Was Photosynthesis at first sustainable?

[Delong]

Hi I was thinking about photosynthesis recently and I know that it first evolved in cyanobacteria like organisms to convert sunlight into chemical energy. It also produced oxygen from water (not CO2) to oxidize the atmosphere. I read that many Eukaryotic organisms need oxygen in order to live and so the increase of oxygen in the atmosphere, called the Great Oxygenic catastrophe, allowed for a greater diversification of life and eventually multicellularity. Anyways I am wondering if photosynthesis was at first sustainable because it seems to create oxygen faster than cellular respiration can use it which explains why oxygen levels increased. Assuming that this is correct, if heterotrophic organisms did not increase as a result of this change would all the carbon dioxide have been used up by photosynthesis before it was produced back by cellular respiration? I think that would be the case and so I wonder why photosynthesis evolved if it was not sustainable or if heterotrophic organisms increased diversity out of necessity to make photosynthesis sustainable.

My own answer is that photosynthesis evolved because it was much more efficient for the cyanobacteria then the axoxygenic photosynthesis that took place before hand because it used water as the reducing agent which was more abundant. It probably produced an excess of glucose since it was so efficient (or did it?) which allowed for greater diversity of autrophic organisms. Meanwhile, the heterotrophic organisms responded to the ecological niche of more oxygen by increasing in diversity. And so the mechanism responsible for making photosynthesis sustainable in the first place was the ecological niche part.

Does this sound correct? Otherwise how could photosynthesis have evolved if it produced oxygen faster than it consumed it and would be unsustainable?

 

[Delong]

could someone please answer this?

 

[JaredJames]

It's the weekend. Please be patient, most people have things to do.

 

[Delong]

ok sorry I'm kind of new.

 

[Delong]

ok...

 

[Delong]

Honestly! is my question stupid or something?!

Ok sorry, maybe I'm being impatient. I don't know how busy people are on here.

 

[Ken Natton]

From the start, Delong, I must declare that I cannot answer your question in any definitive way, but in the hope of attenuating your frustration at the lack of a response, I will offer some thoughts, perhaps they might stimulate a more informed response.

Firstly, I would suggest to you that you have not really offered a clear question. You have given an insight into your thought processes and the issue you are struggling with. It might even help you to think about the problem more clearly if you tried to frame it as a clearer direct question to which the experts might respond.

The heart of your point, it would seem, is that you think that photosynthesis would be unsustainable without sufficient counter-balancing oxygen breathing organisms turning it back to carbon dioxide. My first thought is that you perhaps underestimate just how abundant carbon dioxide is. There is a physical reason, to do with something called ‘ground state’, why atoms of carbon and oxygen have a tendency to gravitate back to carbon dioxide. Oxygen breathing organisms are not the only method by which that happens. I have previously mentioned a clip of Richard Feynman, available to watch on YouTube, explaining fire, which makes it clear that the burning of a log of wood is largely photosynthesis in reverse.

Unsatisfactory as this might be as an answer to you, clearly we are here, and life on Earth works the way it does. It thus seems to me to be reasonable to accept that some kind of balance between photosynthesising organisms and counter balancing processes (not necessarily living ones) was found. That balance might well have been very fragile, environmental science tells us today that the balance remains very delicate. The fact that it has survived for such a long time suggests that there are forces that act to keep it in balance.

 

[alxm]

I'm having difficulties following what you're saying here too. Maybe because I'm not an expert on bacteria or evolution, but I know a bit about the enzymes and mechanisms involved.

You've got two processes here:
1) Cellular respiration. All organisms I know of get their energy from electrochemical redox potential. So you have an electron-transport chain, where you're extracting a bit of redox potential in each step. At the end, you have to dump that electron somewhere, since you don't want random electrons roaming around and reducing stuff. Oxygen is great for that, since you can liberate a good amount of energy from reducing oxygen to water (and say, use it to pump protons, as we do using cytochrome c oxidase)

If you're anaerobic, you have to find something else to reduce. So you have for instance denitrifying bacteria that take nitrates and other nitrogen oxides and reduce that to water and molecular nitrogen. That costs them energy, so it's just an "electron sink" for them.

2) (Oxygenic) Photosynthesis. Now that's at the other end of the electron transport chain. Where are you going to get your electrons from? You could get them from oxidizing food, but unfortunately you're an autotroph and there's not a lot of sulphur around for you to eat.. But the way you've evolved, you're getting more energy out of your electron transport chain than you are from the first step of oxidizing your food, much like how the denitrifying bacteria isn't getting any energy out of the last step of its respiration chain.
So the solution they evolved was to use water as an electron donor. It's the best possible substance, since it's abundant. It's got one drawback though: It means producing molecular oxygen, which take a lot of energy. So much energy, in fact, that you can't do it with a single photon of visible light. (Realistically though, they likely evolved photosynthesis with other electron donors first. Again I'm not an expert on the microbiological/evolutionary aspects)

But what they eventually came up with is Photosystem II, which is one of the most incredible bits of molecular machinery ever evolved. Who knows how many eons it must've taken, but it didn't come about lightly, because Nature only evolved it once. From plants to cyanobacteria to procaryotes, they all have essentially the same PSII. It captures four photons and transfers four electrons, taking them from a cluster of manganese ions, oxidizing them, building up chemical potential. One-two-three-four-bang! Molecular oxygen is formed. They've done beautiful experiments with this, starting with enzymes in a reduced state and subjecting it to laser pulses while measuring O2 concentrations. On every fourth pulse the oxygen concentration spiked.

Anyway, to get the the heart of the question: Obviously, there weren't any aerobic organisms before there was a lot of molecular oxygen around, before oxygenic photosynthesis. So whatever organisms were there at that time weren't using molecular oxygen as the terminal acceptor for their electron transport chain, as aerobic organisms do. So it was sustainable. Even though it cost energy to create it, oxygen was probably just a byproduct, much like the molecular nitrogen from denitrifying bacteria, albeit at a different end of the electron transport chain.

They all get their oxygen from water though. Even though plants have a net uptake of CO2 and a net production of water, the oxygen in CO2 isn't being turned into molecular oxygen, except insofar it becomes water in-between.

 

[phyzguy]

I think the point you're missing is the length of time it took to consume the initial carbon dioxide in the atmosphere and produce the levels of oxygen we see today. Estimates I have read are that this took at least hundreds of millions of years, and possibly over a billion years. So as far as the evolution of the early cyanobacteria was concerned, the Earth's store of carbon dioxide was essentially infinite. Single celled organisms don't evaluate the long term sustainability of their metabolism; if in the near term they are successful, then they proliferate. As long as the rate of depletion of any resource (carbon dioxide in this case) is much longer than the lifetime of an individual, it will have no effect on their evolution. One could argue in a similar fashion that photosynthesis on the Earth is not sustainable in the long term because the sun has a finite lifetime, but this does not keep plants today from capitalizing on the sunshine as long as it lasts.

 

[Delong]

Ok thanks everyone that essentially answers my question. My apologies for it being unclear in my original post. Ultimately my question was: if photosynthesis produced oxygen at a rate faster than it can consume it to produce carbon dioxide then how did it evolve? I think my mistake was in thinking that long term unsustainability would impede the evolution of biological devices that have immense short term benefits. This is the answer Phyzguy gave and to a lesse extent Ken Natton. From Alm's response I can understand that oxygenic Photosynthesis, specifically Photosystem II which is responsible for the oxygen part, evolved in order to have an efficient way to dump extra electrons on Oxygen. This allowed the cyanobacteria to gain a lot more energy from their process possibly to synthesize more glucose for themselves. As a secondary question I wonder how much more glucose this would have produced and what the organism did with all this extra glucose. But I can find that out myself later.

Thanks for the response, your answer was satisfactory Ken Natton for sure. Also thanks for the patient and thorough responses to my question. I have come to understand that there are a lot of questions posed on these forums and experts do not have time to write a lengthy response to them all. Perhaps it would be better if I come to learn these things on my own through my studies at school. I will be more selective of the questions I post on here from now on as to make more reasonable demands on the people of this board. Again thanks for answering my q's and well blessings. Definitely will learn to be more patient and stuff from now on.