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.

A solution for plastic wastes; biodegradation with gamma irradiation to induce photo-oxidation by endophytic fungi

Synthetics polymers derivatives of petroleum are produced all over the world. Approximately 140 million ton per year are introduced in the environment as industrial wastes products, which are accumulated because their degradation is carried out slowly. This has a negative impact in the ecosystems and organisms.

Considering the problematic of plastic wastes, in this study was applied the biodegradation in two types of polymers: low density polyethylene (LDPE) and polypropylene (PP). These polymers are extremely recalcitrant, and the most persistent plastics dumped in the environment. To carry out this biodegradation they used  endophytic fungal strains isolated from two endemic plants Psychotria flavida and Humboldtia brunonis which produce laccase enzymes that have the capacity to degrade these materials, the activity of enzyme was quantified in each species of fungi viz. Cunnighamella echinulata, Pestalotiopsis sp, Hypoxylon anthochroum, Paecilomyces lilacinus, Aspergillus sp, Lasiodiplodia theobromae (Fig.1,2).

To be possible the biodegradation, the microorganism has to join to the polymer forming biofilms and use it as a sole source carbon. In this case were produced biofilms with different dose of irradiated gamma which induce the photo-oxidation in two polymers, the dose was 0-1000 kGy for LPDE and 0-100 kGy for PP (Fig.3,4). This generated a large surface area and produced a great degree of hydrophobicity due to the introduction of carbonyl groups that was used for endophytic fungi in their metabolisms enhancement the biodegradation. The possible mechanisms is the transformation of carbonyl groups to carboxyl groups in presence of oxygen (Fig.5) which undergo to B- oxidation and subsequently undergo to cyclic acid citric, that results in the production of CO₂ and H₂O (Fig.6). The efficiency for degradation of the polymers was measured with the decrease in intrinsic viscosity and average molecular weight of gamma irradiated.       

Figure 1. Shows a) the formation for biofilm fungi in the polymer and b) the presence of enzyme laccase.

Figure 2 Shows the activity of enzyme laccase in each species of fungi where the production level of laccase in L. theobromae was high (10.70 ± 1. U/ml) and in Hypoxylon anthochroum quite low (1.18 ± 0.10 U/ml).

Figura 3 shows percentage weight loss in gamma irradiated fungal inoculated LDPE films.

Figure 4 shows percentage weight loss in gamma irradiated fungal inoculated PP films.

Figure 5 shows (a) control film unirradiated (b) showed the peaks that correspond to addition of oxygen and photoxidation compared with the control film. The gamma irradiated LDPE (1000 kGy) rendered wavenumber 1712 cm^-1 related to carbonyl group formation which was absent in control. The area of peak at wavelength (3100-3500) cm^-1 was broad and bended in irradiated (1000 kGy) fungal treated samples (c) shows the presence of O-H stretching band of hydroxyl groups and adsorbed water corresponding to decrease in hydrophobicity  and attributing to degradation.

Figure 6 show the possible mechanisms for biodegradation a) Gamma radiation and b) the efficiency for degradation by fungi.

The biodegradation through irradiated gamma is a biologic alternative for the problematic of plastic wastes. This study shows that the radiation gamma can increase the capacity of microorganisms for use the polymer as a sole carbon source in their metabolism producing enzymes that degrade these materials.

If you have interested about the subject you can search the next paper “Biodegradation of gamma irradiated low density polyethylene and polypropylene by endophytic fungi”

-Reference

Sheik, S., Chandrashekar, K.R., Swaroop, K., & Somashekarappa, H.M. (2015).Biodegradation of gamma irradiated low density polyethylene and polypropylene by endophytic fungi.International Biodeterioration & Biodegradation.

Effect of simulated tillage on microbial autotrophic CO2 fixation in paddy and upland soils.

The tillage is a common agricultural practice that consist in draw furrows variable depth in the ground with a hand tool or a plow before to cultivate, this technique has been used by centuries, but some investigation say that tillage practice has a negative effect in soil bacterial populations, for that reason was necessary to evaluate the different effect of tillage practice on soil autotrophic bacterial populations and their CO2 assimilation rates at varying soil depths.
Autotrophic bacteria can fix CO2 and are widely distributed in agricultural soils. We know six pathway to fix CO2 but the most common is the Calvin-Benson cycle. To know the effect of tillage on soil, in this study two samples were from paddy soil and two from upland from different region of China (Table 1.). Two sets were taken from each site of soil, in one group tillage was simulated (CT) and in the other one the analysis of soil was direct, without tillage (NT). The samples of soil were incubated and the CO2 were labelled with 14C. The Result of this experiment showed that the amount of CO2 was higher in the tillage treatment, also the samples of all soil was taken from different depth and independently whether tillage was practiced or not, in both cases the CO2 fixed was to more depth, the lees CO2 fixed.

Table1. Basic study site information and corresponding soil physicochemical characteristics. CEC, cation exchange capacity; SOC, soil organic carbon

Figure 1. The 14C-SOC, 14C-MBC and 14C-DOC concentrations recovered at different depths (0–1 cm, 1–5 cm, and 5–17 cm) in conventional tillage (CT) and no-till (NT) soils after 110 days of incubation. Error bars indicate the standard error of the mean (n =4). *indicates significant differences between CT and NT soils at P < 0.05; nd, not detectable. DOC, dissolved organic carbon; MBC, microbial biomass carbon; SOC, soil organic carbon.

The effect of tillage on soil microbial CO2 fixation, can be positive on agricultural practice, if you are interest, you can read more here:

Ge, T., Wu, X., Liu, Q., Zhu, Z., Yuan, H., Wang, W., Whiteley, A.S., and Wu, J. 2016. Effect of simulated tillage on microbial autotrophic CO2 fixation in paddy and upland soils. Scientific Reports.

An excellent alternative to reduce potential atmospheric methane emissions: Anaerobic methane oxidation in freshwater wetlands

Freshwater wetlands (FWW) are characterized by high rates of methanogenesis, and although they occupy a small fraction of the Earth’s surface, represent one of the largest natural sources of methane. Is assumed that anaerobic methane oxidation is the dominant way of the methane consumption in FWW.  This assumption is because the sulfate has been considered the only oxidant for methane in anoxic environments but can be possible that other compounds are being used as electron acceptors, including nitrate/nitrite, iron and manganese.

This study provides the first constraints on both the magnitude and extent of anaerobic methane oxidation (AOM) in FWW, a critical first step to understanding the role of this process in freshwater environments. In this case were sampled three different bio-geographical provinces from Florida, Georgia and Maine, this sites were sampled in two different seasons to captured a range of in situ conditions to illustrate the broad relevance of AOM in FWW.

The measure parameters were the rates of AOM and sulfate reduction, pore water chemistry and stable carbon isotope geochemistry, including microbial lipid biomarker to     evaluate the magnitude of AOM and SR, their role in wetland carbon cycling and the microbial community potentially involved in AOM.

In the table 1 shows the most significant parameters measured to samples used in this study.

In the next figure shows the interaction between sulphate and AOM according to the depth.

Figure 2 sample the relation between SR and AOM in the same way to the previous figure according to the depth.

There is evidence that AOM in FWW is increasing but the rate of direct measurements is not usual so this study includes methanotrophy advances in FWW showing AOM in real time highlight. The potentially large role AOM plays in wetland methane cycling, the zone of maximum AOM activity was 0-3 cm corresponded to the lowest dissolved inorganic carbon values and the highest methane values. Alternative electron acceptors remain feasible sulphate-independent AOM is clearly indicated at some depth horizons in Georgia where rates of AOM exceeded rates of SR so its possible that iron and/or humic substances may play a role: experimental additions of humic substances may prove useful in future investigations.

As FWW ecosystems are responsible for a major portion of global emissions, a better constraint of methane cycling in wetlands is paramount to understanding past and future global methane budgets and the role of FWW in the global methane cycle.

-Segarra, K. E. A., Schubotz, F., Samarkin, V., Yoshinaga, M. Y., Hinrichs, K. U., & Joye, S. B. (2015). High rates of anaerobic methane oxidation in freshwater wetlands reduce potential atmospheric methane emissions. Nature Communications, 6.

If you can read more can search the next paper “High rates of anaerobic methane oxidation in freshwater wetlands reduce potential atmospheric methane emissions”.