水素、ギ酸、酢酸 などの電子を用いて二酸化炭素をメタンまで還元するメタン細菌の反応（特異な酵素および補酵素群）はまだ解明されていないのか？
メタン菌以外の生物はこの代謝系を持っていないというのが古地球環境と関連して面白い。即ち以下の点において。
＊メタン生成系の酵素は極めて酸素に弱く、空気に触れるだけで容易に失活する（Ｗｉｋｉ）。
嫌気系で二酸化炭素にどのような酵素群を用いて電子を供給しメタンを発生させているのか。
www1.uni-frankfurt.de/fb/fb15/institute/inst-3-mol-biowiss/AK-Rother/research
一方、生成メタンから電気を得るにはＣ－Ｈ結合エネルギーの解放であろう。手っ取り早いのは燃焼であるが、ゆっくり燃焼させて電気を得る方法の開発が重要である。
How microbes acquire electricity in
making methane
Date: May 18, 2015 sciencedaily.com
Source: Stanford University
Source: Stanford University
Summary: Scientists have solved a long-standing mystery about methanogens, unique microorganisms that transform electricity and carbon dioxide into methane.
The results could pave the way for microbial 'factories' that produce renewable biofuels and chemicals.
Stanford University scientists have solved a long-standing mystery about methanogens, unique microorganisms that transform electricity and carbon dioxide into methane.
In a new study, the Stanford team demonstrates for the first time how methanogens obtain electrons from solid surfaces.
The discovery could help scientists design electrodes for microbial "factories" that produce methane gas and other compounds sustainably.
"There are several hypotheses to explain how electrons get from an electrode into a methanogen cell,"
said Stanford postdoctoral scholar Jörg Deutzmann, lead author of the study.
"We are the first group to identify the actual mechanism."
The study is published in the current issue of the journal mBio.
"The overall goal is to create large bioreactors where microbes convert atmospheric carbon dioxide and clean electricity from solar, wind or nuclear power into renewable fuels and other valuable chemicals,"
said study co-author Alfred Spormann, a professor of chemical engineering and of civil and environmental engineering at Stanford.
"Now that we understand how methanogens take up electricity, we can re-engineer conventional electrodes to deliver more electrons to more microbes at a faster rate."
The study also provided new insights on microbially influenced corrosion, a biological process that threatens the long-term stability of structures made of iron and steel.
"Biocorrosion is a significant global problem,"
Spormann said.
"The yearly economic loss caused by this process is estimated to be in the $1 billion range."
Methane from microbes
Methane is an important fuel for heating, transportation, cooking and generating electricity.
Most methane comes from natural gas, an abundant fossil fuel extracted from wells. However, burning natural gas emits carbon dioxide, which accelerates global warming.
Methanogens offer a promising alternative. These single-celled organisms resemble bacteria but belong to a genetically distinct domain called Archaea.
Commonly found in sediments and sewage treatment plants, methanogens thrive on carbon dioxide gas and electrons. The byproduct of this primordial meal is pure methane gas, which the microbes excrete into the air.
Researchers are trying to develop large bioreactors where billions of methanogens crank out methane around the clock. These microbial colonies would be fed carbon dioxide from the atmosphere and clean electricity from electrodes.
The entire process would be carbon neutral, Spormann explained.
"When microbial methane is burnt as fuel, carbon dioxide gets recycled back into the atmosphere where it originated,"
he said.
"Natural gas combustion, on the other hand, frees carbon that has been trapped underground for millions of years."
Electron uptake
Producing microbial methane on an industrial scale will require major improvements in efficiency, Deutzmann said.
"Right now the main bottleneck in this process is figuring out how to get more electrons from the electrode into the microbial cell,"
he said.
"To do that, you first have to know how electron uptake works in methanogens. Then you can engineer and enhance the electron-transfer rate and increase methane production."
In nature, methanogens acquire electrons from hydrogen and other molecules that form during the breakdown of organic material or bacterial fermentation.
"These small molecules are food for the microbes,"
Deutzmann said.
"They provide methanogens with electrons to metabolize carbon dioxide and produce methane."
In the Spormann lab, methanogens don't have to worry about food.
Electrons are continuously supplied by a low-voltage current via an electrode.
How those electrons get into the methanogen cell has been the subject of scientific debate.
"The leading hypothesis is that many microbes, including methanogens, take up electrons directly from the electrode,"
Deutzmann said.
"But in a previous study, we found evidence that microbial enzymes and other molecules could also play a role. From an engineering perspective, it makes a difference if you have to design an electrode to accommodate large microbial cells versus enzymes. You can attach a lot more enzymes to the electrode, because enzymes are a lot smaller."
Experiments with enzymes
For the experiment, the Stanford team used a species of methanogen called Methanococcus maripaludis. Cultures of M. maripaludis were grown in flasks equipped with a graphite electrode, which provided a steady supply of electrons. The microbes were also fed carbon dioxide gas.
As expected, methane gas formed inside the flasks, a clear indication that the methanogens were taking up electrons and metabolizing carbon dioxide.
But researchers also detected a build-up of hydrogen gas. Were these molecules of hydrogen shuttling electrons to the methanogens, as occurs in nature?
To find out, the Stanford team repeated the experiment using a genetically engineered strain of M. maripaludis.
These mutant methanogens had six genes deleted from their DNA so they could no longer produce the enzyme hydrogenase, which microbes need to make hydrogen. Although the mutants were grown in the same conditions as normal methanogens, their methane output was significantly lower.
"When hydrogenase was absent from the culture, methane production plummeted 10-fold,"
Spormann said.
"This was a strong indication that hydrogen-producing enzymes are significantly involved in electron uptake."
Further tests without methanogen cells confirmed that hydrogenase and other enzymes take up electrons directly from the electrode surface. The microbial cell itself is not involved in the transfer, as was widely assumed.
"It turns out that all kinds of enzymes are just floating around in the culture medium,"
Deutzmann said.
"These enzymes can attach to the electrode surface and produce small molecules, like hydrogen, which then feed the electrons to the microbes."
Normal methanogen cells produce a variety of enzymes. Stirring, starvation and other biological factors can cause the cells to break open, releasing enzymes into the culture medium, Deutzmann said.
Biocorrosion
"Now that we know that certain enzymes take up electrons, we can engineer them to work better and search for other enzymes that do it even faster,"
he added.
"Another benefit is that we no longer have to design large, porous electrodes to accommodate the entire methanogen cell."
The Stanford team also discovered that methanogen enzymes play a similar role in biocorrosion. The researchers found that granules of iron transfer electrons directly to hydrogenase.
The enzyme uses these electrons to make hydrogen molecules, which, in turn, are consumed by methanogens.
Eliminating hydrogenase from the environment could slow down the rate of corrosion, according to the scientists.
"At first we were surprised by these results, because enzymes were thought to degrade very quickly once they were outside the cell,"
Spormann said.
"But our study showed that free enzymes attached to an electrode surface can remain active for a month or two. Understanding why they are stable for so long could lead to new insights on reducing corrosion and on scaling up the production of microbial methane and other sustainable chemicals."
Story Source:
The above story is based on materials provided by Stanford University. Note: Materials may be edited for content and length.
web.stanford.edu/group/gcep/cgi-bin/gcep-research/all/capturing-electrical-current-via-microbes-to-produce-methane/
dailykos.com/story/2009/12/30/810817/-A-new-efficient-renewable-energy-cycle#
journal.frontiersin.org/article/10.3389/fmicb.2014.00597/full
燃料電池がもたらすのは水素社会よりメタン社会？
河合 基伸＝日経エレクトロニクス2014/01/20 05:00
燃料電池はこれまで、環境意識の高い企業や家庭を中心に、じわじわと導入量を増やしてきました。それがここへ来て、災害時などの非常用電源として、さらには夏場などの電力不足の解消に、そして将来の再生可能エネルギーの余剰電力問題を解決する手段として急速に注目度が高まっています（日経エレクトロニクス 2014年1月20日号 特集「発電所がやってくる」参照）。
2013年11月には米Bloom Energy社が、福岡市に業務用燃料電池システムを設置しました。燃料電池システムを売るのではなく、発電した電力を販売するビジネスモデルです。価格は23～28円/kWhと決して安くはありませんが、非常用電源としても使える利点を強くアピールしています。
この他に岩谷産業は、再生可能エネルギーの余剰電力で水電解して水素を貯蔵し、電力の不足時に燃料電池で発電する実験を北九州市で始めました。
燃料電池車の導入をにらんで、海外から安価な水素を大量に運んでこようという動きも活発になっています。千代田化工建設が有機ハイドライドで、川崎重工業が液化水素での運搬を計画しています。ただし、
「燃料電池車だけでは水素の使用量が少なすぎて、価格が十分に下がらない」
（アナリスト）ため、燃料電池車以外の供給先も確保する必要があります。千代田化工建設は川崎市と共同で、川崎市臨海部の工場や発電所などへ水素を供給することを計画しています。
これらの動きによって、「水素社会」が到来するとされています。
しかし、家庭用や業務用の燃料電池へ水素を直接供給するには、水素の供給網をきめ細かく敷設する必要があります。それには時間がかかりそうです。
今回の特集の取材でも、ガス会社などはあまり乗り気ではありませんでした。自動車や発電所には水素を供給し、家庭や企業には従来通り都市ガスを供給となれば、どちらも量による低コスト化が十分に進みません。
そこでドイツなどで注目を集めているのが、水素からメタンなどの各種の炭化水素を製造する技術です。
風力発電などの余剰電力で水素を製造し、大気中のCO2と反応させてCOと水を生成、さらにCOに再び水素を混ぜてメタンを合成するのです。
このメタンを、そのまま都市ガスのインフラに供給します。既存のインフラを活用できるだけでなく、都市ガスのCO2フリー化が可能になる利点もあります。
水素インフラができるまでの経過措置として、日本でも一時的に水素由来のメタンを利用する「メタン社会」が到来するかもしれません。ひょっとすると、そのまま定着する可能性があると考えるのは、私だけでしょうか。