Methanosarcina acetivorans

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A Microbial Biorealm page on the genus Methanosarcina acetivorans

Classification

Higher order taxa

Archaea; Euryarchaeota; Methanomicrobia; Methanosarcinales; Methanosarcinaceae; Methanosarcina

Species

NCBI: Taxonomy

Methanosarcina acetivorans

Description and significance

Methanosarcina acetivorans is a recently discovered "acetotrophic marine methane-producing bacterium that was isolated from methane-evolving sediments" (1). Early samples of these marine sediments were obtained from the Sumner branch of Scripps Canyon located near La Jolla, CA. It is also important to note that this species has optimal growth at 35 to 40 degrees Celcius and an optimal pH range of 6.5 to 7.0. Additionally, NaCl and Mg2+ are required for its growth (1). After Methanosarcina acetivorans was enriched and isolated, the colonies were found to be blue-green when examined under UV fluorescence microscopy. The colonies that were incubated in roll tubes were pale yellow and found in colonies that were 0.5 mm in diameter (after 14 days), smooth, and circular (1).

Methanosarcina acetivorans was sequenced due to the availability of genetic tools (such as shuttle vectors and shotgun sequencing) that allow for it to be used as a model species for methanogens(3). The C2A strain was isolated from acetate-grown cultures and is the primary strain that is studied for Methanosarcina acetivorans (1). Methanosarcina acetivorans is a unique methanogen because it uses acetate as a source of carbon and as a source of energy. It does so by breaking down acetate to produce carbon dioxide and methane. This property makes it a possible key player in global warming.

Genome structure

The genome of Methanosarcina acetivorans is one of the largest known archaeon genomes and and is comparatively larger than many sequenced prokaryote genomes. It consists of 5,751,492 nucleotides and 4528 protein coding genes (1). Of the protein coding genes, 49% have an assigned putative function, 20% are conserved hypothetical proteins, and 31% are predicted to be proteins with unknown functions (1). This makes it "over three times as large as the two previously sequenced methanogens, Methanobacterium thermoautotrophicum and Methanococcus jannaschii" (2). It is also significant to note that Methanosarcina acetivorans has a linear genome.

The plasmid pC2A was isolated from Methanosarcina acetivorans and thoroughly studied, but the exact function of plasmid pC2A remains unknown. The pC2A plasmid appears to be stable because it was isolated from strains that were maintained in laboratory settings for a long period of time (4). Due to the recent discovery of this plasmid, limited information of plasmid pC2A is available.

Cell structure and metabolism

Methanosarcina acetivorans has a great number and diversity of surface layer proteins in comparison to other archaeal species (1). Although Methanosarcina acetivorans is Gram-negative, there were thin sections that revealed a monolayer cell wall 10nm thick that is characteristic of marine methanogenic bacteria with a protein cell wall (2). Motility has not been observed in any Methanosarcina species, but one flagellin (fla) and two complete chemotaxis (che) gene clusters were found which may suggest that motility could be possible under certain situations (2).

Methanosarcina acetivorans is also capable of methanogenesis (a form of anaerobic respiration) which classifies it as a methanogen (capable of producing methane). Acetic acid is the terminal electron acceptor in this pathway by which methane and carbon dioxide are evolved from acetic acid. Additionally, some species of Methanosarcina acetivorans are capable of growing by using carbon monoxide (CO) as a methanogenic substrate (5).

Ecology

Methanosarcina acetivorans interacts with other organisms and affects the environment primarily through its production of methane (2). Methane is a potent greenhouse gas that reflects heat better than carbon dioxide. Due to this ability of Methanosarcina acetivorans, it is capable of not only affecting organisms that it is in direct contact with, but all organsims that are affected by greenhouse gases (such as humans). Methane is not toxic, but it is highly flammable which causes the oil wells, sewage lagoons, trash dumps, decaying leaves, and stream sediments where Methanosarcina species is typically found, to be potentially explosive. Aside from its environmental influences, not much is known about the interaction of Methanosarcina acetivorans with other microorganisms.

Pathology

Methanosarcina acetivorans is not known to cause any diseases. Refer to ecology section for information on how this microbe affects humans.

Application to Biotechnology

The biotechnological implications of Methanosarcina acetivorans are limited because a precise understanding of the enzymatic processes involved in methanogenesis are still a mystery. A single-subunit enzyme (CO dehydrogenase) that is used by Rhodospirillum rubrum was found to be present on the genome of Methanosarcina acetivorans which allows certain strains to grow on carbon monoxide (2). Methanosarcina acetivorans is a methanogen which means that it is capable of producing methane. Methane is a potential alternative energy source.

Current Research

Current research conducted by the Broad Institute of MIT seeks to understand the role of Methanosarcina acetivorans in the global carbon cycle especially because it is the only known species to possess all three known pathways for methanogenesis (3). Additionally, research of methanogenesis on the genetic level would allow for more detailed analysis of the process; however, the large size of the Methanosarcina acetivorans makes this quite difficult (4). Additionally, the exact mechanism for methanogensis in Methanosarcina acetivorans is still being researched, but could have implications for controlling the amount of methane that is released into the atmosphere.

Research of plasmid pC2A of Methanosarcina acetivorans is also being conducted to find the function of the plasmid and possible applications of this plasmid. The plasmid was found to be approximately 5.1 kilobase pairs in size and there are possibly a low number of plasmids per organism. Plasmid pC2A is one of the first plasmids described in an acetotrophic methanogen, which makes it especially useful for the construction of shuttle vectors to be used in the genetic analysis of other methanogens. The relatively small size of this plasmid allows for it to be easily manipulated with a minimum amount of shearing. Further research will be used to develop a useful vector that is capable of transferring gene fragments (4).

Current research is also being done on the ability of Methanosarcina acetivorans to grow using carbon monoxide (CO). The results of the research indicate that carbon dioxide reduction to methane in Methanosarcina acetivorans plays a role in supporting the carbon monoxide dependent growth in Methanosarcina acetivorans (5). The research suggests that proteins are differentially abundant in carbon monoxide grown Methanosarcina acetivorans in comparison to Methanosarcina acetivorans that were grown from methanol and acetate. Also, the findings seem to support that marine methanogens have evolved pathways that are independent of the use of hydrogen (5). There is also a proposal that the pathway for acetate formation from carbon monoxide is reminiscent of an ancient energy-conservation mechanism that involved the carbon dioxide reduction and acetate fermentation which ultimately led to methanogenesis (5).

References

(1) Sowers, K.R., Baron, S.F., and Ferry, J.G. 1984. Methanosarcina acetivorans sp. nov., an acetotrophic methane-producing bacerium isolated from marine sediments. Appl. Environ. Microbiol. 47: 971-978

(2) Galagan, J.E., et al. "The Genome of M. acetivorans Reveals Extensive Metabolic and Physiological Diversity" Genome Res., Apr 2002, 12: 532 - 542.

(3) Metcalf, W.W., Zhang, J.K., Apolinario, E., Sowers, K.R., and Wolfe, R.S., "A genetic system for Archaea of the genus Methanosarcina: Liposome-mediated transformation and construction of shuttle vectors" Proc Natl Acad Sci U S A. 1997 March 18; 94(6): 2626–2631.

(4) Sowers, K.R., Gunsalus, R.P., "Plasmid DNA from the acetotrophic methanogen Methanosarcina acetivorans." J Bacteriol. 1988 October; 170(10): 4979–4982.

(5) Lessner, D.J., Li, L., Li, Q., Rejtar, T., Andreev, V.P, et al. "An unconventional pathway for reduction of CO2 to methane in CO-grown Methanosarcina acetivorans revealed by proteomics." Proc Natl Acad Sci U S A. 2006 November 21; 103(47): 17921–17926.


Edited by Paul Molina, student of Rachel Larsen and Kit Pogliano

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