Edit: some answerers seem to be slightly misunderstanding the nature of the premise. Remember that the concept here is this; In the Precambrian of Earth, bacteria evolve to extract the abundant chloride in seawater and use it to emit chlorine gas as a defense mechanism. Their predators adapt, and the predators' predators adapt, until all organisms (Only microbes at this stage) emit chlorine in one way or another. With the evolution of multicellular land plants, which occurs after the point-of-divergence with Earth's timeline, all the chlorine-emitting organisms covering the land keep on pumping it into the atmosphere until it is 1% chlorine. They have the same biochemistry as Earth bacteria; albeit an evolved version, but because of all the chlorine in the atmosphere, organic molecules have become heavily chlorinated; hence the PVC instead of cellulose.

In his book World-building, Steven L. Gillett proposes a world called Clorox, where in the planet's life's early history, microbes evolved to use the chloride in the water around them to produce chlorine gas as a defense. An evolutionary arms race ensued, and eventually all of the food chain produces the gas and the atmosphere contains 1% chlorine because of their emissions. The life forms are carbon-based and oxygen-breathing, but are highly tolerant to chlorine. Their cells have thick walls/membranes, and their bones, shells and other hard parts are made of plastics.

Apparently, the plants on this world produce PVC and use it like cellulose, making them have plastic bark, stems and leaves. There are a bunch of other unusual things about the planet, such as its greenish-yellow sky, mild acid freshwater bodies (And mild bleach seas), smogginess, and lack of an ozone layer - the chlorine does the job well enough, however.

I'd like to use a similar idea, but have it in an alternate Earth timeline, where Proterozoic bacteria evolves to emanate chlorine gas instead. One thing I'd like to check though is; Are Gillett's plastic plants viable? Would PVC instead of cellulose work biologically, and are there any repercussions on other aspects of the life it would have? You can leave out the last question if you like, but it'd be all the more appreciated if you didn't. Any other comments on such an atmosphere's effects are welcome.

Please inform me of any errors; and seriously - please don't downvote without leaving criticism. It can get annoying sometimes.

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    $\begingroup$ Viable where? You're talking about an almost complete rebuild of biochemistry as we know it, I'm not sure we can answer that, other than by saying "sure if you say so". $\endgroup$ – Ash Aug 7 '18 at 18:19
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    $\begingroup$ @Ash well, remember that in my proposed project, the plastic flora are descended from Earth bacteria. When I say viable, I mean that such a plant could grow, evolve, survive etc with cellular functions and organelles derived from that of bacteria. $\endgroup$ – SealBoi Aug 7 '18 at 18:30
  • $\begingroup$ Early bacteria could not produce chlorine because it was not in the atmosphere. If, on the other hand, seas were more saline, and bacteria had a much more significant use for Sodium or Potassium (I can not imagine why, but let's not focus on this yet), then yes, significant amount of clorine as a byproduct are possible. $\endgroup$ – Alexander Aug 7 '18 at 20:33
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    $\begingroup$ I really like the idea, mostly because it would necessitate an ecology where plastic eating organisms of micro-and macroscopic scale are common. Which would create all sorts of hilarious problems for an exploration team with our tech, give or take 50 years. $\endgroup$ – Kakturus Aug 8 '18 at 8:27
  • $\begingroup$ @Alexander I'm thinking of marine bacteria here, and there's plenty of chlorine in the oceans. $\endgroup$ – SealBoi Aug 8 '18 at 9:17

I would say your first question isn't really answerable. If by viable you mean, "Can they exist?" then I think the answer is: not with anything similar to current terrestrial biology, but you could handwave it and say evolution has devised a completely new biochemistry. It seems unlikely, but I'll grant it as it's the whole premise of the question. The second question, however, can be explored a little bit.

So, I pose the questions: what do real plants use cellulose for, and what would swapping it out for PVC do to the properties of those plants? Further, what are the long-term consequences for other life and the greater ecology of the planet?

This is the structure for polyvinyl chloride, or PVC:

pvc repeat unit

As you can see, PVC is a fairly straightforward substitution of a chlorine atom for a hydrogen atom in a (long) linear alkane.

This is the structure for cellulose:

cellulose repeat unit

Clearly very different. So our expectation going forward should be that they're going to behave very differently. Now, what do modern plants use cellulose for? To a strong approximation, it's mainly used as a structural component in plants. Thus, we want to know how it's structurally different from PVC. Well, there are many types of wood and other structural cellulosic materials, but I'll do a comparison with softwood (like pine) as a fairly average representative. I'd also like to note that the data I'm looking at is for non-plasticized (rigid) PVC. You can add a plasticizer to the PVC to vastly change its properties.

First of all, PVC is significantly less rigid than softwood. This means that plants likely couldn't grow as tall before their own weight began to bend them over. PVC grass would be droopier, PVC trees would either be short or bendy.

On the other hand, PVC is much stretchier. This will make it difficult for branches to snap off (imagine if tree branches were as tolerant of bending and stretching as plastic packaging!). This likely has only little consequence for plants (better branch retention in high winds?), but would mean that any evolved herbivores would need to use sharp teeth to trim grasses and leaves, rather than pulling them off like many herbivores do today.

PVC actually has a similar density to cellulose, and the density of wood depends a lot on its microstructure. Softwood floats not because cellulose can float (cellulose is more dense than water) but because of its structure. Something similar is likely true for PVC-based biological structures, so aquatic plants and the like are still viable.

An ecological problem: PVC is much more stable than cellulose. This means that the process of sequestration -- whereby an atmospheric gas is captured from the air by plants, turned into a solid, and eventually buried in the ground forever* -- would proceed more quickly with PVC-based plants than cellulose-based ones. This means that CO2 and Cl2 levels in the air will drop more rapidly after plants begin to produce PVC.

CO2 in the atmosphere over time

This graphic shows historical levels of CO2 in the atmosphere. Around 400 MYA land plants first appeared, and you can see the corresponding downward trend in atmospheric CO2 begin around that time. With a PVC-based ecology this drop could be much sharper, and will happen for both CO2 and Cl2. This means climate change on a faster timescale, potentially a much cooler earth in 2018 than we have today, and much less available chlorine in the atmosphere as time went on.

Now, this might not happen. Even bacteria evolved alongside PVC would likely find it tricky to break down, its very chemically stable. However, amyl chloride might be one route to digestion, and similar for aniline, both of which have a degrading effect on PVC. So if bacteria (or, larger scale, animals) evolved with PVC they might be able to produce these chemicals as a sort of specialized digestive fluid. Still, it's possible that even with these adaptations breakdown could be slow enough to have a noticeable effect on sequestration rate.

Okay, this is starting to get rambly and into the realm of wild speculation, so I think I'm done for now. Obviously true answers to this question are unknowable, but by granting the premise I hope I've given you at least a few interesting possibilities to explore.

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    $\begingroup$ The ecological problem also occurred in our cellulose-plant world. The Carboniferous Era got its name because there was a long period of time when dead trees did not break down. Eventually fungi evolved that could break down cellulose. Perhaps a similar pattern will occur in the original poster's world. $\endgroup$ – Jasper Aug 8 '18 at 0:39
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    $\begingroup$ @Jasper Excellent point! $\endgroup$ – realityChemist Aug 8 '18 at 1:28
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    $\begingroup$ The best part about cellulose is that it's a glucose polymer, and plants already have glucose chemistry set up and working. So in order to have PVC plants, I think it's important to explain how they use vinyl chloride first. And that's not easily explained using the biology we know AFAIK. $\endgroup$ – Agent_L Aug 8 '18 at 7:19
  • $\begingroup$ Just in relation to my first question, maybe I could tell you a bit more about the premise, and you could see what you make of it. Basically, in the Precambrian, some species of bacteria evolves to extract the chloride in water, and then use it to emanate chlorine gas. Its predators respond accordingly, and an evolutionary arms race ensues until everything emits chlorine gas. My idea for the plants is that the plants reserve some of this chlorine and produce PVC, as a chlorine-resistant coating. $\endgroup$ – SealBoi Aug 8 '18 at 8:39
  • $\begingroup$ @Agent_L I think that's a strong point, and why I just granted that they do make PVC in answering this question. I still consider the viability question fundamentally unanswerable, but the more discussion goes on the more I lean over toward the, "No," side. Biochemistry would just be so different, it may be that nothing resembling modern land plants even evolve. $\endgroup$ – realityChemist Aug 8 '18 at 14:46

You have to deal with an important problem: Chlorine is way less abundant than Oxygen, thus it makes much harder to build a biochemistry on it.

Oxygen is the third most abundant atom in the solar system after Hydrogen and Helium, while Chlorine is about 4 orders of magnitude less abundant.

It's simply really hard to develop a biochemistry based on a relatively rare element, when you have something else which is more abundant, like it happened with Oxygen on Earth. Mind, I am not saying it is impossible, simply more difficult.

You can have that sort of development in some niches, where Chlorine is constantly more abundant, like what happened for those life forms relying on H2S close to volcanic vents.

Also mind that Chlorine would compete as oxidant with Oxygen, due to its electronegativity. Therefore this add another difficulty to an extensive usage of Chlorine in an Oxygen rich environment.

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    $\begingroup$ I think Gillett's premise was that the plants evolved PVC rather than cellulose because of accumulation of chlorine in their cells, because they produced it themselves. $\endgroup$ – SealBoi Aug 7 '18 at 18:56
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    $\begingroup$ @SealBoi, living cells cannot execute nucleosynthesis. That happens, for chlorine, only in exploding stars. $\endgroup$ – L.Dutch Aug 7 '18 at 19:03
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    $\begingroup$ @L.Dutch, Biological systems are capable of concentrating elements. Carbon makes up 0.18% of the earth's crust and 0.04% of the atmosphere, but makes up roughly 20% of the human body. Chlorine makes up 2% of the ocean and, in this scenario, 1% of the atmosphere. It is perfectly plausible for it to be concentrated to useful levels in plant systems (assuming a completely different biochemistry that isn't killed by chlorine, of course). $\endgroup$ – realityChemist Aug 7 '18 at 19:14
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    $\begingroup$ The question asks about Earth-style life which has slightly better access to chlorine. This answer is about life that uses chlorine instead of oxygen. Flagging as not and answer. $\endgroup$ – Agent_L Aug 8 '18 at 8:59
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    $\begingroup$ I don't think that's how evolution works. Evolution like that relies on random birth mutations, so you can't count on it happening. $\endgroup$ – SealBoi Aug 8 '18 at 10:38

The short answer is that, yes, it's plausible enough for a fictional world, in the sense that ruling it out would take up a whole book by itself.

As @LDutch's answer points out, the economics are against using chlorine, but the chlorine in PVC is there more for industrial-chemistry reasons than for any special property of the final material. There are many plastics that use only carbon and hydrogen (polyethene, polypropylene, polystyrene, polyolefins), and many more that also involve oxygen and nitrogen (polycarbonate, polyurethane, ABS).

In fact, a good question would be why plants don't produce these kinds of plastic. We know of lots of materials that are chemically simpler than cellulose, and have far superior structural properties. In general, biological chemistry seems to favor the use of relatively big and complex building blocks (like saccharides and amino acids) because their chemistry is specific and controllable. The molecules used as building blocks in industrial chemistry (simpler, more reactive molecules like chloroethene), if you put them in a living cell, would tend to react indiscriminately. This is probably why, when you see simple industrial chemicals in living organisms, they are waste products that the organism is trying to get rid of (oxygen, ethanol, methane).

So my hunch is that plants wouldn't make PVC because the monomers are too simple for them to process. But again, biology, can do surprising things if it needs to.

  • $\begingroup$ That's a very good point, and I think this is a good angle to take on OP's question. Out of curiosity I just checked to see if chlorinated cellulose is a thing, and it seems that it is. I'm not sure what kind of material properties it has, but it looks like it's too prone to having the chlorine replaced with another functionality to make a good structural polymer. Still, I wonder if structural chlorinated biopolymers could be a thing in an environment like OP poses. $\endgroup$ – realityChemist Aug 8 '18 at 14:40

Probably not.

There are 2 ways of looking at this, first is the biggest, through marine life.

Ocean water has substantial quantities of chlorine and bromine, both of which are used to produce bioactive compounds, such as the chlorine containing briarenolide J and chloromethane, and the bromine containing bromomethane. Sea water contains about 700 as much chlorine by weight compared to carbon.

There are thousands of naturally occurring organohalogens, many of which are produced by alge, sponges, and corals. Yet none of them make use of halogen compounds as a major component of cell walls or skeletal structure. Given the diversity of species, and the amount of available chlorine, one would expect to see some organohalogens used as a structural compound if it was going to happen, but we don't.

The second is through the properties of PVC. PVC when used to produce a product contains numerous additives to make it usable, otherwise it is quite poor in its material properties. It will dissolve in ketones and aromatic hydrocarbons produced by plants. It degrades when exposed to UV. PVC will also break down and release hydrogen chloride when heated, which is highly corrosive. Chlorinated PVC has superior material properties to PVC, but making it organically may be difficult.

The most likely candidate for PVC based compounds would be complex marine life that lives in cold dark water, the exoskeletons of lobster or crab-like creatures for example. As for using chlorine as a protective agent, some insects produce the chlorinated alkaloid epibatidine, which can be fatal to humans with a dose as small as 1mg. It reasons a plant can also produce this, either by itself or in a symbiotic relationship with bacteria, as the compound is not toxic to plants. Some plants such as Asian Moonseed produce several different chlorinated alkaloids.


in two words? um-possible.

Chlorine is a dead end, if you look at the backbone of large, complex molecules. Carbon, oxygen and nitrogen aren't, because they typically make four, two and three covalent links instead of just one. That's why they are abundant in biochemistry, and chlorine isn't. Is there a single naturally occuring organochloride in the human body? I think not.

Some (for lack of a better word) freakish bacteria produce crazy stuff like this https://de.wikipedia.org/wiki/Bipyrrol_Q1 , but I don't think that's sustainable, or could even be generalised.

  • $\begingroup$ You need to tell us why it's a dead end. $\endgroup$ – RonJohn Aug 7 '18 at 19:42
  • $\begingroup$ @RonJohn I think you need to have a look at you old high school chemistry lecture notes. ;-) en.wikipedia.org/wiki/Organochloride#Natural_occurrence $\endgroup$ – Karl Aug 7 '18 at 19:49
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    $\begingroup$ You're missing the point, @Karl. This site requires answers to have reasons beyond "because I say so, and "read your HS chemistry notes." It's guaranteed to get you down voted even though you're right about chlorine being a dead end. $\endgroup$ – RonJohn Aug 7 '18 at 20:22
  • $\begingroup$ The question was asking about PVC, where carbon does the heavy work of linking the molecule together. $\endgroup$ – Stig Hemmer Aug 8 '18 at 8:45
  • $\begingroup$ @StigHemmer Not the point. Oxygen in organic molecules can have different oxidation states, occur as alcohol, ether, carboxy group, while chlorine is stuck with a single bond at oxidation number -1. It's just not very useful, biochemically. $\endgroup$ – Karl Aug 8 '18 at 17:22

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