Metabolic, phylogenetic, and ecological diversity of the methanogenic archaea.
01 Mar 2008-Annals of the New York Academy of Sciences (Wiley/Blackwell (10.1111))-Vol. 1125, Iss: 1, pp 171-189
TL;DR: The ecology of methanogens highlights their complex interactions with other anaerobes and the physical and chemical factors controlling their function.
Abstract: Although of limited metabolic diversity, methanogenic archaea or methanogens possess great phylogenetic and ecological diversity. Only three types of methanogenic pathways are known: CO(2)-reduction, methyl-group reduction, and the aceticlastic reaction. Cultured methanogens are grouped into five orders based upon their phylogeny and phenotypic properties. In addition, uncultured methanogens that may represent new orders are present in many environments. The ecology of methanogens highlights their complex interactions with other anaerobes and the physical and chemical factors controlling their function.
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TL;DR: An up-to-date synthesis of estimates of global CH4 emissions from wetlands and other freshwater aquatic ecosystems is provided, major biogeophysical controls over CH4 emitters from wetlands are summarized, new frontiers in CH4 biogeochemistry are suggested, and relationships between methanogen community structure and CH4 dynamics in situ are examined.
Abstract: Understanding the dynamics of methane (CH4) emissions is of paramount importance because CH4 has 25 times the global warming potential of carbon dioxide (CO2) and is currently the second most important anthropogenic greenhouse gas. Wetlands are the single largest natural CH4 source with median emissions from published studies of 164 Tg yr 1 , which is about a third of total global emissions. We provide a perspective on important new frontiers in obtaining a better understanding of CH4 dynamics in natural systems, with a focus on wetlands. One of the most exciting recent developments in this field is the attempt to integrate the different methodologies and spatial scales of biogeochemistry, molecular microbiology, and modeling, and thus this is a major focus of this review. Our specific objectives are to provide an up-to-date synthesis of estimates of global CH4 emissions from wetlands and other freshwater aquatic ecosystems, briefly summarize major biogeophysical controls over CH4 emissions from wetlands, suggest new frontiers in CH4 biogeochemistry, examine relationships between methanogen community structure and CH4 dynamics in situ, and to review the current generation of CH4 models. We highlight throughout some of the most pressing issues concerning global change and feedbacks on CH4 emissions from natural ecosystems. Major uncertainties in estimating current and future CH4 emissions from natural ecosystems include the following: (i) A number of important controls over CH4 production, consumption, and transport have not been, or are inadequately, incorporated into existing CH4 biogeochemistry models. (ii) Significant errors in regional and global emission estimates are derived from large spatial-scale extrapolations from highly heterogeneous and often poorly mapped wetland complexes. (iii) The limited number of observations of CH4 fluxes and their associated environmental variables loosely constrains the parameterization of process-based biogeochemistry models.
847 citations
Cites background from "Metabolic, phylogenetic, and ecolog..."
...As several aspects of the phylogeny, biochemistry, and ecology of methanogens have been reviewed elsewhere (Liu & Whitman, 2008), we focus here on recent advances in the understanding of methanogen community dynamics in freshwater ecosystems....
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...6), although some © 2012 Blackwell Publishing Ltd, Global Change Biology, 19, 1325–1346 hydrogenotrophic methanogens require acetate for growth but do not make it into CH4 (Br€auer et al., 2006; Liu & Whitman, 2008; Sakai et al., 2012)....
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...However, Methanosarcinaceae are outcompeted in low acetate conditions by the other known acetoclastic group, Methanosaetaceae, which can use acetate at concentrations as low as 5–20 lM (Liu & Whitman, 2008)....
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...For example, Methanosarcinaceae isolates require minimum acetate levels near 1 mM and thus should be abundant in environments with high acetate availability (Liu & Whitman, 2008)....
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...Methanosarcinaceae are the most metabolically versatile group of methanogens, consuming acetate and capable of using methanol, methylamines, and for some terrestrial species also H2 (Galagan et al., 2002; Liu & Whitman, 2008)....
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TL;DR: A critical review that summarizes state-of-the-art technologies for biogas upgrading and enhancement with particular attention to the emerging biological methanation processes.
815 citations
Cites background from "Metabolic, phylogenetic, and ecolog..."
...On contrary, high H2 levels (> 10 Pa) inhibit the anaerobic digestion, and promote the accumulation of electron sinks such as lactate, ethanol, propionate, and butyrate (Liu and Whitman, 2008)....
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TL;DR: This review highlights recent progress in the research of anaerobic CH4 oxidation, ofCH4 production in the plant rhizosphere, of CH4 serving as substrate for the aquatic trophic food chain and the discovery of novel aerobic methanotrophs.
Abstract: Summary
The global budget of atmospheric CH4, which is on the order of 500–600 Tg CH4 per year, is mainly the result of environmental microbial processes, such as archaeal methanogenesis in wetlands, rice fields, ruminant and termite digestive systems and of microbial methane oxidation under anoxic and oxic conditions. This review highlights recent progress in the research of anaerobic CH4 oxidation, of CH4 production in the plant rhizosphere, of CH4 serving as substrate for the aquatic trophic food chain and the discovery of novel aerobic methanotrophs. It also emphasizes progress and deficiencies in our knowledge of microbial utilization of low atmospheric CH4 concentrations in soil, CH4 production in the plant canopy, intestinal methanogenesis and CH4 production in pelagic water.
729 citations
TL;DR: The objectives of this review are to evaluate options that have been demonstrated to mitigate enteric CH4 emissions per unit of ECM (CH4/ECM) from dairy cattle on a quantitative basis and in a sustained manner and to integrate approaches in genetics, feeding and nutrition, physiology, and health to emphasize why herd productivity, not individual animal productivity, is important to environmental sustainability.
638 citations
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...To date, 3 major genera and 3 minor genera of methanogens belonging to the Archaea domain have been identified, although it is likely that more exist (Wright et al., 2006; Janssen and Kirs, 2008; Liu and Whitman, 2008; Kong et al., 2013; Poulsen et al., 2013)....
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TL;DR: A strong positive interaction was found between protozoal numbers and methane emissions, and because this group is possibly not essential for rumen function, protozoa might be a target for methane mitigation.
Abstract: Ruminant production is under increased public scrutiny in terms of the importance of cattle and other ruminants as major producers of the greenhouse gas methane. Methanogenesis is performed by methanogenic archaea, a specialised group of microbes present in several anaerobic environments including the rumen. In the rumen, methanogens utilise predominantly H2 and CO2 as substrates to produce methane, filling an important functional niche in the ecosystem. However, in addition to methanogens, other microbes also have an influence on methane production either because they are involved in hydrogen (H2) metabolism or because they affect the numbers of methanogens or other members of the microbiota. This study explores the relationship between some of these microbes and methanogenesis and highlights some functional groups that could play a role in decreasing methane emissions. Dihydrogen (‘H2’ from this point on) is the key element that drives methane production in the rumen. Among H2 producers, protozoa have a prominent position, which is strengthened by their close physical association with methanogens, which favours H2 transfer from one to the other. A strong positive interaction was found between protozoal numbers and methane emissions, and because this group is possibly not essential for rumen function, protozoa might be a target for methane mitigation. An important function that is associated with production of H2 is the degradation of fibrous plant material. However, not all members of the rumen fibrolytic community produce H2. Increasing the proportion of non-H2 producing fibrolytic microorganisms might decrease methane production without affecting forage degradability. Alternative pathways that use electron acceptors other than CO2 to oxidise H2 also exist in the rumen. Bacteria with this type of metabolism normally occupy a distinct ecological niche and are not dominant members of the microbiota; however, their numbers can increase if the right potential electron acceptor is present in the diet. Nitrate is an alternative electron sinks that can promote the growth of particular bacteria able to compete with methanogens. Because of the toxicity of the intermediate product, nitrite, the use of nitrate has not been fully explored, but in adapted animals, nitrite does not accumulate and nitrate supplementation may be an alternative under some dietary conditions that deserves to be further studied. In conclusion, methanogens in the rumen co-exist with other microbes, which have contrasting activities. A better understanding of these populations and the pathways that compete with methanogenesis may provide novel targets for emissions abatement in ruminant production.
487 citations
Cites background from "Metabolic, phylogenetic, and ecolog..."
...Methane is also produced from acetate via the aceticlastic pathway and this pathway appears to be limited to members of the order Methanosarcinales (Liu and Whitman, 2008)....
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...Methane is produced in the gastrointestinal tract of ruminants and in particular within the rumen by a specialised group of microbes, the methanogenic archaea (Janssen and Kirs, 2008; Liu and Whitman, 2008)....
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...from the order Methanobacteriales (Liu and Whitman, 2008)....
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...Most rumen methanogens do not contain cytochromes and although they are less efficient at obtaining energy through the production of methane than their cytochrome-containing relatives of the order Methanosarcinales (Thauer et al., 2008), they are better adapted to the environmental conditions prevailing in the rumen....
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...There are three major substrates used by methanogens to produce methane: CO2, compounds containing a methyl group or acetate (Liu and Whitman, 2008)....
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References
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TL;DR: In this article, the complete 1.66-megabase pair genome sequence of an autotrophic archaeon, Methanococcus jannaschii, and its 58 and 16-kilobase pair extrachromosomal elements are described.
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TL;DR: Knowing the factors that impact methane production can result in the development of mitigation strategies to reduce methane losses by cattle and implementation of these strategies should result in enhanced animal productivity and decreased contributions by cattle to the atmospheric methane budget.
Abstract: Increasing atmospheric concentrations of methane have led scientists to examine its sources of origin. Ruminant livestock can produce 250 to 500 L of methane per day. This level of production results in estimates of the contribution by cattle to global warming that may occur in the next 50 to 100 yr to be a little less than 2%. Many factors influence methane emissions from cattle and include the following: level of feed intake, type of carbohydrate in the diet, feed processing, addition of lipids or ionophores to the diet, and alterations in the ruminal microflora. Manipulation of these factors can reduce methane emissions from cattle. Many techniques exist to quantify methane emissions from individual or groups of animals. Enclosure techniques are precise but require trained animals and may limit animal movement. Isotopic and nonisotopic tracer techniques may also be used effectively. Prediction equations based on fermentation balance or feed characteristics have been used to estimate methane production. These equations are useful, but the assumptions and conditions that must be met for each equation limit their ability to accurately predict methane production. Methane production from groups of animals can be measured by mass balance, micrometeorological, or tracer methods. These techniques can measure methane emissions from animals in either indoor or outdoor enclosures. Use of these techniques and knowledge of the factors that impact methane production can result in the development of mitigation strategies to reduce methane losses by cattle. Implementation of these strategies should result in enhanced animal productivity and decreased contributions by cattle to the atmospheric methane budget.
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