Some time around 1975-77, the early evening news and magazine programme ‘Nationwide’ did a Christmas special about life in the year 2000. I can remember a few details. The cod was considered an endangered species or extinct, there was a test tube with an embryo in it and women were no longer familiar with the idea of skirts or dresses. It’s seemingly impossible to track down, but since Richard Stilgoe was involved maybe that’s just as well, but then so was Valerie Singleton. Anyway, one of the things it featured was a TV schedule including ‘The Universe About Us’ as a parody of the well-known natural history programme ‘The World About Us’, which was about life native to asteroids and how they coped without an atmosphere, and it was this that really piqued my interest.
At the time, I used to exercise my imagination in rather a limited way by a kind of analogical method. For instance, I used to think that what was happening with audio at the time would happen with video two decades later, so the ubiquity of cassette recorders in 1977 I imagined to extend to video recorders with built in screens and cameras in 1997. I also used to extend two dimensions to three and replace rotary motion with linear, so if I’d done a session on two-dimensional tessellation I would try to imagine how that would work in three, and try to think of ways of replacing wheels with the likes of linear induction motors. I was actually a little concerned that this process of analogising was a bit lazy and wanted to come up with another way of imagining things which was a bit more flexible and original, but of course it did bear a limited amount of fruit.
I did this with the idea of organisms who didn’t breathe oxygen by imagining an airless planet or moon to be like a desert on Earth, except that the environment in question was effectively an oxygen desert, where not only water but also oxygen was scarce. I don’t remember too much about it, but one thing I do recall was the idea of trees with deep roots to reach subterranean water deposits as a basis for life forms who sought out oxygen deposits deep underground in a similar way. There will be a notebook somewhere with further details in it. I also eventually came up with the idea of a Martian whose body was based on similar principles. It had a large dome on top of its body covered in holes which it used to inhale air, which it then compressed to breathable density using piston organs. The problem with this is that there is practically no oxygen in the Martian atmosphere and it would have to be “cost-effective”: that is, in an atmosphere with a usable amount of oxygen in it, the energy expended in compression would have to be lower than the energy released by respiration. This is actually a practical problem with respiratory diseases. If your lungs are unable to function without a lot of respiratory effort, you can actually end up losing weight because you burn so many calories by the energy spent on breathing, and of course ultimately you could go so far from breaking even that it would actually be fatal.
This assumes, of course, that life requires oxygen, which is by no means so. It so happens that our own metabolism is built around the famous Krebs cycle which liberates energy from glucose by carefully controlled oxidation, with a small bit at the beginning called glycolysis which only releases a small amount of energy without needing oxygen, and there are plenty of completely anærobic organisms – ones who do not require oxygen – and even ones for whom oxygen is toxic. However, for a living thing to rely only on anærobic respiration would be much less efficient than using oxygen and they would be unable to compete well with species occupying similar niches which could avail themselves thereof.
The only reason there is much free molecular oxygen in our atmosphere is that æons ago, cyanobacteria evolved which were able to combine carbon dioxide and water to store energy and produced oxygen as a by-product. This actually ended up poisoning most of the life around on this planet which had thriven up until then and plunged it into a global ice age where there were even glaciers at the equator due to the lack of a greenhouse effect from the carbon dioxide which had been removed from the atmosphere. It was actually a bit of a disaster, and it demonstrates very clearly that oxygen can be a liability for life rather than essential to it. It is in fact implicated in the kind of damage associated with aging, and if life like us could survive without respiring in an oxygen-rich environment we might end up living a lot longer, barring accidents. However, it remains to be seen how we would manage to derive energy to do all that living, and perhaps if we were only able to use anærobic respiration we would take a lot longer to get things done and life might end up seeming about the same length anyway.
Photosynthesis is not the only way free oxygen can arise in an atmosphere. The Jovian moon Europa and Saturn’s moon Enceladus both have extremely thin oxygen atmospheres because of the breakdown of ice and in the latter case water vapour, and in the case of Enceladus this oxygen is actually transported to Titan’s much thicker atmosphere. It’s thought that a very common type of planet would be the “water world”, which could form in several different ways but consist of an ocean hundreds of kilometres deep over a layer of ice which is only there due to compression and not cold. Such a world would start off with a water vapour atmosphere but ultraviolet radiation from its sun would split up the molecules and the hydrogen would escape into space, leaving the oxygen behind, probably at breathable levels. However, life as we know it on such a planet is another question because depending on how thick the ice is, volcanic activity and rock could be deeply buried under the ocean bed and heavier elements wouldn’t be available, so it’s likely that larger such planets would be lifeless due to lack of material resources. On smaller worlds, the oddity may be that even though photosynthesis might never have evolved, heterotrophs such as fungi and animals might, without needing plants, but there would still need to be producers for them to eat.
It’s also been suggested that although organisms benefit from oxidation or other chemical processes to release energy, other forms of carbon-based biochemistry might use other elements than oxygen to do it. In fact it isn’t necessary to go as far as another planet to see this happening because even here there are sulphur bacteria which use that element instead. In fact sulphur is used metabolically in a number of different organisms in various ways. There are two opposite processes chemically referred to as oxidation and reduction. Oxidation is the loss of electrons whereas reduction is gain, and sulphur bacteria are a big personal reason why I didn’t go into marine biology. As a teenager, I did field work on a mud flat in Kent which was rich in anærobic bacteria releasing stinky hydrogen sulphide living in a black, tarry layer under the mud in which I got completely covered, which seriously put me off doing any more of that kind of thing. I wonder, in fact, whether this was part of the point of the activity. Anyway, from the comfort of this urban East Midlands sofa, I am able to pontificate on the matter in a more detached manner. Sulphur bacteria occur in several different types and use sulphur for various purposes. The element was present in quantity on this planet before oxygen respiration evolved and would have been an ample source of energy. Some archæa do the same. They may actually “breathe” sulphate rather than sulphur as such, and whereas when oxygen is breathed it’s reduced to water, sulphur produces hydrogen sulphide. However, both elemental sulphur and various sulphur compounds are used. Sulphur, being in the same column of the periodic table as oxygen, has certain similar properties, although its valency, unlike oxygen’s which is always two, varies. Further down that column are selenium, tellurium and polonium, and all but the last perform useful functions in some living things, the function of polonium being of course to kill things and be extremely dangerous, but none of them are abundant enough to be used for respiration. Sulphur is a solid at room temperature and at sea level pressure it only melts at 239°C, so it’s unlikely to be a respiratory gas. An ecosystem based on sulphur would therefore probably be completely aquatic. However, sulphur is the fifth most common element on the surface of this planet and the tenth most common cosmically and it crops up all over the Solar System, such as in the clouds of Venus, as sulphuric acid oceans on early Mars and all over Io both as an element and as frozen sulphur dioxide. All of this suggests that there are many worlds out there in the Universe with sulphuric acid cycling through the atmosphere in the same way as water does on Earth, and depending on its concentration it could be very hostile to the development of life, which sadly could also apply to Mars and Venus. Nonetheless, the worlds themselves could be quite interesting geologically and chemically.
A popular science fictional choice of another option to oxygen is chlorine, which I’m pretty sure I’ve mentioned before on here. The potential for marine organisms to produce elemental chlorine gas is considerable because of the salt content of the oceans, and it may be that whereas we on this planet have gone down the oxygen route, others will have a large amount of chlorine in their atmospheres. If this is so, and their oceans are like ours in other ways, they will also contain a lot of caustic soda, so from our perspective if there’s any life there at all it will be in some way extremophile. Such oceans might also be high in elemental iron, as were Earth’s before the oxygen catastrophe, as it’s known. For me, the issue with chlorine is that it’s liable to produce “dead ends” in molecules. Oxygen, being bivalent, can participate in groups which join both to the main part of an organic molecule and other elements such as hydrogen, and can also occur in rings, but chlorine only has a valency of one and therefore terminates a group and can neither form part of a carbon chain or ring. This would give chlorine a different function in such biochemistry and there might still be a rôle for oxygen in it anyway, though not as a breathing gas. If the parallel to oxygen was close, photosynthesis would involve the combination of tetrachloromethane with hydrochloric acid, or rather hydrogen chloride, to form a partially substituted chlorinated hydrocarbon as an energy store and respiration would involve the production of tetrachloromethane. At our atmospheric pressures, tetrachloromethane is only gaseous above 77°C although it melts at -22, but chlorine is a powerful greenhouse gas so it’s feasible that a planet with a high-chlorine atmosphere would be quite warm and have water on its surface above our own boiling point, or again the possibility exists of aquatic life only. Incidentally, it hasn’t escaped my attention that in the above word equation I assumed hydrochloric acid or hydrogen chloride to be the main constituent of the oceans rather than water, which may be incompatible with life. This, however, is just a straight naïve substitution of chlorine for oxygen, which might not parallel a genuine viable set of processes upon which biochemistry could be based. For instance, and again this is tinkering, retaining water in that equation still leads to free chlorine and tetrachloromethane in the atmosphere but also a kind of chlorinated “sugar”. The real processes of photosynthesis and ærobic respiration are a lot more complex than that famous equation suggests, and there may be flexibility in there somewhere.
The collaborative science fiction project Orion’s Arm has had a go at creating a chlorine-based planet class, claiming that it’s unlikely that the process could take place easily and that they’re likely to be either rare or the result of something like a terraforming process by intelligent aliens. However, they do turn up in science fiction quite often. John Christopher’s ‘Tripods’ trilogy depicts aliens who aim to convert our atmosphere to one high in chlorine so they can settle our planet. Isaac Asimov’s ‘C-Chute’ describes a human spacecraft which gets taken over by chlorine breathers during a war and the human attempt to reclaim it in a toxic atmosphere. Getting back to the Orion’s Arm article, I agree that weathering would be more pronounced on such a planet and that photosynthetic pigments are likely to be purple because of the greenness of chlorine gas, but in fact it’s also theorised that chlorophyll is a second generation pigment on this planet necessitated by prior purple microörganisms using up the rest of the spectrum, so in fact it might well be the case that even most habitable planets would have purple vegetation and that Earth is unusual in having green plants.
Another option I’ve wondered about but am almost sure is not viable is fluorine. This is the element after oxygen in the periodic table and also the most chemically reactive of all elements. Physically, it has similarities with oxygen, with a similar boiling point, although it’s yellow. This is by contrast with chlorine which at our sea level pressure is only -34.1°C, meaning incidentally that chlorine planets would have to be hotter than Earth to be viable unless they had something like lakes of pure molten chlorine at the poles. However, fluorine is so reactive that it would be difficult to dislodge from its bonding. For a long time it seemed entirely unfeasible to me that any planet could have free fluorine in its atmosphere, but in fact it is possible, though in small amounts and probably only locally. Fluorite mineral is locally common here in the English East Midlands. This is calcium fluoride, which releases hydrogen fluoride, or hydrofluoric acid, when sulphuric acid acts on it. This leads to the disturbing situation of a planet with pools of hydrofluoric acid at least briefly on its surface, before it eats through the rocks and makes its way towards the mantle. Once it encounters heat, however, it would dissociate into hydrogen and fluorine, or when struck by lightning it might also separate. It would then combine very easily, to the extent that it could even form xenon fluoride in small amounts. Hence I think a planet with a little free fluorine in its atmosphere is possible, but it would be quite short-lived and incompatible with life. That said, fluorine does exist in terrestrial biochemistry in teeth and bones where fluoride content is high in water, and also in krill for some reason I don’t understand.
At the top of this post, I gave the impression it was going to be about space camels, and it is. That is, it’s about the possibility of alien animals who can thrive in an atmosphere rich in their respiratory gas for long periods of time, and I am still going to do that. The point here is that such animals may not breathe oxygen in the first place.
Among the simplest and most easily plausible situations is simply an ecosystem like ours but no oxygen respiration, just glycolysis. There are animals who don’t breathe on our own planet. There is a cnidarian parasitic on salmon who doesn’t breathe. In our cells, like those of most other animals, there are symbiotic organelles descended from bacteria called mitochondria which are largely responsible for processing glucose to release energy in combination with oxygen. Henneguya salminicola is a microscopic relative of jellyfish whose mitochondria don’t do this. There’s also a whole phylum of animals, the Loricifera, which includes species who never come in contact with free oxygen, living in Mediterranean sediment, and may also lack mitochondria. The famous Cryptosporidium, a pathogenic alveolate which I unfortunately have considerable personal experience with due to its presence in water in Leeds in the 1990s, does not respire using oxygen. There are also innumerable species of anærobic bacteria and archæa. On this planet, all of the larger organisms live in special and restricted environments, and although they are larger, they’re still pretty small compared to us. It does, however, at least prove that there can be animals who don’t breathe oxygen and are fine, and that would be one option for evolution, or indeed a path that the whole of evolution could take on a planet with no oxygen in its atmosphere, perhaps using a different energy source than light to power its biosphere. Very many aspects of our anatomy and physiology do depend on our need for oxygen, such as a circulatory system including a heart, and of course lungs, but it isn’t clear that an animal who doesn’t breathe at all wouldn’t need one if larger than a certain size because there would still be a need to move nutrition and waste products around, and there might even be lungs because of the need to vocalise for communication, or perhaps to exhale nitrogenous waste such as ammonia. Presumably organisms evolving in an oxygen-free environment right from the start would also have many bodily compounds which would react, perhaps even violently, with oxygen if they were to come in contact with it, possibly even being highly inflammable.
Another very common and straightforward technique for surviving without breathing is found among whales, dolphins, seals, sirenians and possibly early humans. These are simply good at holding their breath, and are in that sense “oxygen camels”. Sperm whales, for example, can hold their breath for up to an hour and a half, and a lower metabolic rate could cause this to increase to several hours, so it’s interesting to speculate whether the likes of ichthyosaurs and plesiosaurs might have gone for hours without breathing. In a way, then, oxygen camels not only exist but we may even be them ourselves. We have the diving reflex, where our heart rate slows down when we are immersed in water. All vertebrates, as far as I know, can also store oxygen using a hæmoglobin-like pigment in their muscles so that it can be readily available for use when needed rather than having to rely instantly on blood oxygen.
Another possibility, which I’ve explored elsewhere in collaboration with someone else, is of an animal consisting largely of a thin “skin” which performs many different biological functions but is bladder-like, containing sacs of air like a lilo. Such an animal takes a similar approach to oxygen as a succulent plant does to water, storing it when plentiful and calling on reserves when needed. However, the volume of gas could make this rather ungainly. Perhaps there could be airship-like animals on some planets who do this though. Sky whales, as it were.
A more elegant approach would involve storing oxygen, sulphur or chlorine chemically and releasing it when needed, and if space camels exist this is, I suspect, the most widely adopted solution, probably in combination with greater than usual reliance on anærobic respiration, or perhaps “achloric” respiration in some cases. This would involve relatively dense solid compounds which could be induced to release oxygen or chlorine at manageably slow rates, rather like fat deposits can be called upon to release energy for metabolism. Camels partly rely on the water content of their humps in the sense that the adipose tissue stores water rather than the humps actually being “water tanks”, but this is not the most important store of water in their bodies, which is a combination of the bloodstream and one of their stomachs along with dry fæces and viscous, low-water urine. However, it isn’t clear how much this could be extended compared to breathing. Another possibility is something like hibernation when oxygen or chlorine levels are low, or perhaps the ability to switch over to another respiratory element such as the much more compact sulphur by changing the respiratory pathway and storing sulphur compounds.
Why, though, would a situation arise where a respiratory element varied in availability? This happens on our own planet because we have air-breathing animals who have returned to the water. Perhaps on another planet with plateaux above the level of breathable oxygen it would be necessary for animals venturing onto them, perhaps to exploit an ecological niche too extreme for their lowland colleagues, to have such adaptations. A similar situation might emerge in the upper atmosphere, with the airship-like animals, although it should be borne in mind here that they would need to employ a lighter-than-air gas such as hydrogen to maintain their altitude, perhaps consuming aerial flora. Or, bird-like animals might fly into the upper atmosphere and glide, becoming dormant for a while perhaps to avoid predators or harsh environmental conditions, although what could be harsher than the upper atmosphere? Incidentally, this is still in the troposphere, so in a sense it would not be the “upper atmosphere” as lift and drag would still have to apply.
Applying camel physiology to a low-oxygen (assuming it is oxygen) environment, there’s the efficient use of oxygen in the body, akin to the low level of water in the urine, the storage of oxygen in special corpuscles which are somewhat like red blood corpuscles but hold onto their oxygen for longer and the chemical conversion of compounds in storage to release molecular dioxygen. On the subject of dioxygen, ozone would be a slightly more efficient way of packaging oxygen and hydrogen peroxide considerably more efficient, although it would have to be protected from catalase and the body would have to be protected from it, which occurs in white blood cells. The human body is 65% oxygen by mass, although little of this could be usefully released without causing fatal chemical reactions. A space camel could also have an extra lung used solely for storage, which could exhale into the other lungs when needed. As it stands, most of the oxygen inhaled into human lungs emerges from them unused. This could be remedied by compression and the removal of carbon dioxide.
Therefore, I think there could be space camels, and environments in which that would be a useful adaptation, if there are aliens at all, but they might not be able to breathe oxygen and might even burst into flames if they landed here unprotected. Or, they could be like enormous inflatable camel balloons floating through the stratosphere. Burning giraffe anyone?
A Box Of Chocolates 