The Equations of Life

by Charles S. Cockell

  The educational objective of the lecture is to get students to question whether the view we have of habitability and the clement conditions for life on Earth are limited by our own knowledge or whether our planet is truly a universal template for all life in the universe. By producing an elaborate fifty-minute story where all the bits seem to cross-check and yet lead to a conclusion that Earth is uninhabitable, I want to challenge them to think about whether our view of the evolution of life on Earth is simply a giant Just So story we have assembled into a jigsaw about something that was serendipitous, a chance result of one specific evolutionary route.

  My own view, as I am sure you have now guessed, is that life on Earth tells us something fundamental and universal. There are many flaws I can find in my own elaboration of the unlikelihood of life on Naknar 3 and the idea of the sulfate-reducing intelligence. Being a sulfate-reducing intelligence really would require almost constantly eating sulfate to generate enough energy to power a brain or a body that can walk. However, besides this inconvenience, the problem with sulfate reduction is that it requires geologically abundant and readily available sources of sulfate to allow for the emergence of multicellular life. I cannot rule out that on some distant world where there is a vast quantity of sulfate piled up in ancient mounds, there might just be scope for herds of sulfate-reducing pigs gnawing away at gypsum mounds, grunting and snorting at their intelligent alien handlers who return to their homes to eat gypsum pie. For now, the sulfate-reducing intelligence must remain speculation, and the low amounts of energy to be gathered through this metabolism make the squealing gypsum-eating pigs unlikely. Regardless, even if the squealing pigs do exist, they are still using electron transport to harvest energy, conforming to the principles we have already identified.

  Even on our own planet, though, we do know of complex animal life that tentatively explores unconventional sources of electrons for energy. Giant tubeworms, Riftia pachyptila, inhabit hydrothermal vents in the deep oceans, where boiling hot water bubbles up from the Earth’s crust, often spewing black sulfide-rich minerals into the ocean. These worms contain within them consortia of bacteria that use hydrogen sulfide (rotten egg gas) from the vents and oxidize it using oxygen gas dissolved in the water. There is enough energy there to produce worms over two meters long and about four centimeters in diameter, the strange white cylinders with their dark-red heads swaying and shimmering in the warm water venting from the rocky chimneys that surround them.

  At these vents, the concentrations of hydrogen sulfide gas, which are usually unmeasurable in the atmosphere, are so high that the bacteria in the worms’ guts can forage enough to use it as an electron donor to make energy and thus synthesize organic compounds, which the worms use to grow to their gargantuan size. Although the electron transport chain still lies at the heart of this extraordinary symbiosis, these bizarre, twisting tubes show us that sometimes on Earth, where rare chemicals and gases reach unusual concentrations, animal life can take on strange alliances to sustain itself by exploring different forms of energy.

  These endearing worms show that evolutionary possibilities can be multiplied by the enormous variation in the geochemical availability of the raw materials of life. So far, I have discussed how physical principles greatly constrain the products of evolution within some very narrow bounds. One way in which the scope of evolutionary possibility can be changed within the strictures of physics is by changing the geochemical availability of things needed to drive innovations. Even without wild science-fiction stories about sulfate-reducing lecturers on our own world, we have abundant evidence, not least in the deep-ocean worms, for how geochemical availability can redirect the course and products of evolution. The worms show us how life is bounded by physics all along the evolutionary path, but perturbations in the geology of planets can open new evolutionary vistas, particularly when it comes to the chemicals that can drive energy acquisition.

  Nevertheless, the rise of oxygen in the Earth’s atmosphere, itself a result of life, made aerobic respiration more widely available. Thus, the rise of oxygen enabled life to access vast quantities of energy, opening the way to multicellular life and intelligence once the earliest animal cells tapped into these opportunities. More energy begets bigger and more-complex machines, triggering the emergence of a biosphere that reached beyond the simplicity of single-celled life.

  Woven throughout this evolutionary story, from the earliest microbes to the rise of animals, are electron transfer chains. More than this, the energy available from different electron donors and acceptors, the energy released from these chains, explains much about the transitions that have occurred throughout evolutionary history. Early life soldiered on, extracting energy from rocks, gases, and organic materials. Then concentrations of oxygen gas rose, driven by the physics of splitting apart water and the use of electron chains to capture energy in sunlight. From this momentous innovation, the rise of animals was linked to the thermodynamics of how much energy is released when electrons are transferred from organic matter to this newfound oxygen. Microbes and monkeys roam the Earth, powered by energy released from movements of a subatomic particle, the electron.

  It is appropriate at this stage to wonder whether there are any other energy-yielding reactions that life might use as alternatives to electron transfer chains. Perhaps other forms of energy could get around the problem of the limited amounts available in oxygen-free environments. Can we escape oxygen altogether yet gather large amounts of energy to drive a complex biosphere? To say that life is constrained by physics is not to say that it is limited to only one method of collecting energy. As with humans, from wind power to nuclear power, perhaps a self-replicating, evolving system has more than one card up its sleeve.

  Certainly, we know that some cells can get away without using an electron transport chain. Instead of all that rigmarole with membranes and electron donors and acceptors, an alternative approach is to just take a phosphorus-containing chemical group going spare on a molecule and tag it directly onto ADP to make that energy-storing molecule ATP. This process is at the heart of fermentation, an extraordinarily versatile set of metabolic pathways that permeate our lives. Fermentation turns sugars into acids, which we use to pickle vegetables, or you can use it to make alcohols, the basis of beer, wine, and much else besides. This same process goes on in your own body as sugar transforms into lactic acid, giving you cramps if you do some sudden exercise and you cannot get enough oxygen to the muscles. Although this process does not use an electron transport chain per se, it still involves chemical reactions that are essentially electrons moving around. And despite the relative simplicity of fermentation, it produces only about a tenth of the energy yield of the electron transport chains. The low energy yield explains why your body prefers to get oxygen and not cramps and why many microbes, when offered the chance, switch to aerobic respiration over fermentation.

  However, perhaps we are still not being imaginative enough. What about something much more radical? One line of thinking might be to consider whether there are other particles in an atom, aside from electrons, from which we might get energy. Besides the electrons, atoms have energy bound up in their nucleus. Is the nucleus a possible source of energy for life?

  Gathering energy from nuclear fission, the decay of the nucleus in some unstable elements such as uranium, could be one way for life to ramp up its options from the rather paltry pickings available in electrons. Unfortunately, the atomic nucleus is very difficult to control.

  We could generate a vast amount of energy by producing a nuclear chain reaction like in a nuclear reactor, but such a reaction requires a lot of uranium or a similar fissile element. There are seldom piles of such an element just lying around in the environment. Aside from using technological ingenuity as we do in nuclear reactors, we would have difficulty seeing how life forms without technology could use this energy and, if they did, how they would control a nuclear reaction without essentially vaporizing in a meltdown or an explosion.

  An
other way life could tap into nuclear fission is to make use of the by-product of fission—ionizing radiation. As unstable elements such as uranium and thorium decay, so do they release high-energy emissions in the form of alpha, beta, and gamma radiation. Could this radiation power life?

  We already know that ionizing radiation has enough energy to split water. The radiation produced from the decay of fissile elements in the planet’s crust blasts apart water molecules to make hydrogen. As we have seen, hydrogen can be an electron donor to make energy. Nevertheless, even here, the electron transport chain is at work, taking the products of ionizing radiation and shuttling them through the cell. Nuclear fission is merely an add-on precursor for making electron-containing hydrogen, food for life.

  There are even more bizarre environments where we might look for evidence of life that uses ionizing radiation. Human society releases this radiation, unfortunately sometimes unintentionally. One of the more remarkable claims to have emerged from devastated nuclear disaster sites concerns fungi living near Ukraine’s Chernobyl nuclear reactor, which blew its top in 1986. Researchers in Russia carried out laboratory experiments using fungi exposed to intense radiation there. The black melanin pigment the fungi contain became better at electron transfer reactions when it was irradiated. Fungi that contained the pigment were more metabolically active, raising the extraordinary possibility that these fungi, and other organisms like them, may even benefit from the radiation streaming out from decaying and devastated nuclear reactor cores. Nevertheless, within this postapocalyptic finding, electron transfer reactions still reign supreme. Indeed, it is very difficult to think of a way that highly energetic radiation from nuclear fission, with a tendency to destroy compounds, can be harnessed by life other than by breaking down molecules into bite-sized pieces that can be fed into the more sedate and kindly electron transport chains, or by changing the electron transfer properties of molecules.

  As the fungi at Chernobyl suggest, energy can be harvested as atomic nuclei fall apart in fission reactions, but alternatively, energy is also given up when some atoms are smashed together. Unfortunately, unleashing energy in nuclear fusion to power life seems even less straightforward than fission. Vast amounts of untamed energy can be freed when nuclei of atoms are fused; nuclear fusion is the basis of energy production in the Sun. However, the conditions required to get the reaction going are extreme: temperatures of many millions of degrees Celsius must be reached to coax nuclei to bind. Even brown dwarfs, planets tens of times larger than Jupiter, cannot reach temperatures in their cores to ignite fusion reactions. Efforts to do this using technology require elaborate nuclear fusion reactors to contain plasmas at these unconventional temperatures. Nuclear fusion seems an unlikely source of energy for life (other than, of course, the bright light that is produced from a star’s nuclear fusion reactions and that here on Earth is used in photosynthesis). For now, then, gathering the energy of the atomic nucleus directly, bypassing electrons altogether, seems unlikely.

  Probing around in the atomic structure to find new sources of energy is one way to seek alternatives. Another way is to think about physical processes that might yield some accessible energy. In an elegant stroll through some imaginative alternatives, astrobiologists Dirk Schulze-Makuch and Louis Irwin proposed some alternative ways of subsisting. Kinetic energy, the energy of movement, for example, in tides or currents in oceans, might be harnessed using small hairs that wave around in the water like the little hairs on the surface of some protozoa. The bending of the hairs in the water currents would trigger the opening of channels that drive ion movement and, like the electron transport pathways, the capturing of energy.

  Perhaps thermal energy might be used to take advantage of the huge heat gradients in hydrothermal vents, where temperatures drop from hundreds of degrees to just a few degrees above freezing as the erupting fluids from the crust come into contact with the oceans. Some forms of life might harness magnetic fields to separate ions and, by changing their alignment with a magnetic field, might use the resulting movement in ions to drive energy harvesting. Schulze-Makuch and Irwin also considered osmotic gradients, pressure gradients, and gravitation as ways to drive the movement of ions and molecules and so collect energy to do work.

  Before anyone scoffs at these ideas, remember that the basis of the chemiosmosis mechanism is a whirring, rotating machine, ATP synthase. Ultimately, the whole purpose of electron transport is to build up a proton gradient that makes ATP synthase go round and round when the protons flow through it. The mechanical principle is not different from steam being used to turn a turbine; the only difference is that from the turbine, we make electricity, whereas in the cell, we use the changing shape of ATP synthase as it rotates to snap ADP and phosphate groups together to make ATP.

  The cell does not care what makes that ATP synthase go on its merry-go-round. With an open mind, we can imagine life separating its rotating ATP synthase from electron transport and proton gradients and perhaps directly tapping into the movement of ions driven by other gradients: gravity, pressure, heat, and magnetic fields.

  However, as Schulze-Makuch and Irwin recognized, many of these radical energy sources have problems. Gravitational energy is really too small to move things around at the microbial scale. The same is true with pressure gradients. The difference in pressure between the surface of the Earth and the deep interior of the planet is enormous, but at the microbial scale, it is paltry (0.01-pascal difference between one end of a one-micron-long bacterium and the other if it is pointed downward). Can life use this differential to do anything useful? An apparatus that would use such a small gradient seems unlikely. Magnetic fields on present-day Earth are so small that it is difficult to see how they could be used to make energy, although some microbes and animals do detect these fields and use them to navigate. On other planets, stronger magnetic fields might offer greater potential for energy generation.

  Other problems confounding these ideas are the special environmental conditions for them. Large thermal gradients are common around deep-ocean hydrothermal vents, in deeply heated rocks, and on the surface of the Earth warmed by the Sun, but they are not ubiquitous. These gradients would probably have to be highly stable, intense, and reliable for microbial communities to develop and be sustained over long periods. Osmotic gradients using salts and ions also depend on a wide diversity and persistence of these gradients.

  No doubt, the physics of alternative methods of gathering energy in cells is well worthy of investigation. Their demonstrated plausibility, even discovery, in the environment would expand our appreciation of how life can, within the constraints of physics, exploit free energy in the environment. We could better say whether chemiosmosis results from strange idiosyncrasies in the Earth’s evolutionary experiment or if it reflects a much more fundamental and predictable path in the physics of energy acquisition by life. However, even assuming that we have not found these other pathways because we have not looked hard enough (and there are many unassigned genes in the DNA of living things), none have jumped out at microbiologists and molecular biologists.

  With these other possibilities in mind, do we consider Mitchell’s chemiosmosis system pure chance at work, an example of where predictions from physics must be left behind to allow for the vagaries of randomness? Was it a contingent invention, stumbled across and then locked into life as a historical accident? Would other methods of energy acquisition dominate on other planets? I do not think so. As much of this chapter has shown, the success of the electron transfer chain is the ability to tap into a wide diversity of electron donors and acceptors and even pure electrons themselves. Living things that can use electrons this way have at their disposal a vast array of elements and compounds from which they might collect energy anywhere, from the surface of a planet down into its interior. On the surface, electron transport can be used to harness the tremendous energy available in photosynthesis. As environmental conditions change, life forms that can plug and play different electron donors
and acceptors have a huge advantage in moving into new habitats where resources are unexploited. Someone else has eaten the hydrogen? That’s no problem; I’ll eat iron. The sulfate has all gone? No worry; I’ll eat nitrates. The ability to strip electrons off the raw materials of planets or the by-products of other living things to make energy has a spectacular flexibility that few other energy sources have.

  Certainly, we can imagine bacteria that eat ionizing radiation or long, tubular life forms that use thermal gradients across a hydrothermal vent. Nevertheless, we cannot help but conclude that on any planet, access to the multifarious forms of electrons in anything—from water to uranium—would give any life form an advantage. The ability to tap into this energy would give life a great step forward in colonizing the diverse environments that any geologically active planet produces. We must always be careful about confirmation bias, but I think there is a logic to, a reason for, the success of electron transfer as the core energy machinery of life on the planet, and I suspect it would be the case elsewhere.

  Like life on the macroscopic scale, we can imagine variation. We know that sodium ions have been used instead of protons for making a gradient across a cell membrane; perhaps other ions might be used. Maybe proteins in the electron transfer chain might have different structures from the ones we know on Earth, mirroring the huge diversity of these proteins even on our planet, but still containing iron, sulfur, or other elements suitable for shuttling electrons. These variations are analogous to the different colors and modifications in shapes of animals on Earth at the scale more familiar to you and me. But at the core, at the bottom of all this machinery, is the manipulation of a subatomic particle, the electron, to gather free energy from the universe. Nothing reflects so beautifully the potential universality of living systems, their link to the most basic particles and physical principles in the cosmos, and that physics and biology are utterly inseparable.