Hydrogen metabolizing life...

[Moontanman]

Could complex life exist on a rocky world with an extensive hydrogen atmosphere? This wiki article suggests hydrogen breathers might possibly exist..

 

http://en.wikipedia.org/wiki/Hydrogenosomes

 

A rocky planet with more mass than the earth but with a hydrogen atmosphere would retain heat due to the super greenhouse effect of hydrogen and could maintain liquid water on it's surface much further away than the Earth is from our sun...

In fact, I am going to speculate that hydrogen breathing life forms are likely more abundant than oxygen breathing life forms... We have already found several large rocky exoplanets, so far they seem to be more common than small rocky worlds.

 

The abundance of small red stars with large rocky planets is suggestive that such planets might be better abodes of life, a hydrogen atmosphere large rocky planet could orbit far enough away from a small red star that it wouldn't be tidally locked as a world with oxygen would have to be to maintain liquid water on it's surface...

[CharonY]

Actually hydrogenosomes produce hydrogen as a byproduct of energy generation. They do not actually respire it.

Respiration involves the transfer of an electron from a donor (usually NADH) to an acceptor, such as O2, nitrate, fumarate, Fe3+ to name a few examples. The redox potential between donor and acceptor drives the process, which in turn powers the transporters involved in creating a membrane potential necessary for ATP generation.

 

As such hydrogen can be an electron donor in some organisms, but not an acceptor.

Edited September 25, 2013 by CharonY

[Moontanman]

Actually hydrogenosomes produce hydrogen as a byproduct of energy generation. They do not actually respire it.

Respiration involves the transfer of an electron from a donor (usually NADH) to an acceptor, such as O2, nitrate, fumarate, Fe3+ to name a few examples. The redox potential between donor and acceptor drives the process, which in turn powers the transporters involved in creating a membrane potential necessary for ATP generation.

 

As such hydrogen can be an electron donor in some organisms, but not an acceptor.

 

 

OK, but how does this process allow multicellular organisms to live in anoxic environments?

Ok here is sort of what i am talking about...

 

http://io9.com/5555072/saturns-moon-titan-is-probably-home-to-hydrogen-breathing-microbes

 

Because Titan is so cold, it has lakes of methane and ethane where there would be lakes of liquid water on Earth. Still, the presence of such lakes at all suggests Titan is potentially complex enough to support some exotic form of life, and in 2005 two scientists theorized microbes could in fact live in these lakes if they were able to breathe hydrogen gas and consume the organic hydrocarbon acetylene. This process is a bit like certain kinds of anaerobic respiration used by bacteria and microbes on Earth, which also use hydrogen in place of oxygen.

 

[CharonY]

The part in bold is simply wrong, again hydrogen is not an electron acceptor. Whoever wrote it clearly misunderstood the process of hydrogen use (as electron donor, i.e. pretty much on the opposite side of the role of oxygen).

 

With regards to hydrogen production, as I mentioned, it is a byproduct of anaerobic energy generation. One way using substrate-level phosphorylation (i.e. a process to generate ATP that is independent of respiration). I am not an expert on the precise pathwys that hydrogenosomes take, but looking at the scheme it appears ferredoxins are being reduced (assuming that the graphic on wiki is correct). Then the transfer to H+ and generation of H2 is for the regeneration (i.e. re-oxidation) of ferredoxin and drives the production of acetyl-coA.

 

The actual energy generation happens in a different step. Again, we have microbes producing hydrogen (often to regenerate e.g. NAD) or which utilize it as electron donor. Just not as the role that oxygen has (i.e. electron acceptor. The redoxpotential is simply on the wrong end of the spectrum.

[Moontanman]

Ok, so the idea of a planet with the animals and plants using hydrogen as a breathing gas and plants using methane to produce hydrogen is bogus?

[CharonY]

Actually, if one wants to look at fancy energetics, one should not look at animals or plants, but at bacteria instead. They pretty much figured everything out that is energetically possible. Now if you focus on respiration and use methane and hydrogen analogous to water and oxygen then it is clearly energetically impossible. You cannot get energy out of that the way you can from oxygen (essentially like willing a ball to rolll up a hill on its own).

That being said, there are ways bacteria either produce or consume hydrogen or methane to survive. As mentioned, some are able to use hydrogen as electron donor (and then something else with a higher redox potential as acceptor). And as in the above example hydrogen production is used to regenerate reduced compounds (ferredoxins, NAD+) that are needed for substrate-level energy generation.

 

For methane we have also something. Methanotrophes are also able to use methane as electron donor (and also carbon source) . Then we have methanogenesis. In these cases methane is emitted as a consequence of the final step of the electron transfer chain (i.e. analogous to the water production in oxygen respiration). In this case electrons are transferred generally to either acetate or CO2.

As you can see, hydrogen and methane are only usable really at the front end of energy generation (i.e. electron donors) or are produced as a side aspect of respiration/fermentation processes.

 

Both cannot be linked in a chain to yield energy. It is a bit like placing a ball on a flat surface and willing it to accelerate.

 

However, with additional components it is possible to sustain life with these as core processes (as we have on Earth). Just the analogous use of hydrogen -> oxygen and methane ->water does not make much sense.

 

Typed this in a hurry so it may not be easily understandable. Need coffee.

Edited September 26, 2013 by CharonY

[Moontanman]

I remember as a teenager reading a book by Isaac Asimov where he suggested a sun powered saturation unsaturation cycle with saturated fats replacing proteins but I can't find a reference to it... except for me talking about on this forum 4 years ago...

[CharonY]

Well, at the minimum you would need a nitrogen source and some other precursors are needed.

[John Cuthber]

"A rocky planet with more mass than the earth but with a hydrogen atmosphere would retain heat due to the super greenhouse effect of hydrogen "

Hydrogen is not a greenhouse gas- it doesn't absorb IR radiation at all.

[Moontanman]

"A rocky planet with more mass than the earth but with a hydrogen atmosphere would retain heat due to the super greenhouse effect of hydrogen "

Hydrogen is not a greenhouse gas- it doesn't absorb IR radiation at all.

 

 

Sara Seager would seem to have a different take on that...

 

http://hoffman.cm.utexas.edu/courses/Science-2013-Seager-577-81.pdf

 

A Major Extension of the Habitable Zone

For our qualitative assessment of habitability, we

therefore focus on the dominant planetary atmospheric greenhouse gases and how

they delimit the habitable zone (Fig. 2).

The most important atmospheric greenhouse gas that extends the

habitable zone to large planet-star

separation is molecular hydrogen

(H2

). Planets are expected to form

with some primordial light gases,

either H2

(from interior outgassing)

(26, 31) or H2 and He (from gravitational capture of gas from the

surrounding protoplanetary disk).

Although small planets like Earth,

Venus, and Mars are unable to retain these light gases, more massive

or colder exoplanets are expected

to be able to do so. H2

is a formidable greenhouse gas, because it

can absorb radiation over a wide,

continuous wavelength range. Most

molecules absorb in discreet bands.

As a homonuclear molecule, H2 does

not have a dipole moment and therefore lacks the typical rotationalvibrational bands that absorb light

at near-IR wavelengths. However,

a momentary dipole is induced by

collisions, and thus at high enough

pressures, frequent collisions induce

very broadband absorption (32, 33).

Furthermore, H2 does not condense

until tens of kelvin at 1- to 100-bar

pressures (in comparison, CO2 condenses at about 190 to 250 K for 1-

to 10-bar pressures and is therefore

a cutoff for the cold end of conventional planet

habitability). The potency of H2 as a greenhouse

gas means that planets can have surface liquid

water at a factor of several times larger planetsun separations than planets with CO2

atmospheres (34) and even possibly extending to

rogue planets that were ejected from their birth

planetary system and are now floating through

the galaxy (35).

 

 

I can't seem to get the illustration to download but if you go to the link it is there...

Fig. 2. The habitable zone. The light blue region depicts the “conventional” habitable zone for

planets with N2-CO2-H2

O atmospheres (9, 10). The yellow region shows the habitable zone as extended

inward for dry planets (36, 37), as dry as 1% relative humidity (37). The outer darker blue region shows

the outer extension of the habitable zone for hydrogen-rich atmospheres (34) and can extend even out

to free-floating planets with no host star (35). The solar system planets are shown with images. Known

exoplanets are shown with symbols [here, planets with a mass or minimum mass less than 10 Earth

masses or a radius less than 2.5 Earth radii taken from (66)].

[John Cuthber]

I will have to look at that in more detail but...

" thus at high enough pressures, frequent collisions induce very broadband absorption "

I suspect that, under those conditions of temperature and pressure the world would be sufficiently different from ours to make any comparison unreliable.

Liquid hydrogen is about a thousand times denser than the gas at normal atmospheric conditions and it's transparent.

The data for H2 absorbing radiation are given here

http://www.astro.ku.dk/~aborysow/programs/AApaper.2002.pdf

but the units are not ones I'm familliar with so I will have to convert them to something I can compare to other materials

[Moontanman]

Some hydrogen metabolizing organisms:

 

http://www.phschool.com/science/science_news/articles/hungry_for_hydrogen.html

 

Pace and his colleagues describe their results in an upcoming Proceedings of the National Academy of Sciences.

Although hydrogen-consuming bacteria have been found in other low-oxygen environments, such as rice paddies and cow rumens, the new study hints that such microbes may be surprisingly prevalent.

"This will popularize the importance of hydrogen metabolism in the environment. Most people don't think about it very much," says Pace. The finding also implies that many bacteria in deep-sea hydrothermal vents, for instance, may metabolize hydrogen instead of sulfur.

Although he finds the Colorado group's findings surprising, Ken Nealson of the University of Southern California in Los Angeles says that they also make a lot of sense. "Hydrogen is the most abundant element in the universe," he notes. "Any wise organism would choose hydrogen over sulfur."

Nealson says that Pace's results may not only cause researchers to take a closer look at the microbes on Earth but also stimulate new ideas for scientists searching for life on other planets. For instance, volcanic activity and other hydrogen-generating geochemical processes that have been observed in the solar system might host microbial life, says Nealson.

 

[Moontanman]

More on hydrogen "breathing" organisms.

 

https://en.wikipedia.org/wiki/Methanogen

 

 

 

Physical description[edit]

Methanogens are coccoid (spherical shaped) or bacilli (rod shaped). There are over 50 described species of methanogens, which do not form a monophyletic group, although all methanogens belong to Archaea. They are anaerobic organisms and cannot function under aerobic conditions. They are very sensitive to the presence of oxygen even at trace level. Usually, they cannot sustain oxygen stress for a prolonged time. However, Methanosarcina barkeri is exceptional in possessing a superoxide dismutase (SOD) enzyme, and may survive longer than the others in the presence of O₂.^[4][5] Some methanogens, called hydrogenotrophic, use carbon dioxide (CO₂) as a source of carbon, and hydrogen as a reducing agent.

The reduction of carbon dioxide into methane in the presence of hydrogen can be expressed as follows:

CO₂ + 4 H₂ → CH₄ + 2H₂O

Some of the CO₂ is reacted with the hydrogen to produce methane, which creates an electrochemical gradient across cell membrane, used to generate ATP through chemiosmosis. In contrast, plants and algae use water as their reducing agent.

Methanogens lack peptidoglycan, a polymer that is found in the cell walls of the Bacteria but not in those of Archaea. Some methanogens have a cell wall that is composed of pseudopeptidoglycan. Other methanogens do not, but have at least one paracrystalline array (S-layer) made up of proteins that fit together like a jigsaw puzzle.