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Utopia Talk / Politics / Life on Dark Exoplanets
swordtail
Anarchist Prime
Tue Jul 10 20:17:44

Life on Dark Exoplanets

John Michael Godier

Published on Jun 23, 2018


An exploration of the possibilities of low-light microbial life and how some life on earth indicates that photosynthesis at low energies is possible, and may be common in the universe.

http://www.youtube.com/watch?v=ep6IU5rX5V4
Nimatzo
iChihuaha
Wed Jul 11 05:33:25
A bit hard to get excited over the potential fact that the life that may litter the universe is some kind of anemic germ.
Hot Rod
Revved Up
Wed Jul 11 07:33:39

I wonder if they will melt the flesh of our astronauts?

hood
Member
Wed Jul 11 07:50:44
Unless there's some very specific chemistry in photosynthesis that theoretically prevents it from occurring at different wavelengths, I don't know why plants wouldn't be able to photosynthesize non-visible light. The only reason plants on Earth specialize in visible light is because the sun maximizes energy in visible wavelengths, so by photosynthesizing in those wavelengths, plants give themselves access to the most energy. It's quite literally just natural selection. The healthiest plants with the easiest access to food (energy from photosynthesis) survived.

Hell, that plants are green makes no logical sense at first blush. Plants being green means they reflect green light. But green light is where the Suns energy spectrum peaks. That means the plants we know reject the most abundant source of food the sun has to offer. Sounds pretty backwards without any context.
Seb
Member
Wed Jul 11 08:06:10
IIRC there are actually limits on wavelengths that work for photosynthesis.

Also, the fact chlorophyll is specifically green means there are wavelength limits imposed by chemistry and entropy considerations.

e.g there is way more radiation power in non visible light so if that could be harvested... and if all visible light could be harvested, then leaves would appear black.

Dukhat
Member
Wed Jul 11 08:10:07
It does make sense most life is mostly germs. Complex life in terms of multi-cellularity didn't take off on earth until hundreds of millions of years ago which is only a small fraction of our history as a habitable planet.

Most planets in the goldilocks zone will be tidally locked to a red giant as well which means less diverse biomes versus the numerous kinds we see on earth today that helped propel evolution forward.
hood
Member
Wed Jul 11 08:12:37
Chlorophyll is green because it doesn't use green wavelengths. The reason for this is because in protoearth, airborne microbes first developed photosynthesis and they evolved to harness the most intense wavelengths from the sun: the green where the sun peaks. As such, plants weren't getting enough green wavelengths and had to adapt to use the blue/red parts of the spectrum. Which again, is related to where the sun peaks in intensity. I'm sure you're familiar with black body diagrams:

http://apo...2/graphics/earth_sun_plank.jpg


So, is photosynthesis specifically, chemically limited to visible light, or was visible light just more plentiful? I'd gladly take a source suggesting that visible light is indeed a general you accepted limit and not just a limit of our circumstance in rotating around our sun.

Actually, that would be an interesting experiment - see if one could breed plants that specialize in non-visible light photosynthesis.
hood
Member
Wed Jul 11 08:13:27
"tidally locked to a red giant"

I hope you meant red dwarf.
Wrath of Orion
Member
Wed Jul 11 08:40:15
NIR and beyond is not energetic enough to drive photosynthesis as we know it. It would need to be some other system to function at longer wavelengths.

If you're talking shorter wavelengths (UV and below), they are too energetic and generally cause more damage than is worth it. Again, for plants as we know them.

If you want to utilize wavelengths other than visible, you'd need to come up with an entirely new set of photochemical reactions.
hood
Member
Wed Jul 11 08:44:37
Thanks WoO.

So discovering microbes that photosynthesize UV isn't necessarily too surprising because a microbe requires less energy than a fully fledged plant. But growing a full plant would be unlikely as we know it. Correct?
Seb
Member
Wed Jul 11 08:47:47
Also, the visible spectrum peaks in green, which PS doesn't use (green being reflected/transmitted, but red and blue absorbed, hence the green colour).

IIRC Photosynthesis works because chlorophyll absorbs two photons to get to the energy required to disassociate a proton from water.

Using additional photons is strongly selected against as the cross section for successful absorption of all required photons will go down exponentially with n where n is the number of photons required.

So I assume for whatever reason it has evolved to use a high and a low energy visible photon, leaving green alone. Presumably that means that there is something that selects against green given more energy is available in the green band - probably the chemistry involved.
Wrath of Orion
Member
Wed Jul 11 08:47:52
I assume you mean NIR, which is the lower energy radiation?
Seb
Member
Wed Jul 11 08:53:37
This reply above was written immediately after the first one.

RE airborne microbes using photosynthesis in the green, are you sure? It seems unlikely this would have a huge impact on the spectrum reaching the earth.

Wrath of Orion
Member
Wed Jul 11 08:59:19
Anyway, going on that assumption, NIR generally does not have enough energy per quanta to drive the photochemical reactions. It doesn't really matter how "much" NIR you use, the energy per quanta is not there (Seb touched on some issues with basing reactions on simultaneously absorbing many quanta to drive a reaction). "Low light" generally means very low amounts of visible light.

That said, there is some work on bacteria that seem to be able to use NIR to some extent. I don't work in that area and have done limited reading on it.

Here is one I found just on a quick search. I only glanced through it as I'm working on other stuff atm.

http://arxiv.org/ftp/arxiv/papers/1405/1405.4420.pdf
hood
Member
Wed Jul 11 09:06:11
http://www...rth-purple-study-suggests.html

I might have misremembered/been misinformed about the airborne aspect. But above link argues for green light absorbing plants.
Seb
Member
Wed Jul 11 09:22:22
Interesting - it does seem quite speculative (authors admit as such) but it looks like there is no compelling argument why *not* use green. My guess was there would be something along the lines of, e.g.

"the chemical structure to get the various modes to allow the two photons energies to be absorbed means X,y,z structure which is metabolically expensive to create".

Wrath of Orion
Member
Wed Jul 11 09:44:49
One thing to keep in mind is that plants generally receive far more PAR (photosynthetically active radiation - visible light) than they can use. As the sun climbs higher in the sky, photo-protective mechanisms kick in to deal with the excess absorbed energy. So if they're already getting more energy than they need, there may not be an advantage to focusing on green light.

That said, green light is strongly absorbed by plants. Reflectance levels for green light typically peak at 4-7% (transmission can be assumed to be similar), while red and blue light sees typical reflectance levels around 2-3%. Of course, this varies greatly by species, plant status, etc. But still, green is heavily absorbed in plants, just not as heavily as red and blue.

One other thing to consider is that the atmospheric composition during various stages of evolution would need to be taken into account. The atmospheric composition greatly alters the surface's irradiance spectrum, so that may have played a role as well. The theory of airborne green absorbing bacteria would apply here, but I'm more talking about atmospheric gas composition and density.
hood
Member
Wed Jul 11 09:45:29
Yeah, there are several theories I've come across in the past hour or so. One was that green would be too energetic (to the point of damage). Another was that by using a process of red/blue light, plants actually absorb about 85% of the visible spectrum (although why green couldn't be a part of it isn't explained by this).

This is all pretty relevant to a setting for one of my story ruminations, hence my interest.
Wrath of Orion
Member
Wed Jul 11 09:46:14
Erm, concentration.
Wrath of Orion
Member
Wed Jul 11 10:02:34
You may find this interesting.

http://www.plantphysiol.org/content/154/2/434.short
Sam Adams
Member
Wed Jul 11 10:45:39
Woo is correct. Most leaves still absorb lots of green light. Not as much as the other wavelengths but enough. Spectrally averaged most living canopies are quite dark with albedos near 0.1. They are not reflecting much energy even in green.
Seb
Member
Wed Jul 11 15:35:54
Yeah but they aren't photosynthesising with most of it either.

Sam Adams
Member
Wed Jul 11 15:55:10
True. Most solar energy hitting even a deep canopy is neither reflected nor photosynthesized.
State Department
Member
Thu Jul 12 16:05:00
"if all visible light could be harvested, then leaves would appear black"

Harvesting...appearing black. Ruh roh.
Nimatzo
iChihuaha
Fri Jul 13 03:49:17
"One thing to keep in mind is that plants generally receive far more PAR (photosynthetically active radiation - visible light) than they can use."

To use this extra energy (only possible in controlled settings) you will have to add extra CO2.
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