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Noclevername
2013-Apr-21, 03:37 AM
I'm working currently on an SF story that has a human-habitable exoplanet.

In order to avoid having the native life be too Earthlike, I have a few questions regarding biochemistry.

1) Would it be possible to have a free-oxygen-producing process with a chemistry other than chlorophyl? I have read that purple bacteria conduct photosynthesis using sulphur compounds rather than CO2, water and hydrocarbons, so I can't use that, and other pigments like melanin react to light, but I'm looking specifically for something that could lead to breathable air.

2) If not, could it happen for chlorophyl to evolve independently in parallel without straining the bounds of coincidence too much? I've heard it mentioned somewhere that chlorophyl-like precursor compounds have been observed in space bodies, but I'm not sure that is true or that it might lead to the same result.

3) Also, I plan for my my alien pseudoplants lack cellulose and instead develop glass-sponge-like silica spicules. Would this work for land life?

(Out of idle curiousity, are there any Earth microbes that use chlorophyl but not cellulose?)

Colin Robinson
2013-Apr-21, 07:36 AM
I'm working currently on an SF story that has a human-habitable exoplanet.

In order to avoid having the native life be too Earthlike, I have a few questions regarding biochemistry.

1) Would it be possible to have a free-oxygen-producing process with a chemistry other than chlorophyl? I have read that purple bacteria conduct photosynthesis using sulphur compounds rather than CO2, water and hydrocarbons, so I can't use that, and other pigments like melanin react to light, but I'm looking specifically for something that could lead to breathable air.

2) If not, could it happen for chlorophyl to evolve independently in parallel without straining the bounds of coincidence too much? I've heard it mentioned somewhere that chlorophyl-like precursor compounds have been observed in space bodies, but I'm not sure that is true or that it might lead to the same result.

I would have thought both of these scenarios are quite plausible.

I don't think we know whether plants on Earth use chlorophyl because it is the only or optimal way of doing photosynthesis, or whether it is because it's one of several workable solutions, and happens to be the solution which their common ancestor reached by chance.

Even on Earth, different organisms have different versions of chlorophyl. If plant-like organisms on another planet have chlorophyl also, presumably it would be different again.


3) Also, I plan for my my alien pseudoplants lack cellulose and instead develop glass-sponge-like silica spicules. Would this work for land life?

I can't see why it couldn't work.


(Out of idle curiousity, are there any Earth microbes that use chlorophyl but not cellulose?)

Diatoms come to mind. They do use some cellulose, but the character of their cell walls is largely due to silica.

ravens_cry
2013-Apr-21, 03:42 PM
While they don't use chlorophyll, true fungi use chitin in their cell walls, and it is not terribly hard, at least to my limited understanding, to imagine a creature that uses chitin like fungi but chlorophyll like a plant.

Noclevername
2013-Apr-22, 10:41 AM
What kind of size limits would these changes imply? (The planet setting has 1.2 G)

chornedsnorkack
2013-Apr-23, 07:00 AM
A problem with chitin is that it differs from cellulose in incorporating nitrogen. Fungi who are eating nitrogen-rich biomass anyway can afford it; plants are often short of nitrogen, so cellulose is a cheap material for them.

Silica has been a limiting nutrient in ocean since the spread of diatoms in Mesozoic. Land plants capable of using silica might have roots that actively dissolve and transport silica... but this would mean that the land under a tree will be sinking.

Noclevername
2013-Apr-23, 07:14 AM
A problem with chitin is that it differs from cellulose in incorporating nitrogen. Fungi who are eating nitrogen-rich biomass anyway can afford it; plants are often short of nitrogen, so cellulose is a cheap material for them.


A nitrogen-fixing bacteria-analog should solve that problem. In addition, it'll make the alien soil more fertile for planting Earth crops.


Silica has been a limiting nutrient in ocean since the spread of diatoms in Mesozoic. Land plants capable of using silica might have roots that actively dissolve and transport silica... but this would mean that the land under a tree will be sinking.

But silica from dead plantoids should make its way back into the ground as the organic parts decay, I think. There are Earth plants with silica components, like most grasses.

trinitree88
2013-Apr-24, 08:43 PM
I'm working currently on an SF story that has a human-habitable exoplanet.

In order to avoid having the native life be too Earthlike, I have a few questions regarding biochemistry.

1) Would it be possible to have a free-oxygen-producing process with a chemistry other than chlorophyl? I have read that purple bacteria conduct photosynthesis using sulphur compounds rather than CO2, water and hydrocarbons, so I can't use that, and other pigments like melanin react to light, but I'm looking specifically for something that could lead to breathable air.

2) If not, could it happen for chlorophyl to evolve independently in parallel without straining the bounds of coincidence too much? I've heard it mentioned somewhere that chlorophyl-like precursor compounds have been observed in space bodies, but I'm not sure that is true or that it might lead to the same result.

3) Also, I plan for my my alien pseudoplants lack cellulose and instead develop glass-sponge-like silica spicules. Would this work for land life?

(Out of idle curiousity, are there any Earth microbes that use chlorophyl but not cellulose?)


Noclevername. The history of photochemistry in organism began with brown algae, using the red end of the visible spectrum. Then it was red algae, I believe, using the blue end of the spectrum. When green algae evolved with chlorophyll.......it used both ends and hindered further evolution of both of it's predecessors, nice evolutionary niche. So you could invoke the old ppigments.


When I'd walk the beach in georgetown maine @ reid state park with my kids.....you'd have a billion years of evolution washing around in the tide pools...pete.

Noclevername
2013-Apr-24, 09:13 PM
Noclevername. The history of photochemistry in organism began with brown algae, using the red end of the visible spectrum. Then it was red algae, I believe, using the blue end of the spectrum. When green algae evolved with chlorophyll.......it used both ends and hindered further evolution of both of it's predecessors, nice evolutionary niche. So you could invoke the old ppigments.


Brown algae and red algae both use chlorophyl in chloroplasts, so that's probably not the exact terms you were looking for.

Did the old pigments produce a breatheable atmosphere by human standards? IIRC photosynthesizing life produced little free oxygen until after the chlorophyls came along.

cjameshuff
2013-Apr-24, 10:50 PM
A problem with chitin is that it differs from cellulose in incorporating nitrogen. Fungi who are eating nitrogen-rich biomass anyway can afford it; plants are often short of nitrogen, so cellulose is a cheap material for them.

Earth plants are short of nitrogen, there's no reason alien plants couldn't fix it themselves.

Colin Robinson
2013-Apr-25, 02:39 AM
Earth plants are short of nitrogen, there's no reason alien plants couldn't fix it themselves.

If an organism combines characteristics of a plant and a fungus, perhaps we could call it a flant. Or a plungus?

Nick Theodorakis
2013-Apr-25, 12:45 PM
Brown algae and red algae both use chlorophyl in chloroplasts, so that's probably not the exact terms you were looking for.

Did the old pigments produce a breatheable atmosphere by human standards? IIRC photosynthesizing life produced little free oxygen until after the chlorophyls came along.

Non-oxygen evolving photosynthetic bacteria are still among us, and they typically use related pigments called bacteriochlorophylls. It's the whole light absorbing photosystem complex that's involved in capturing the the light and using the energy to extract an electron from water (thus splitting it into oxygen and H+). Non-oxygen evolving photosynthetic bacteria pull electrons from other molecules like hydrogen sulfide. It's much harder to split water to get electrons but once it was figured out, those bacteria were at a tremendous advantage because water is everywhere. I suppose it would have been possible to have evolved other pigments.

Now, some photosynthetic archaea use a completely different system based on bacteriorhodopsin, which is actually related to the light absorbing proteins, rhodopsins, in our eyes, and use retinal as the light capturing molecule. They don't split water or anything else as far as I remember, but rather pump hydrogen ions across the membrane to generate a proton gradient that can be used for energy. (Bacterial or chloroplast photosynthesis also makes a proton gradient, but they do it in a more complicated way).

Nick

Noclevername
2013-Apr-30, 07:16 PM
Bump! Anyone have ideas on how tall such plantoids (chitin or spicule supported) could grow on land?

FarmMarsNow
2013-May-01, 12:32 AM
Wild guess! Your 'Plants' might metabolize energy by using their silica spicules to catalyze the production of Phosgene which they then break down with an endo-thermic reaction into dichloromethane and O2. While they might utilize light to direct their life processes, that is not the basis of their energy source. Instead of gathering energy like we do and dispersing it as heat, they must absorb heat directly to live and convert it to chemical bond energy which they then expel. One of the results of this phenol based life could be nylon or polycarbonate or a silicon polymer with phenols, so your plants would have silicon based organs and plastic skin. They'd be very toxic but they would produce oxygen. Would all of the above chemistry work? Probably not, but its a wild guess.

Noclevername
2013-May-01, 12:49 AM
They'd be very toxic but they would produce oxygen.

Like that Star Trek episode with the space hippies? Yeah, brother!

FarmMarsNow
2013-May-01, 04:17 AM
Like that Star Trek episode with the space hippies? Yeah, brother! I've seen many but not that one. No...wait I did see it. That's the one with the hypno-flowers.

Noclevername
2013-May-01, 04:32 AM
I've seen many but not that one. No...wait I did see it. That's the one with the hypno-flowers.

Nope! The other space hippies (http://en.memory-alpha.org/wiki/The_Way_to_Eden_(episode)).

FarmMarsNow
2013-May-03, 04:39 AM
Herbert, Herbert, Herbert.

Delvo
2013-May-12, 11:49 PM
Any process which separates oxygen from a compound and uses the compound's other atom(s) will yield free oxygen. Plants break up carbon dioxide to get the carbon. Since carbon's usefulness to biochemistry is pretty well established, and carbon dioxide is a pretty common compound on planets, it's entirely reasonable to expect alien life forms to evolve to use carbon dioxide as a source of carbon. And as soon as they're doing that, there's physically no way they could fail to be releasing free oxygen.

This could be done with a variety of pigments, and is on earth, but there's also some reason to expect chlorophyll to be the dominant one in general. And that's just because it is here, in plants that are known to produce and use other pigments as well, so we know chlorophyll didn't just squeak by due to lack of alternatives. If one of those others that are found in the very same leaf with chlorophyll were more useful to our plants here, then they would be producing and using that more-useful alternative in larger amounts, but they don't.

Nick Theodorakis
2013-May-13, 12:32 AM
Any process which separates oxygen from a compound and uses the compound's other atom(s) will yield free oxygen. Plants break up carbon dioxide to get the carbon. Since carbon's usefulness to biochemistry is pretty well established, and carbon dioxide is a pretty common compound on planets, it's entirely reasonable to expect alien life forms to evolve to use carbon dioxide as a source of carbon. And as soon as they're doing that, there's physically no way they could fail to be releasing free oxygen.
...

That's not where the oxygen comes from, and that's not how carbon is fixed from CO2. Oxygen comes from the splitting of water in photosystem I or II during the capture of photons. CO2 is fixed by the enzyme ribulose-bisiphosphate carboxylase (Rubisco) by converting one molecule of CO2 and one molecule of ribulose-bis-phosphate into 2 molecules 3-phosphoglycerate during the Calvin cycle. No oxygen is evolved in that reaction, and even non-oxygen-evolving photosynthetic bacteria use this reaction.

Nick

eburacum45
2013-May-13, 11:41 AM
Photosynthesis can happen near warm deep-sea vents, using only infra-red radiation and in complete darkness as far as human eyes can see.
http://www.phschool.com/science/science_news/articles/grow_in_dark.html
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1166624/

There could be warm deep-sea vents on Europa, or on a Europa-type planet in deep space, and photosynthetic life could occur in these locations. Whether it does or not is another matter.

Delvo
2013-May-13, 12:08 PM
No oxygen is evolved in that reaction...I wasn't just talking about the immediate reaction when the molecule is first trapped. I'm talking about the whole net effect of the plant by the time the atoms get back out. All three kinds of atoms in carbon dioxide and water molecules get separated from each other somewhere along the way and incorporated into a lot of different organic compounds, one reaction after another after another, until some of them eventually get put back together in new combinations and released. The individual oxygen atoms in a plant's body come from both kinds of molecule originally. I simply didn't mention the water part of it because the carbon cycle would still be relevant even on a planet where life somehow used a different standard solvent instead of water.

Nick Theodorakis
2013-May-13, 12:26 PM
Not sure what you are getting at. There can be carbon fixation without any free molecular oxygen production at all. In those cases, some other molecule such as hydrogen sulfide is split instead of water (even though the organisms live in water, of course).

Nick Theodorakis
2013-May-13, 12:27 PM
Photosynthesis can happen near warm deep-sea vents, using only infra-red radiation and in complete darkness as far as human eyes can see.
http://www.phschool.com/science/science_news/articles/grow_in_dark.html
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1166624/

There could be warm deep-sea vents on Europa, or on a Europa-type planet in deep space, and photosynthetic life could occur in these locations. Whether it does or not is another matter.

Those are pretty cool since it looks like they use the same bacteriochlorophylls that related green sulfur bacteria do, just using a different part of the absorption spectrum.

Nick

Delvo
2013-May-14, 12:38 PM
Not sure what you are getting at.Each carbon atom that enters a plant is associated with two oxygen atoms, but organic compounds do not consist of anywhere near a 2:1 oxygen:carbon ratio. There's actually more carbon than oxygen in organic matter, not only by atom count but even by weight despite carbon being lighter. Those extra oxygen atoms coming in in carbon dioxide have to be somewhere, and if they're not staying in the plant, that means they're leaving it.

chornedsnorkack
2013-May-14, 01:32 PM
Bump! Anyone have ideas on how tall such plantoids (chitin or spicule supported) could grow on land?

What is the main limit on the height of the living trees - wood strength, availability of water, or something else?

Nick Theodorakis
2013-May-14, 02:53 PM
Each carbon atom that enters a plant is associated with two oxygen atoms, but organic compounds do not consist of anywhere near a 2:1 oxygen:carbon ratio. There's actually more carbon than oxygen in organic matter, not only by atom count but even by weight despite carbon being lighter. Those extra oxygen atoms coming in in carbon dioxide have to be somewhere, and if they're not staying in the plant, that means they're leaving it.

Not necessarily as molecular oxygen; oxygen can be transferred, for example, to phosphate during the numerous phosphoryl transfer reactions that occur or simply to water. I can't recall exactly in the Calvin cycle where they go, but it's not molecular oxygen.

Nick

cjameshuff
2013-May-14, 03:06 PM
Each carbon atom that enters a plant is associated with two oxygen atoms, but organic compounds do not consist of anywhere near a 2:1 oxygen:carbon ratio. There's actually more carbon than oxygen in organic matter, not only by atom count but even by weight despite carbon being lighter. Those extra oxygen atoms coming in in carbon dioxide have to be somewhere, and if they're not staying in the plant, that means they're leaving it.

You're only looking at half of the problem. Oxygen-producing plants combine CO2 and H2O to form carbohydrates, releasing the excess oxygen. Some other photosynthetic organisms fix CO2 with HS instead. They produce water and elemental sulfur, not oxygen.



What is the main limit on the height of the living trees - wood strength, availability of water, or something else?

Ultimately, the limiting factor may be evolution. A supertree could trap atmospheric moisture. Material strength doesn't give a hard limit, but you can't just scale a tree up, the width at the base needs to grow faster to support the weight of the tree. A supertree would need a lot of specialization for extreme size, and the sheer amount of time required for supertrees to grow means fewer generations to develop those specializations in, slower adaptation relative to pests and disease, etc. The huge size means a lower population as well, meaning less diversity in the gene pool. If they're just a few anomalies with most of the population being smaller, then there's not much evolutionary pressure to develop the specializations required for extreme size.

So the limit isn't necessarily a fundamental physical or biological one, but how well a smaller tree can be adapted to continued growth. A tree that looks like a forested mountain might be biologically possible, but evolutionarily extremely improbable.

ravens_cry
2013-May-16, 12:05 AM
Actually, I believe there is a biological and physical limit, how high water can be raised.

Noclevername
2013-May-16, 12:49 AM
Well, I can't picture a mountain-sized sponge or fungus forming, so there must be physical limits to the supporting strength of chitin and silica spicules, and I'm guessing they are less than that of cellulose; any idea what that limit might be?

cjameshuff
2013-May-16, 03:00 AM
Actually, I believe there is a biological and physical limit, how high water can be raised.

Cloud height.

cjameshuff
2013-May-16, 03:08 AM
Well, I can't picture a mountain-sized sponge or fungus forming, so there must be physical limits to the supporting strength of chitin and silica spicules, and I'm guessing they are less than that of cellulose; any idea what that limit might be?

Material strength really only determines how wide the base has to be for a given height. And something under strong selection pressure to increase its size isn't going to stay identical in composition and structure if those are limiting its size.

Noclevername
2013-May-16, 03:16 AM
Material strength really only determines how wide the base has to be for a given height. And something under strong selection pressure to increase its size isn't going to stay identical in composition and structure if those are limiting its size.

So they end up as cone shaped, or as wide slabs. I think.

What selective pressures could result in large size?

Colin Robinson
2013-May-16, 03:26 AM
So they end up as cone shaped, or as wide slabs. I think.

What selective pressures could result in large size?

For plants/organisms that photosynthesize, competition for direct sunlight.

Noclevername
2013-May-16, 03:43 AM
For plants/organisms that photosynthesize, competition for direct sunlight.

Well, yes, but it wouldn't have to be mountain-tall to do that, just on top of something else that's less efficient. So you could get layered lifeforms each trying to smother the rest.

Colin Robinson
2013-May-16, 08:02 AM
Well, yes, but it wouldn't have to be mountain-tall to do that, just on top of something else that's less efficient. So you could get layered lifeforms each trying to smother the rest.

That is what happens here on Earth. Except that I haven't heard the term "smother" to describe what happens to plants when other plants get in the way of the sunlight. I think the usual term is "shaded out".