Taxonomic, Phylogenetic, and Ecological Diversity of Methanogenic Archaea

In this work, H2 production by anaerobic mixed cultures was reviewed. First, the different anaerobic microbial communities that have a direct relation with the generation or consumption of H2 are discussed. Then, the different methods used to inhibit the H2-consuming bacteria are analyzed (mainly in the methanogenesis phase) such as biokinetic control (low pH and short hydraulic retention time), heat-shock treatment and chemical inhibitors along with their advantages/disadvantages for their application on an industrial scale. After that, biochemical pathways of carbohydrate degradation to H2, organic acids and solvents are showed. Fourth, structure, diversity and dynamics of H2-producers communities are detailed. Later, the hydrogenase structure and activity is related with H2 production. Also, the causes for H2 production inhibition are analyzed along with strategies to avoid it. Finally, immobilized-cells systems are presented as a way to enhance H2 production.

Hydrogen production by fermentative consortia

Methanogenic archaea reside primarily in the rumen and the lower segments of the intestines of ruminants, where they utilize the reducing equivalents derived from rumen fermentation to reduce carbon dioxide, formic acid, or methylamines to methane (CH4). Research on methanogens in the rumen has attracted great interest in the last decade because CH4 emission from ruminants contributes to global greenhouse gas emission and represents a loss of feed energy. Some DNA-based phylogenetic studies have depicted a diverse and dynamic community of methanogens in the rumen. In the past decade, researchers have focused on elucidating the underpinning that determines and affects the diversity, composition, structure, and dynamics of methanogen community of the rumen. Concurrently, many researchers have attempted to develop and evaluate interventions to mitigate enteric CH4 emission. Although much work has been done using plant secondary metabolites, other approaches such as using nitrate and 3-nitrooxy propanol have also yielded promising results. Most of these antimethanogenic compounds or substances often show inconsistent results among studies and also lead to adverse effects on feed intake and digestion and other aspects of rumen fermentation when fed at doses high enough to achieve effective mitigation. This review provides a brief overview of the rumen methanogens and then an appraisal of most of the antimethanogenic compounds and substances that have been evaluated both in vitro and in vivo. Knowledge gaps and future research needs are also discussed with a focus on methanogens and methane mitigation.

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Rumen methanogens and mitigation of methane emission by anti-methanogenic compounds and substances

Selective inhibition of methanogens for the improvement of biohydrogen production in microbial electrolysis cells

This mini-review summarizes the category, characteristics, and the application fields of the chemical methanogenic inhibitors. Usually, the chemical methanogenic inhibitors can be divided into "specific" and nonspecific inhibitors. The former group includes the structural analogs of coenzyme M and HMG-CoA inhibitors. The nonspecific group includes many chemicals which can inhibit the activity of both methanogens and non-methanogens. The chemical inhibitors of methanogenesis have been widely used in the fields of understanding methane production and consumption in pure culture or in complex natural environment, production of value-added substances, such as volatile fatty acids and hydrogen, and reduction of energy loss and improvement of the efficiency of ruminal energetic transformations. Finally, with an increasing understanding of the mechanistic effects of the chemical inhibitors of methanogenesis, it is possible that some could be used to develop into promising feed additives to reduce losses associated with enteric methane production or as useful tools to screen microbial consortia from various biotechnological applications to enhance hydrogen and acid production.

Chemical inhibitors of methanogenesis and putative applications.

The pterin compound lumazine [2, 4-(1H, 3H)-pteridinedione] inhibited the growth of several methanogenic archaea completely at a concentration of ≤ 0.6 mM and was bacteriocidal for Methanobacterium thermoautotrophicum strain Marburg. In contrast, growth of two non-methanogenic archaea, several eubacteria, and one eukaryote was not strongly affected at much higher concentrations. In washed-cell suspensions, methanogenesis from H2 and CO2 by Mb. thermoautotrophicum or from H2 and methanol by Methanosarcina barkeri was inhibited by addition of lumazine. In cell-free extracts of Mb. thermoautotrophicum, H2-driven methane production from CO2 or CH3-S-CoM was completely inhibited by 0.6 mM lumazine. The results suggest that the compound may be useful in probing the methanogenesis pathway or in selecting against methanogens.

Ross Teal

Papers

The pterin lumazine inhibits growth of methanogens and methane formation