When reading David Brin’s first three books in the Uplift universe, I was intrigued by the mention of hydrogen-breathers. There is not much about them because of limited interaction with the oxygen-breathing life-forms in the civilization of the Five Galaxies according to the books.
What does it mean to be a hydrogen-breather? Why or how are they different from oxygen-breathers? It turns out that, right here on planet Earth, there are organisms that make their living using hydrogen (H2) as an energy source. They tend to be tiny micro-organisms living in anaerobic (“lack of free oxygen (O2)”) conditions. Oxygen-breathers, on the other hand, come in all shapes and sizes on our planet from tiny bacteria to blue whales and networks of fungi. Is there something special about oxygen? Why yes! It’s a surprising molecule with an anomalously weak O=O double-bond, thus thermodynamically unstable, but is anomalously kinetically stable because of quantum mechanics!
But most importantly, because of the weak O=O bond, O2 is a “high-energy” molecule that can react chemically to form “low-energy” molecules with stronger chemical bonds. The energy released in these chemical reactions can go towards, not just maintaining life, but growing it! Organisms got bigger and seemingly more complex (although bacteria are also complex) thanks to an oxygen-fueled metabolism that provides much more energy than practically anything else that isn’t too unstable. (Substances that are too unstable don’t hang around long enough!) On planet Earth we have established a remarkable symbiosis between oxygen-producing photosynthesis and oxygen-consuming respiration.
Let’s examine the energetics. The single H–H bond in H2 is ~435 kJ/mol. Hence, two equivalents of H2 would “store” ~870 kJ in its chemical bonds. The O=O double bond is ~500 kJ/mol. When two equivalents of H2 react with one equivalent of O2 to form two equivalents of water (chemically: 2 H2 + 1 O2 --> 2 H2O), four O–H bonds are created, each worth ~460 kJ/mol. The net energy change in this reaction is 870 + 500 – 4(460) = –470 kJ, ignoring entropy and phase-changes. (Technically, this is the enthalpy, and the majority contributor.) The negative sign indicates that energy is released in this reaction. 2 H2O is lower energy compared to 2 H2 + 1 O2 by 470 kJ. The lion’s share of this difference comes from the weak O=O bond. The H–H bond, while weaker than the O–H bond, is not substantially weaker. Counting bonds in pairs, this would be comparing 870 kJ to 920 kJ, or a 50 kJ difference. The O=O bond at 500 kJ is significantly weaker than the ~900 kJ of a H–H or O–H pair, and this difference accounts for most of the energy released.
If free O2 is not present, organisms trying to make a living don’t have a good “high-energy” option that could serve as a fuel. I study the chemical origins of life, and the fundamental carbon input into the biochemistry of life is CO2, at least on our planet. Carbon dioxide can be represented as O=C=O, i.e., it has two C=O bonds worth ~800 kJ each. A hydrogen-breather could carry out the following reaction: CO2 + 4 H2 --> CH4 + 2 H2O. The bond-energy analysis (with C–H being ~410 kJ/mol) yields a net energy change of –140 kJ, but this comes from breaking four pairs of bonds to make four new pairs. On a per pair basis, this would be a measly 35 kJ compared to the 235 kJ per pair from reacting H2 with O2.
You might quibble that I didn’t pick a good set of initial reactants, even though CO2 and H2 are the main entry points into building biomass at the origin-of-life here on planet Earth. You’d be hard-pressed to find a more reasonable starting point given the environmental conditions and what is available in sufficient supply. What about the choice of methane (CH4) as the product? That’s what gives the highest energy output given the reactants. A number of microorganisms known as methanogens (e.g., inside cows) continue to perform this reaction for their energy needs. We humans think of methane (main component of “natural gas”) as a fuel, but for those hydrogen-breathing microorganisms trying to make a living, it’s a waste product. We think it’s a fuel because we can burn it in O2 for energy. But methane is quite stable thermodynamically. It’s O2 that’s “high-energy” and unstable – that’s the fuel!
Some oxygen-breathing micro-organisms can switch to anaerobic mechanisms when their oxygen fuel is in scarce supply. It’s almost as if they go into hibernation. Metabolism is slowed down considerably to require less energy to survive. I find this phenomena of cryptobiosis fascinating – it’s the strange boundary between life and death, a liminal space. What do you do if conditions are terrible and you’re trying to stay alive? Breathe hydrogen! If this idea intrigues you, a mini-review article by Morita (Microb. Ecol. 2000, 38, 307-320) that you might find interesting is titled “Is H2 the Universal Energy Source for Long-Term Survival?”
What if environmental conditions were different? Perhaps on the cold moon of Titan orbiting Saturn where there are hydrocarbon oceans. To keep things simple and maintain the analogy with O=O and O=C=O, let’s consider the molecule C2H4 which has a C=C bond (615 kJ/mol) to break as an energy source. Then, for the reaction C2H4 + 2 H2 --> 2 CH4, the energy change would be –175 kJ. Given that four of the C–H bonds remain “intact” in this transformation, the exchange is two pairs of bonds or ~90 kJ per pair. Not as good as O2 but better than CO2. Whether you can generate a metabolism from hydrocarbons is less clear since we don’t yet understand proto-metabolism nor how to define it exactly. But astrobiologists can speculate. For a recent mini-review, see: “Out of Thin Air? Astrobiology and Atmospheric Chemotrophy” (Cowan, Ferrari, McKay, Astrobiology 2021, DOI: 10.1089/ast.2021.0066)
If there are hydrogen-breathers on alien worlds, they’d be hard pressed to thrive and grow with a lower chemical energy input, unless they are able to find some other source. Nitrogen oxides might work as higher-energy reactants, but they have other problems, and you might as well decompose them to N2 and O2 and then use the latter to fuel your metabolism. The F2 bond is anomalously weak, but you’ll have problems recycling the fluorine and it’s also not as abundant in the universe as an element. Similar issues abound as you go to lower rows in the periodic table.
We think of life as being carbon-based, and there’s good reason for it in terms of the diversity of structures and types of chemical bonds with other abundant elements. (Tetravalent silicon has other limitations.) But the structural elements cannot be built without an energy source, and oxygen beats its rivals easily. Coupled with hydrogen, the most abundant element, and carbon may simply be a carrier for energy flux.
While we might not think of many organisms as hydrogen-breathers, ultimately it’s the coupling of hydrogen and oxygen that powers organisms like us humans on planet Earth. We’re hydrogen-breathers at a distance, via intermediaries. But we’re also oxygen-breathers, and perhaps that’s what makes the difference!
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