Hydrogenic photosynthesis reduces methane and water to build biomass ($\text{CH}_2\text{O}$) and releases hydrogen:

$$\text{CH}_4 + \text{H}_2\text{O} + \text{photons} \to \text{CH}_2\text{O} + 2\text{H}_2$$

For reference, oxygenic photosynthesis is:

$$n \text{ CO}_2 + n \text{ H}_2\text{O} + \text{photons} \to (\text{CH}_2\text{O})n + n \text{O}_2$$

According to this excellent paper by Bains et al, the hydrogenic process is some four times as efficient as the oxygenic version, allowing four times the amount of biomass to be constructed for the same quantity of light (see note *1).

The linked paper describes how large planets could hold onto a hydrogen atmosphere, but this question is not about that.

My question is about strategies for animal evolution, since the flip side of it being 4 times as easy for autotrophs to build mass, is that heterotrophic consumers get 4 times less energy from breaking down one gram of this hydrogenic biomass. Here are the authors words:

"From a purely human point of view, the evolution of hydrogenic photosynthesis might be a disappointing discovery on another world, for reasons implicit in Figure 1. Just as making biomass in an oxidized environment requires more energy, breaking down biomass in an oxidized environment releases more energy. In particular, oxidizing biomass using molecular oxygen releases substantially more energy than reducing it using molecular hydrogen. A commonly-held explanation for the rise of complex animals in the late Pre-Cambrian and Cambrian periods was the rise in atmospheric oxygen that allowed their energy-intensive lifestyles "

My question is; how does the change in 'balance of power' between autotrophs and heterotrophs affect the evolution of both and what is the appropriate animal metabolism to allow animals to display the types of abilities (which rely on storing concentrated energy see note *2) that earth animals display?

Please note - any answer that addresses the fourfold animal vs plant imbalance is valid - PhD level biochemistry answers will be much appreciated but I am not expecting to get many of them!

End of question: what follows is supporting material from the paper that you can treat as **TL;DR.

Note *1

Here is the passage from the paper that makes the claim about reduced biomass generation requirements.

"Comparison of Gibbs energies of formation of CO2 (gas ~ −394 kJ/mol, aq ~−385 kJ/mol) and CH4 (gas ~ −50 kJ/mol, aq ~ −35 kJ/mol) [65] shows that any reaction involving CO2 as the C-bearing reactant will almost always have a more positive Gibbs energy of reaction than a similar reaction with CH4 as the reactant. The quantitative difference between the reactions will depend on the products of the reaction, as illustrated in Figure 1. On average, for the set of chemicals in Figure 1, making the chemical from CH4 takes ~20% the energy needed to make it from CO2. This suggests that building biomass in a CH4/H2-dominated environment would require only ~20% of the energy needed in our CO2-dominated environment."

Note *2

The linked paper mentions that maybe these animals could make use of dimethylsulfonium proprionate (DMSP) to store energy rather than carbohydrate but I don't really understand this process or what its implications are...

  • 1
    $\begingroup$ I'm no chemist but I don't follow the logic of heterotrophic consumers getting 4 times less energy from the biomass. Since the produced Formaldehyde molecule is the same in both cases, why would the relative efficiency of the production process change the stored energy for a given unit mass? $\endgroup$ Commented Sep 13, 2015 at 15:28
  • $\begingroup$ @KillingTime its a fair question and I have to say I don't know the answer - its too far out of my comfort zone to paraphrase the authors arguments. However I have edited my answer to include a quotation from part of the paper that motivated my question. $\endgroup$
    – rumguff
    Commented Sep 13, 2015 at 15:37
  • $\begingroup$ I'm not sure about the claim that heterotrophs get 4x less energy per gram. The chemical outputs of both types of photosynthesis are the same so why would there be less of energy available for heterotrophs? $\endgroup$
    – Green
    Commented Sep 13, 2015 at 15:52
  • 3
    $\begingroup$ @Green Because there is no free oxygen available? Remember that plants release hydrogen now. Even if there is an alternate oxygen source, oxygen in a hydrogen atmosphere is not a good sign at all... $\endgroup$ Commented Sep 13, 2015 at 15:59
  • $\begingroup$ @green I added the relevant section of the paper that addresses your point. I think they key is that the reference equations at the top of my post are highly simplified pictures... $\endgroup$
    – rumguff
    Commented Sep 13, 2015 at 16:06

2 Answers 2


If I've understood your question correctly I'm going to basically ignore the biochemical science and jump straight to what I feel is the meat (actually, veg) of the question:

What happens if plants grow 4x faster, but animals get 4x less nutrition from them

Please note that above I'm using 'plant' as a synonym for autotroph and 'animal' as a synonym for heterotroph. I'm doing this simply because it feels more natural as a form of address. I'll use the correct terms later as it's important to make the distinction.

So: Moving on.

The period for which single celled life dominates will become shorter. Your single cells are more likely to be autotrophic, and as such will multiply much more quickly. In this sort of high-energy high population environment any heterotrophs that do emerge will have a glut of food, but won't be as much of an impactor on the autotrophs as they were in our history (as they reproduce at a quarter of the rate). The autotrophs therefore will compete with each other, and the high population density will lead to cellular co-operation faster.

When it comes to multicellular plantlife: competition will be fierce. I mean, genuinely fierce. These plants will have 4x the energy, and therefore 4x the capacity to reproduce, grow and generally do what plants do. Tall trees, resource sapping and funky seed dispersal techniques will blossom as all the plants will have more energy to 'waste'.

Animals on the other hand will have to move slower by necessity. They still have an advantage in that they don't need the sun, and they still have an advantage in that they're eating a richer energy source, but we won't be seeing purely carnivorous predators anytime soon as the amount of acreage required for a single predator would go up 16 fold (4x for the herbivores, then another 4x for the pure carnivores) Omnivores would likely do the best, but still, slower creatures would do better.

As the disparity between the amount of energy that can be gained from the sun vs the amount of energy gained from eating other plants is much smaller lifeforms exhibiting both autotrophic and heterotrophic behaviour would be considerably more prolific. Parasitic and carnivorous plants would be more common, and I'd expect a whole range of adaptations (Jellyfish vines, climbing bananas, Cuckoo-Elm?) and being photoheterotrophic (using sunlight to help fix carbon but not photosynthesising directly) would be a strong evolutionary choice.

If you want to see an earthlike system then your animals are going to have to have some serious metabolic mojo. For starters the herbivores will have to eat at least 4x more vegetation, and that's assuming metabolic efficiency works the same way. As previously mentioned any fast carnivores are going to be ravenously hungry, and would also have to evolve some major parenting skills as they won't have the energy to employ a 'fire and forget' strategy and then worry about all the competitors they just spawned. I'm unsure as to whether the same argument about parenting applies to the herbivores.

One last, rather intriguing (though contradictory) thought: Underwater the apex predator would probably be Coral...

  • 2
    $\begingroup$ Great answer; I must admit now you mention it the implication about the widening of the producer/herbivore/carnivore pyramid seems obvious but I hadn't actually worked that out for myself. That ravenous apex hunter would be a magnificent beast (and probably subject to significant pressure to develop real intelligence). Nice point also about the photoheterotrophs. Likewise there is probably a big niche for fungi, which I also like. Hope to get more answers like this. $\endgroup$
    – rumguff
    Commented Sep 16, 2015 at 22:48
  • $\begingroup$ actually to accommodate that widely spread food chain I can just make the planet vast - a super-earth, which fits in nicely with retaining a H2 atmosphere. Will need to think about how animals can cover the larger distances though. $\endgroup$
    – rumguff
    Commented Sep 16, 2015 at 22:51
  • 2
    $\begingroup$ If you increase the prey density you can decrease the range required. If the plants grow incredibly densely then the prey will be denser so your predator can use ambush tactics and let the prey come to him. As long as he doesn't stay still long enough for the Greater Spotted Climbing Banana to get it's tendrils into him... $\endgroup$
    – Joe Bloggs
    Commented Sep 17, 2015 at 8:24

Get your oxidizers here! Get them while they're hot!

The fundamental question is where do you get your oxidizer from? All oxygen on this methane+H2 planet is wrapped up in water or something else. Candidate oxidizers might be Fluorine or chlorine but both have their problems. Fluorine is so reactive it never stays free for long. Chlorine is also never found free in the atmosphere. With so much methane and hydrogen floating around, any oxidizer is going to get captured quickly. We only have it on Earth because there's so much life pumping out oxygen.

This leave us with two options. First, we develop a reciprocal metabolism that doesn't require an oxidizer and runs on hydrogen. (The world of chemistry is broad. It could probably be done.) I don't know near enough chemistry to even guess at candidate reactions.

Or, second, we recycle the oxidizers within the autotroph after consuming them from terrestrial carbonate, perhaps calcium carbonate which has three oxygen atoms for one calcium atom. I don't know the energy penalty in acquiring an oxidizer this way but it seems convenient. Perhaps a fluorine catalyst of some kind?

CO2 is also removed from the atmosphere by conversion to carbonate, at a rate that depends on surface chemistry.

This atmosphere is the inverse of Earth. On Earth, the oxidizer is freely available and the fuel is in short supply.

  • 2
    $\begingroup$ the lack of oxidisers is why this is referred to as a reducing atmosphere. I quote from the paper: "In a reducing environment, highly oxidized compounds could be stored as energy storage materials, having the highest energy density when reduced with hydrogen, or other compounds with roles comparable to DMSP could be accumulated and be used as high-energy food. The absence of oxygen does not therefore preclude the possibility that other biomass components could be metabolized to yield lots of energy per gram." Oxidising accumulated biomass is not the only way of producing energy it seems. $\endgroup$
    – rumguff
    Commented Sep 15, 2015 at 21:06
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    $\begingroup$ Ha! :) You got a middle-schooler's answer. Sorry I can't do better. $\endgroup$
    – Green
    Commented Sep 15, 2015 at 21:12

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