out by microbial activity. The production of 14CO2 from 14C-labelled was measured to demonstrate the microbial decomposition and the maximal evolution occurs in the first 14 days (corresponded to 7.4 ± 3.5% of HA) (Figure 1), after this time the production of CO2 decreased due to an increase in the recalcitrance of polycatechol to biodegradation.

Figure 1. 14CO2 evolution in anaerobic culture. Inoculated culture (HSAI-1) showed the major increment
 in day 14 (filled circles), while uninoculated control cultures (open circles) remain low production.

The decomposition of HAs during 28 days was demonstrated using high-performance size-exclusion chromatography. Delays in retention times reflect reduction in molecular mass (figure 2). This molecular mass was measured in day 0 and day 28 with the inoculated and inoculated conditions, showing a major decomposition in the inoculated culture (table 1).

Figure 2. HA decomposition. Chromatograms from uninoculated cultures are represented in blue,
and those from uninoculated cultures are shown in magenta. Retention times (7.8-8.3 min) are
indicated with vertical lines (dotted line: uninoculated cultures, solid line: inoculated cultures).

Table 1. Calculated molecular masses of HA obtained from HPSEC analysis. The HAs were analysis in
day 0 and they 28 to verify the anaerobic decomposition

In this study is assumed that HA acts as terminal electron acceptors under anaerobic conditions, however, they could not determinate whether HA was utilize as an electron acceptor.

The best-studied anaerobic bacterium Clostridium thermocellum is able to degrade lignocellulose and ferment the produced sugars in ethanol, in this form, is considered that strain HSAI-1 can be able to degrade biopolymers under anaerobic conditions. Is considered that molecular structure of HSs is similar to lignin structure, it is because lignin is the principal material in the formation of HSs, and the produced compounds after degradation of HAs will be studied in the future to compare them with the product in lignin degradation.

Reference
Ueno, A., Shimizu, S., Tamamura, S., Okuyama, H., Naganuma, T., & Kaneko, K. (2016). Anaerobic decomposition of humic substances by Clostridium from the deep subsurface. Scientific reports, 6.