Astrobiology IV: Photosynthesis and energy

by CellBioGuy2 min read17th Oct 20168 comments


Personal Blog

Originally I sat down to write about the large-scale history of Earth, and line up the big developments that our biosphere has undergone in the last 4 billion years.  But after writing about the reason that Earth is unique in our solar system (that is, photosynthesis being an option here), I guess I needed to explore photosynthesis and other forms of metabolism on Earth in a little more detail and before I knew it I’d written more than 3000 words about it.  So, here we are, taking a deep dive into photosynthesis and energy metabolism, and trying to determine if the origin of photosynthesis is a rare event or likely anywhere you get a biosphere with light falling on it.  Warning:  gets a little technical.

In short, I think it’s clear from the fact that there are multiple origins of it that phototrophy, using light for energy, is likely to show up anywhere there is light and life.  I suspect, but cannot rigorously prove, that even though photosynthesis of biomass only emerged once it was an early development in life on Earth emerging very near the root of the Bacterial tree and just produced a very strong first-mover advantage crowding out secondary origins of it, and would probably also show up where there is life and light.  As for oxygen-producing photosynthesis, its origin from more mundane other forms of photosynthesis is still being studied.  It required a strange chaining together of multiple modes of photosynthesis to make it work, and only ever happened once as well.  Its time of emergence, early or late, is pretty unconstrained and I don’t think there’s sufficient evidence to say one way or another if it is likely to happen anywhere there is photosynthesis.  It could be subject to the same ‘first mover advantage’ situation that other photosynthesis may have encountered as well.  But once it got going, it would naturally take over biomass production and crowd out other forms of photosynthesis due to the inherent chemical advantages it has on any wet planet (that have nothing to do with making oxygen) and its effects on other forms of photosynthesis.

Oxygen in the atmosphere had some important side effects, one which most people care about being allowing big complicated energy-gobbling organisms like animals – all that energy that organisms can get burning biomass in oxygen lets organisms that do so do a lot of interesting stuff.  Looking for oxygen in the atmospheres of other terrestrial planets would be an extremely informative experiment, as the presence of this substance would suggest that a process very similar to the process that created our huge diverse and active biosphere were underway.


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Dang. Some information I've been pointed to since publishing this suggests that there are multiple groups out there that consider it likely that photosynthesis was present very close to the root of the bacterial tree, and that large numbers of bacterial groups may have lost it rather than it going all around the tree by horizonal transfer. This would put photosynthesis as one of the rather earlier metabolic pathways on Earth.

I've also been pointed towards more modern evidence that oxygenic photosynthesis originated in the original bacterium that created both energy-producing and sulfer-creating photosynthesis rather than the two coming together via horizontal transfer later.

Timing of next post uncertain, three weeks of insane teaching and grant-writing and yeast-poking ahead.

What about dissolved oxygen in water, which also supports large animals? Could it happen in underground oceans of icy moons?

Icy moons would need oxygen to come down from their surfaces, where ultraviolet light and particle radiation spatters hydrogen out of the ice and off into space leaving oxidized molecules (not just oxygen) behind in the ice. This is possible, as on Europa the surface is young and it is believed to recycle surface ice down towards the internal ocean on megayear timescales (and Europa has an exceedingly thin oxygen 'atmosphere', one trillionth of a bar, from radiation-split water).

Figures I've seen (see "Energy, Chemical Disequilibrium, and Geological Constraints on Europa" by Hand et al) suggest that on Europa, a maximum of 5*10^9 moles of 'oxidants' may be delivered to the interior of Europa from the ice crust per year. Let's assume that's all oxygen - in that case, it's about a millionth of the photosynthetic oxygen flux of the Earth, and if we assume it is oxidizing hydrogen sulfide provides an energy flux of only about 45 megawatts to the interior.

That's less than a hundred thousandth the geothermal energy flux of the moon (and thus probably much smaller than the geochemical energy flux), but like the geothermal/geochemical energy flux it would not be even, there would be areas of downwelling ice with oxidizing agents slowly oozing out as it melted where this energy would be concentrated.

Great article here on the latest deep bio, tho i can't read original..

Many Worlds, Subterranean Edition

and a panspermia update, with link in comments to a sweet little pdf

"And how was it that these sophisticated life processes emerged not all that long (in astronomical or geological terms) after Earth cooled enough to be habitable? “Either life developed here super-fast or it came full-on as DNA life from afar,” Ruvkun said."

and a new paper in Nature on some new archeans, had a great takeaway that may def help the panspermia argument, as the little buggers seem to be primed to move up the chain in anerobic enviros.

"We found that Asgard archaea share many genes uniquely with eukaryotes, including several genes that are involved in the formation of structures that give eukaryotic cells their complex character. Such genes had thus far only been found in eukaryotes, indicating that these archaea were somehow primed to become complex.

I am continually confused as to why people find seven hundred million years (4.4 to 3.7 billion years ago, the date at which we have both the oldest intact rocks on earth and not coincidentally the oldest good evidence for living things) an insufficient time to develop biochemical complexity. The Hadean is more unknown (due to no solid rocks surviving from that era due to geological reprocessing) than hellish, at least to a microbe over evolutionary time. There was liquid water and hydrothermal systems, there was basalt, there was more granitic rock, there was an atmosphere, and equilibrium temperatures were almost certainly not exactly extreme. The late heavy bombardment would NOT have touched the subsurface biosphere, there should be continuity of livable environments for that whole time, and could've actually increased the number of interesting hydrothermal environments that chemosynthetic bacteria love overall. Major biochemical inventions and evolutionary transitions can indeed happen remarkably fast, especially when there is no competition to constrain you to a particular ecological niche or established players crowding out newcomers to a process. The genetic code itself shows evidence of an extremely rapid period of evolution with many divergent lineages, of which only one survives in a runaway winner-take-all fashion (more on this in a later post on the nature of LUCA). I see no need to invoke panspermia especially when you have the deep domain split between the bacteria and archaea on Earth, which might be a relic of some deep differences between lineages that invented some important stuff separately.

This being said, GOD YES I want sequencers on other planets in case there is life there with a common biochemistry. Spread of microbes from world to world is not an impossible thing within our solar system. I would highly HIGHLY doubt it between star systems.

The Asgardian archaea are indeed fascinating. They somewhat overstate the case that there are major eukaryotic components in these guys a bit (a lot of the domains are separate rather than strung together the way they are in eukaryotes). There are so many eukaryogenesis models that are consistent with these guys existing though that a lot more research is needed. An alternative hypothesis to them being 'primed to be complex' is that they are abortive linages that peeled away from the eukaryogenesis pathway that lead to us and underwent reductive evolution back towards a standard archaean niche. Things can simplify too over evolutionary time.

Yes, but two of the hydrogens/electrons ripped from water in the process of photosynthesis effectively reduce one of the oxygens from the CO2, regenerating one water: 2 H2O + CO2 -> O2 + CH2O + H2O (multiply atom numbers by various numbers to get the actual biomolecules that comes out the other side of the various carbon fixation pathways). Still needed since the oxygen production all happens from the photosystems splitting water rather than from splitting CO2, the photosystems never touch the carbon directly. That water is produced no matter what you're using as your electron donor.