Carbon Chapter 69 Methanogen

In order to make it

Methanogenesis or biomethanation is the formation of methane by microbes known as methanogens. Organisms capable of producing methane have been identified only from the domain Archaea, a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with anaerobic bacteria. The production of methane is an important and widespread form of microbial metabolism. In anoxic environments, it is the final step in the decomposition of biomass. Methanogenesis is responsible for significant amounts of natural gas accumulations, the remainder being thermogenic.[1][2][3]

Methanogenesis in microbes is a form of anaerobic respiration.[4] Methanogens do not use oxygen to respire; in fact, oxygen inhibits the growth of methanogens. The terminal electron acceptor in methanogenesis is not oxygen, but carbon. The carbon can occur in a small number of organic compounds, all with low molecular weights. The two best described pathways involve the use of acetic acid or inorganic carbon dioxide as terminal electron acceptors:

CO2 + 4 H2 → CH4 + 2 H2OCH3COOH → CH4 + CO2

During anaerobic respiration of carbohydrates, H2 and acetate are formed in a ratio of 2:1 or lower, so H2 contributes only ca. 33% to methanogenesis, with acetate contributing the greater proportion. In some circumstances, for instance in the rumen, where acetate is largely absorbed into the bloodstream of the host, the contribution of H2 to methanogenesis is greater.[5]

However, depending on pH and temperature, methanogenesis has been shown to use carbon from other small organic compounds, such as formic acid (formate), methanol, methylamines, tetramethylammonium, dimethyl sulfide, and methanethiol. The catabolism of the methyl compounds is mediated by methyl transferases to give methyl coenzyme M.[4]

Proposed mechanismEdit

The biochemistry of methanogenesis involves the following coenzymes and cofactors: F420, coenzyme B, coenzyme M, methanofuran, and methanopterin.

The mechanism for the conversion of CH

3–S bond into methane involves a ternary complex of methyl coenzyme M and coenzyme B fit into a channel terminated by the axial site on nickel of the cofactor F430. One proposed mechanism invokes electron transfer from Ni(I) (to give Ni(II)), which initiates formation of CH

4. Coupling of the coenzyme M thiyl radical (RS.) with HS coenzyme B releases a proton and re-reduces Ni(II) by one-electron, regenerating Ni(I).[6]

Reverse methanogenesisEdit

Some organisms can oxidize methane, functionally reversing the process of methanogenesis, also referred to as the anaerobic oxidation of methane (AOM). Organisms performing AOM have been found in multiple marine and freshwater environments including methane seeps, hydrothermal vents, coastal sediments and sulfate-methane transition zones.[7] These organisms may accomplish reverse methanogenesis using a nickel-containing protein similar to methyl-coenzyme M reductase used by methanogenic archaea.[8] Reverse methanogenesis occurs according to the reaction:

SO42− + CH4 → HCO3− + HS− + H2O[9]

Importance in carbon cycleEdit

Methanogenesis is the final step in the decay of organic matter. During the decay process, electron acceptors (such as oxygen, ferric iron, sulfate, and nitrate) become depleted, while hydrogen (H2) and carbon dioxide accumulate. Light organics produced by fermentation also accumulate. During advanced stages of organic decay, all electron acceptors become depleted except carbon dioxide. Carbon dioxide is a product of most catabolic processes, so it is not depleted like other potential electron acceptors.

Only methanogenesis and fermentation can occur in the absence of electron acceptors other than carbon. Fermentation only allows the breakdown of larger organic compounds, and produces small organic compounds. Methanogenesis effectively removes the semi-final products of decay: hydrogen, small organics, and carbon dioxide. Without methanogenesis, a great deal of carbon (in the form of fermentation products) would accumulate in anaerobic environments.

8.03.6.2.1 Methanogens

Methanogen lipids have been intensively studied and characterized due to their structures being one of the most remarkable features that distinguish the Archaea from all other organisms (Woese et al., 1990). The polar lipids of methanogens comprise both di- and tetra-ethers of glycerol and isoprenoid alcohols with most compounds being based on the core lipids archaeol (12) or caldarchaeol (13). Minor core lipids are sn-2- and sn-3-hydroxyarchaeol and macrocyclic archaeol (Koga et al., 1993). As discussed earlier (Section 8.03.5.5), nonpolar lipids are also distinctive with many methanogens having high contents of hydrocarbons including the characteristic irregularly branched compound PMI (18) and structurally related analogs (e.g., Risatti et al., 1984; Schouten et al., 1997; Tornabene et al., 1979).

Methanogens

Methanogens are exclusively Archaea, and are one of the most common anaerobic microbes in highly reducing conditions in close association with decomposing organic material.

From: Encyclopedia of Caves (Second Edition), 2012

Related terms:

Methanogenesis

Acetate

Fermentation

Hydrogen

Methane

Micro-Organism

Oxidation

Sulphate

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Biogeochemistry

J.J. Brocks, R.E. Summons, in Treatise on Geochemistry, 2003

8.03.6.2.1 Methanogens

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Biogeochemistry

J.P. Megonigal, ... P.T. Visscher, in Treatise on Geochemistry, 2003

8.08.4.2 Methanogen Diversity and Metabolism

Methanogens are strict anaerobes that produce CH4 as a waste product of energy metabolism. Several characteristics set the methanogens apart from most other microorganisms. They are the largest and most diverse group in the Archea, which is phylogenetically distinct from the other two domains of life, the Bacteria and Eukaryota. Many members of the Archea grow at extremes of temperature, pH, or salinity, although the distribution of methanogens is fairly cosmopolitan. Methanogens have a number of unique coenzymes (Wolfe, 1996) and differ from Bacteria in the construction of their cell walls, a characteristic that makes them insensitive to penicillin and other antibiotics. The methanogens and other Archea can be identified by an electron carrier, F420, that autofluoresces at 420 nm under UV light (Edwards and McBride, 1975). Many methanogens lack cytochromes and other features of electron transport chains such as quinones (Fenchel and Finlay, 1995). It has been suggested that a quinone-like role in electron transfer may be filled by phenazine compounds in the Methanosarcinales (Abken et al., 1998; Deppenmeier et al., 1999). In a detailed taxonomic treatment, Boone et al. (1993) defined 26 genera and 74 species of methanogens.

Despite a great deal of taxonomic diversity, the methanogens use a limited variety of simple energy sources compared to the other major forms of anaerobic metabolism (Zinder, 1993). The compounds that support energy conservation for growth are H2, acetate, formate, some alcohols, and methylated compounds. The most important of these are generally H2 and acetate. About 73% of methanogenic species consume H2 (Garcia et al., 2000):

(3)4H2+CO2→CH4+2H2O

Hydrogenotrophic methanogenesis (also known as CO2/H2 reduction or H2-dependent methanogenesis) is a chemoautotrophic process in which H2 is the source of both energy and electrons, and CO2 is often both an electron sink and the source of cellular carbon. Some hydrogenotrophic methanogens require an additional organic carbon source for growth (Vogels et al., 1988). CO2/H2 reduction requires a 4:1 molar ratio of H2 to CO2, yet H2 is typically at nM concentrations while CO2 is at mM concentrations in natural systems. Thus, substrate limitation of hydrogenotrophic methanogenesis must always be caused by a lack of the electron donor, H2. Roughly 45% of all hydrogenotrophic methanogens can substitute formate for H2 in reaction (3) (Garcia et al., 2000; Thiele and Zeikus, 1988).

Whereas hydrogenotrophy is wide spread among the methanogens, acetotrophy (also known as acetate fermentation or acetoclastic methanogenesis) is restricted to just two genera, the Methanosarcina and Methanosaeta (formerly Methanothrix), which comprise ∼10% of methanogenic species:

(4)CH3COOH→CO2+CH4

The Methanosarcina use a wide variety of substrates and have high potential growth rates, but their affinity for acetate is low (Jetten et al., 1992). In contrast, the Methanosaeta specialize in using acetate and have a high affinity for the substrate, but their potential growth rate is low. Despite its limited taxonomic distribution, acetotrophy is the dominant methanogenic pathway in many ecosystems. Acetotrophic methanogenesis can be considered a special case of methylotrophy whereby a portion of the substrate molecule is oxidized to CO2, while a methyl group on the same molecule is reduced to CH4 (Fenchel and Finlay, 1995; Hornibrook et al., 2000; Pine and Barker, 1956). In this case, the methyl group is the electron donor and the carboxyl group is the electron acceptor.

About 26% of methanogenic species can use methylated substrates other than acetate, such as methanol, methylated amines, and methylated sulfur compounds (Hippe et al., 1979; Kiene, 1991b). Because the process does not require an external electron acceptor, methylotrophy is a type of fermentation. The contribution of these organisms to CH4 production in natural ecosystems appears to be minor, but they may contribute substantially to the metabolism of methylated sulfur compounds. Scholten et al. (2003) provided thermodynamic constraints on the feasibility of such reactions.

Some methanogens are dependent on a single substrate, such as acetate or methanol, while others are able to grow on two or more alternative substrates. All known formate-oxidizing methanogens are also hydrogenotrophs (Garcia et al., 2000), and at least one methanogen also grows by fermenting pyruvate (Bock and Scho¨nheit, 1995). Members of the Methanosaetaceae use acetate but not H2, whereas the Methanosarcinaceae can use both substrates. Both genera fall in the order Methanosarcinales.

Methanogens metabolize several substrates that do not support growth. Methanosarcina barkeri was able to lower the redox potential by dissimilatory reduction of Fe(III) until the redox potential reached 50 mV, at which point methanogenesis began (Fetzer and Conrad, 1993). In a survey of five methanogenic species drawn from a wide range of phylogenetic and physiological types, all the hydrogenotrophs could reduce Fe(III) (Bond and Lovley, 2002). The ability to grow on Fe(III) was not investigated. Carbon monoxide is converted to CH4, but the physiological role this substrate plays is uncertain (Vogels et al., 1988). Rich and King (1999) measured maximum potential uptake velocities of 1–2 nmol CO cm−3 sediment h−1 in anaerobic soils, but the response to amendments of SO42−, Fe(III), and the methanogen inhibitor bromoethanesulfonic acid (BES), suggested that no more than 30% of the oxidation activity was due to methanogens. Methanogens degrade chlorinated pollutants and have been used for bioremediation (Fathepure and Boyd, 1988; Mikesell and Boyd, 1990).

Methanogens grow at temperatures ranging from 4 °C to 100 °C, salinities from freshwater to brine, and pH from 3 to 9. Most grow optimally at temperatures ≥30 °C, but thermophilic methanogens have temperature optima near 100 °C, and a few isolates are adapted to frigid conditions (Franzmann et al., 1997). In a survey of 68 methanogenic species, most species grew best in a pH range from 6 to 8, and none could grow at pH <5.6 (Garcia et al., 2000). Observations of CH4 production in acidic environments suggests that there are uncultured methanogens that can grow at pH<5.6 (Walker, 1998).

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Emission of Greenhouse Gases and Their Warming Effect

S.K. Jalota, ... Samanpreet Kaur, in Understanding Climate Change Impacts on Crop Productivity and Water Balance, 2018

Effect of Increased Temperature

Under warmer conditions, when other factors are not limiting, activities of both methanogens and methanotrophs are increased, and CH4 emission is stimulated. For instance, a global rise in temperature of 3.4°C has been predicted to increase CH4 emissions from wetlands by 78% (Shindell, Walter, & Faluvegi, 2004). Effect of warming is influenced by soil aeration status. Warming may lead to a substantial increase in net CH4 emissions from anaerobic permafrosts and wetlands at high latitudes because of partial inhibition of microbial respiration, emerged from inactivation of biological oxidation systems due to decreased soil redox potential. However, in aerobic surface soils (oxygen and atmospheric CH4), CH4 emission from soil is decreased due to increased CH4 oxidation rates and net CH4 uptake by soil microorganisms with increased gas diffusion rates and microbial access. Increasing temperature generally offsets the stimulatory effect of increased CO2 levels on CH4 flux.

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Greenhouse Gases Formation and Emission

Antonio C. Barbera, ... Carmelo Maucieri, in Encyclopedia of Ecology (Second Edition), 2019

Methane production

Methanogenesis is operated by strictly anaerobic bacteria which requires negative oxydo-reduction potentials (Eh < − 200 mV). Methanogens belong to the domain Archaea which have a limited trophic spectrum comprised of a small number of simple substrates: H2 + CO2, acetate, formate, methylated compounds (methanol, methylamines, dimethylsulphur), and primary and secondary alcohols. This allows to distinguish five trophic groups of methanogens: hydrogenotrophs, formatotrophs, acetotrophs, methylotrophs, and alcoholotrophs. The two major pathways of CH4 production in most environments where organic matter decomposition is significant are acetotrophy and CO2 reduction by H2.

The possible pathway for CH4 emission from soil are: (i) diffusion of dissolved CH4 along the concentration gradient, (ii) release of CH4-containing gas bubbles (ebullition), and iii) transport via the aerenchyma of vascular plants (plant-mediated transport).

The first process, diffusion, takes place because of the formation of a CH4 concentration gradient from deeper soil layers, where the production of CH4 is large, to the atmosphere, while oxidation of CH4 occurs in upper layers. Diffusion is a slow process compared to the other two transport mechanisms, that is, ebullition and plant-mediated transport, but it is biogeochemically important because it extends the contact between CH4 and methanotrophic bacteria in the upper aerobic layer, promoting CH4 oxidation.

The second process, ebullition, takes place when CH4 production is large. Gas bubbles are formed and emigrate to the surface. As this process is fast, CH4 oxidation is absent or negligible.

The third process, plant mediated transport, takes place through an internal system of continuous air spaces named aerenchyma, a structure which is developed by vascular plants to adapt to flooded environments. The basic function of this structure is to transport the O2 necessary for root respiration and cell division in submerged organs, but it is also used for CH4 transportation from the rhizosphere to the atmosphere, bypassing the aerobic, CH4-oxidizing layers. This process involves two major mechanisms: molecular diffusion and bulk flow. The gradient of CH4 concentration formed inside the aerenchyma conduits is the driving force for CH4 diffusion from the peat root zone to the aerial parts of the plant. The other plant-mediated transport mechanism, bulk transportation, involves the migration of CH4 along the plant, also through the aerenchyma structure, from the leaves to the rhizome and back to the atmosphere through old leaves or horizontal rhizomes connected to other shoots. The driving force for this process is a pressure gradient generated by differences in temperature or water vapor pressure between the internal air spaces in plants and the surrounding atmosphere. This is a very efficient and rapid mechanism of CH4 transportation and, in consequence, it is responsible for most of CH4 emissions (> 95%) to the atmosphere from rice paddies.

The factors controlling CH4 production in soil are anaerobic conditions and redox potential, electron acceptors, substrate availability, temperature, diffusion, water availability and water table, soil pH and salinity, fertilizer and manure additions and amendments, trace metals, competitive inhibition, vegetation, plant species and cultivars, and elevated CO2 concentrations.

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Biotechnological Tools for Remediation of Acid Mine Drainage (Removal of Metals From Wastewater and Leachate)

Bhupinder Dhir, in Bio-Geotechnologies for Mine Site Rehabilitation, 2018

4.3.4.1 Microbial Community Structure

SRB alone in conjunction with other microbial communities help in the neutralization of the effluent. Microbial communities that take part in the process include methanogens, acetogens, and sulfate reducers (Neculita et al., 2007). Factors such as availability of nutrients, growth stage, physiological state of bacterial cells, environmental conditions (pH, ionic strength, and temperature), presence of competitive ions, and concentration of the biomass influence the efficiency of microbes (El Bayoumy et al., 1997; Utgikar et al., 2000; Santos et al., 2004). Anaerobic degradation of complex organic carbon compounds to simpler molecules by other microflora limits the availability of substrates to SRB (Zagury et al., 2006; Neculita et al., 2007).

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Biogeochemistry

E.G. Nisbet, C.M.R. Fowler, in Treatise on Geochemistry, 2003

8.01.6.2 Methanogenesis: Impact on the Environment

Life operates on a global scale. On a geological timescale, once the first cell had replicated, all habitats on the planet would immediately be filled. This would rapidly have consequences for the atmosphere. In particular, methanogens are likely very ancient, and may long predate methanotrophic bacteria. Methanogens most probably predate photosynthesizers if the evolutionary lengths in the standard models of molecular palaeontology (Woese, 1987; Barnes et al., 1996; Pace, 1997) have value. Possibly they also predate the methane oxidizing archaea. These operate by anaerobic oxidation of methane against sulfate, to produce bicarbonate, HS− and water: their impact would have been limited by the supply of sulfate oxidant. Once methanogens had evolved, they would have occupied proximal and distal hydrothermal habitats, and then perhaps wider habitats such as open ocean (Sansone et al., 2001) and tidal habitats. Possibly methanotrophs evolved quickly following the arrival of methanogens, to exploit the new opportunity: but, in the likely absence of abundant free molecular oxygen, they would have been severely limited by the supply of oxidant.

Methanogenesis on a scale large enough to affect the atmosphere would have been possible if the hydrogen supply from inorganic and organic sources (and hence methanogenesis) had been adequate: given the likely abundance of ultramafic rock near the surface, interacting with hydrothermal water, it is not unreasonable to suppose a major flux of inorganic hydrogen. If so, and there was a surplus of methane, then much of the methane formed by the first methanogens would have been emitted directly to atmosphere. In the dry air on a cold glacial planet, this methane might rapidly overwhelm the OH. Over a few tens of millennia, the atmospheric methane burden would build up and have a major greenhouse impact (see Pavlov et al., 2000), until enough ice melted to permit OH in air and thus control the methane.

Methane may have played a crucial role in allowing the early Earth to be habitable (Pavlov et al., 2000, 2001). Methane emitted by organisms would have had a substantial greenhouse effect, and if the methane/carbon dioxide ratio in the air were high, methane could have fostered an organic smog that protected shallow-level life against ultraviolet radiation in sunlight (Lovelock, 1988). Thus, there is a possible progression here, from the first methanogens, few in total number and confined to the immediate vicinity of hydrothermal systems on a very cold planet, then a warming trend, then development of planktonic life and much more widely spread methanogens, increasing the warming.

Catling et al. (2001) pointed out that in the early Archean, biogenic methane may have saved the Earth from permanent glaciation. On the modern Earth, on a 20 yr timescale, emission of methane has an incremental greenhouse impact nearly 60 times, weight-for-weight, or 21 times, molecule-for-molecule, that of carbon dioxide. On the Archean planet, this ratio would have been very different, and the difference is nonlinear with burden. But whatever the greenhouse impact was, methane is a very powerful greenhouse gas. Indeed, unless abundant methane existed in the air it is difficult to imagine how intense global glaciation was avoided. Thus geologically likely models of the early Archean atmosphere, that are consistent with the Isua evidence for water-eroded and water-transported sediment, would be expected to invoke high methane concentrations (102–103 ppmv—compared to modern air with less than 2 ppmv CH4 and ∼375 ppmv of CO2). Such high levels of methane would lead to hydrogen escape by photolysis and loss from the top of the atmosphere, and hence irreversible net oxidation of the planetary surface environment (Catling et al., 2001), though not necessarily to significant ambient O2 at any particular time.

Methanogenesis may have had the interesting consequence of triggering the evolution of nitrogen fixation (Navarro-Gonzalez et al., 2001; Kasting and Siefert, 2001). On an early planet with CO2 present in the air, nitrogen fixation would have occurred in lightning strikes, which would have used oxygen atoms from the carbon dioxide (or from water) to form NO. However, if CO2 levels declined and CH4 rose, the oxygen supply would be reduced, limiting the synthesis of NO. This would have created a crisis for the biosphere as usable nitrogen is essential. Out of this crisis, Navarro-Gonzalez et al. suggested, may have come what now appears to be the essentially "altruistic" process of nitrogen fixation, which is very expensive in energy.

Another, not necessarily incompatible hypothesis is that nitrogenase first evolved as a manager of excess ammonia in the lower, anaerobic part of microbial mats, where hydrogen is present. The product, dinitrogen, could be safely bubbled away. Had a crisis occurred, in which there was a shortage of fixed nitrogen, any cell or consortium of cells able to reverse the process would have been advantaged. It is perhaps notable that in nitrogenase the N2 is bound to a cluster of Mo–3Fe–3S. Molybdenum, iron and sulfur are likely to be abundant together at hydrothermal systems, especially around andesite volcanoes, and this may be a protein with a hydrothermal heritage. Falkowski (1997) points out that the requirement for iron, and the need for anoxia, would have put severe limits on nitrogen fixation, such that fixed nitrogen supply (and hence the availability of iron), not phosphorus, may be the chief limitation on the productivity of the biosphere. Indeed, the vast scale of human fixation of nitrogen, and perhaps the pH change of the ocean, may some day be seen as the greatest peril of global climate change: not the greenhouse.

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Chemolithoautotrophy

Annette Summers Engel, in Encyclopedia of Caves (Third Edition), 2019

Hydrogen

Hydrogen gas accumulates from the anaerobic breakdown of organic molecules by fermentative (heterotrophic) bacteria and serves as an important energy source for aerobic and anaerobic archaeal and bacterial chemolithoautotrophs. In most cases, H2 is rapidly consumed under anaerobic conditions by methanogens, acetogens, and sulfate-reducers. However, if anaerobic growth is slower than production of H2 by heterotrophs or chemoorganotrophs, then H2 will diffuse into the aerobic environment where it can be oxidized by chemolithoautotrophic H2-oxidizing microorganisms with the membrane-bound enzyme hydrogenase. These organisms fix CO2 via the CBB or rTCA cycle (Table 1) and include a wide variety of gram-negative and gram-positive bacteria like Alcaligenes, Hydrogenobacter, Pseudomonas, and Aquifex.

The anaerobic microbial groups that require H2 as an electron donor follow predictable redox chemistry regarding the utilization of available terminal electron acceptors (Table 1). But, for the most part, reduction of nitrate, sulfate, iron, or manganese during anaerobic respiration is typically done by heterotrophs or chemoorganotrophs, such as the dissimilatory sulfate-reducing microbes. These microbes reduce sulfate to hydrogen sulfide (H2S) and belong predominately to the Deltaproteobacteria class of the Proteobacteria phylum, but there are also sulfate reducers in the phyla Firmicutes Nitrospirae, Thermodesulfobacteria, and Thermodesulfobium, as well as within the Archaea, such as the genera Archeoglobus, Thermocladium, and Caldivirga. Chemolithoautotrophic sulfate reducers use H2 as the electron donor, sulfate as the electron acceptor, and CO2 as the sole carbon source. Chemolithoautotrophs use the reductive acetyl-CoA pathway, while the rTCA cycle is used by chemoorganotrophs. If sulfate concentrations are high, then sulfate-reducing bacteria completely oxidize fermentation by-products to CO2. In low-sulfate anaerobic environments, however, sulfate-reducing bacteria compete with methanogens for H2 and organic compounds. The reduction of elemental sulfur is also possible, and chemolithoautotrophic sulfur-reducing Archaea include Thermoproteus, Acidianus, and Desulfurolobus, which are common in acidic environments. Although not exclusively a chemolithoautotrophic pathway, the disproportionation of elemental sulfur is also possible, whereby elemental sulfur is split into two different compounds, typically sulfite and thiosulfate, to produce both H2S and sulfate. Desulfocapsa is a common sulfur-disproportionating bacterial genus.

Methanogens are exclusively Archaea and are one of the most common anaerobic microbes in highly reducing conditions in close association with decomposing organic material. Methanogens oxidize H2 as the electron donor while reducing CO2 to methane using the reductive acetyl-CoA pathway (Table 1). These methanogens are usually found in close association (syntrophy or interspecies hydrogen transfer) with fermenting microbial groups because of the need for the continuous H2 supply provided through fermentation. Chemolithoautotrophic methanogenesis is less efficient than those utilizing formate, acetate, methanol and other alcohols, carbon monoxide, or even elemental iron, as alternative electron donors. The physiologies of these different types of methanogens (chemolithoautotrophic vs chemoorganotrophic) are distinct, and there are at least seven major groups of methanogens based on this physiology, including the genera Methanobacterium, Methanosaeta, Methanococcus, and Methanolobus.

Acetogenesis results in less overall cellular energy than methanogenesis, but both metabolic groups are found in similar habitats. Acteogenic bacteria, such as Clostridium and Acetobacterium, are obligate anaerobes that form acetate from the oxidation of H2 using the reductive acetyl-CoA pathway for CO2 fixation. These organisms are known as homoacetogens. H2 is the common electron donor, but donors can also be from sugars, organic acids, and amino acids. Many acetogens also reduce nitrate and thiosulfate, and are tolerant of low pH, thereby making them more versatile than other anaerobes.

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Energy from Wastewater Treatment

S.Z. Ahammad, T.R. Sreekrishnan, in Bioremediation and Bioeconomy, 2016

9 Hybrid Anaerobic Reactor

Very slow growth rate of anaerobic consortia is a bottleneck to enhance the treatment rate. In most of the cases, during anaerobic treatment of wastewaters, the methanogenesis step is considered as the rate-limiting one. To enhance the degradation rate, higher methanogen concentration in the reactor is desired. In HARs, the biomass concentration is enhanced through the use of biogranules, which consist of all the four different groups of anaerobic microorganisms. Higher superficial liquid velocity is maintained in the reactor to fluidize the biogranules (Ahammad et al., 2010; Saravanan and Sreekrishnan, 2005). Though maintenance of higher upflow velocity requires extra energy, it enhances the mass transfer rate to a substantial level. In a typical fluidized bed reactor, the biogranules are developed by growing biofilm onto inert support material. As inert support has its own weight, it demands extra energy to fluidize. In an HAR, use of self-immobilized granules can eliminate extra energy demand while fluidizing the granules. In an HAR this is made possible by growing self-immobilized anaerobic granules, which consist of all different types of bacteria and archaea required for anaerobic treatment, and they are present at a very high concentration in the compact granules. It results in lower energy usage (than in AFBR) in operating such reactor as well as improving the anaerobic treatment and subsequently increasing production of biogas (Ahammad et al., 2010). Net energy obtained from such a system makes it an energy-positive wastewater treatment option.

The energy audit is always done considering the existing practices in different treatment facilities. There is no doubt that the activated sludge process (ASP), an aerobic process, is the most common and widely used treatment technology despite its high demand on energy for aeration. Its robustness and simplicity of design and operation are often overshadowed by its drawbacks associated with the high energy input. Status of energy use in different processes is changing due to increase in the awareness of carbon footprints associated with the process, and efforts are in place to reduce carbon footprints. More and more processes are adapting greener, low carbon footprint technologies by working to eliminate the problems associated with energy-neutral and energy-positive technologies.

Understanding of the ASP would help to choose the appropriate alternatives to replace it. The main features of ASP are discussed in the following section.

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Biological Processes Affecting Contaminants Transport and Fate

R.M. Maier, in Environmental and Pollution Science (Third Edition), 2019

9.4.2.2 Aromatic Hydrocarbons

Recent evidence indicates that nonsubstituted aromatics like benzene can be degraded only very slowly under anaerobic conditions. However, like aliphatic hydrocarbons, substituted aromatic compounds can be rapidly and completely degraded under anaerobic conditions (Fig. 9.16). Anaerobic mineralization of aromatics often requires a mixed microbial community whose populations work together under different redox potentials. For example, mineralization of benzoate can be achieved by growing an anaerobic benzoate degrader in coculture with an aerobic methanogen or sulfate reducer. In this consortium, benzoate is transformed by one or more anaerobes to yield aromatic acids, which in turn are transformed to methanogenic precursors such as acetate, carbon dioxide, or formate. These small molecules can then be utilized by methanogens (Fig. 9.17). This process can be described as an anaerobic food chain because the organisms higher in the food chain cannot utilize acetate or other methanogenic precursors, while the methanogens cannot utilize larger molecules such as benzoate. Methanogens utilize carbon dioxide as a terminal electron acceptor, thereby forming methane. (Note: Methanogens should not be confused with methanotrophic bacteria, which aerobically oxidize methane to carbon dioxide.)

Fig. 9.16. Anaerobic biodegradation of benzoate. Note that the intermediate benzoyl-CoA is a common intermediate found in the anaerobic degradation of aromatic compounds. Further note (in contrast to aerobic conditions) that anaerobic microbes completely saturate the ring during biodegradation before it is opened.

Fig. 9.17. An anaerobic food chain. Shown is the formation of simple compounds from benzoate by a population of anaerobic bacteria and the subsequent utilization of the newly available substrate by a second anaerobic population, the methanogenic bacteria.

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Methanogens

Methanogens are exclusively Archaea, and are one of the most common anaerobic microbes in highly reducing conditions in close association with decomposing organic material.

From: Encyclopedia of Caves (Second Edition), 2012

Related terms:

Methanogenesis

Acetate

Fermentation

Hydrogen

Methane

Micro-Organism

Oxidation

Sulphate

View all Topics

Biogeochemistry

J.J. Brocks, R.E. Summons, in Treatise on Geochemistry, 2003

8.03.6.2.1 Methanogens

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Biogeochemistry

J.P. Megonigal, ... P.T. Visscher, in Treatise on Geochemistry, 2003

8.08.4.2 Methanogen Diversity and Metabolism

(3)4H2+CO2→CH4+2H2O

(4)CH3COOH→CO2+CH4

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Emission of Greenhouse Gases and Their Warming Effect

S.K. Jalota, ... Samanpreet Kaur, in Understanding Climate Change Impacts on Crop Productivity and Water Balance, 2018

Effect of Increased Temperature

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Greenhouse Gases Formation and Emission

Antonio C. Barbera, ... Carmelo Maucieri, in Encyclopedia of Ecology (Second Edition), 2019

Methane production

The second process, ebullition, takes place when CH4 production is large. Gas bubbles are formed and emigrate to the surface. As this process is fast, CH4 oxidation is absent or negligible.

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Biotechnological Tools for Remediation of Acid Mine Drainage (Removal of Metals From Wastewater and Leachate)

Bhupinder Dhir, in Bio-Geotechnologies for Mine Site Rehabilitation, 2018

4.3.4.1 Microbial Community Structure

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Biogeochemistry

E.G. Nisbet, C.M.R. Fowler, in Treatise on Geochemistry, 2003

8.01.6.2 Methanogenesis: Impact on the Environment

Chemolithoautotrophy

Annette Summers Engel, in Encyclopedia of Caves (Third Edition),

Hydrogen

View chapterPurchase book

Energy from Wastewater Treatment

S.Z. Ahammad, T.R. Sreekrishnan, in Bioremediation and Bioeconomy, 2016

9 Hybrid Anaerobic Reactor

Understanding of the ASP would help to choose the appropriate alternatives to replace it. The main features of ASP are discussed in the following section.

View chapterPurchase book

Biological Processes Affecting Contaminants Transport and Fate

R.M. Maier, in Environmental and Pollution Science (Third Edition), 2019

9.4.2.2 Aromatic Hydrocarbons

An anaerobic food chain. Shown is the formation of simple compounds from benzoate by a population of anaerobic bacteria and the subsequent utilization of the newly available substrate by a second anaerobic population, the methanogenic bacteria.

Chapter 69 - Methanogen

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