# What are some biochemical alternatives to carbon?

I am making a universe and I thought that it would be unrealistic for all life forms in my entire universe to be based upon carbon.

I am aware of silicon as a possible replacement, but I'm looking for a few more to have some variety.

My question is:
What other elements could replace carbon as the base element of life?

In this previous answer to Life on a molten world, I provided several biochemical regimes dependent upon temperature.

## At 400+ C, Fluorosilicones (silicon based macromolecule)

Each of the suggested biochemical regimes includes carbon as the backbone molecule for the chemical chain, except that of the highest temperature which is Fluorosilicone chemical chains dissolved in fluorosilicone solvent (our chemistry is proteins [carbon chains] dissolved in water) for temperatures in the 400 C - 500 C range.

$$\begin{array}{|c|c|} \hline \text{Temp Range} & \text{Macromolecule in Solvent} \\ \hline \text{400° C to 500°? C} & \text{Fluorosilicones in Fluorosilicones} \\ \hline \text{113° C to 445° C } & \text{Fluorocarbons in molten Sulfur} \\ \hline \text{0° C to 100° C} & \text{Proteins in Water} \\ \hline \text{-77.7° C to -33.4° C} & \text{Proteins in Liquid Ammonia} \\ \hline \text{-183.6° C to -161.6° C} & \text{Lipids in Liquid Methane} \\ \hline \text{-253° C to -240° C} & \text{Lipids in Liquid Hydrogen} \\ \hline \end{array}$$

This table suggests that our familiar protein in water form of life is only appropriate for a certain range of temperatures. When you develop worlds in other temperature ranges, native life will develop for the temperature range of that planet.

e.g. Mars (average temperature of -65 C) might require proteins in liquid ammonia while Titan (average temperature -180 C) might require life using lipids in liquid methane.

• Interesting, maybe if Venus was smaller with a thinner atmosphere it could support life based on Fluorocarbons or Fluorosilicones – Stephanie Mar 5 '16 at 18:56
• Or since Venus' surface temperature is ~465 C, perhaps there's fluorosilicone based life suspended in fluorosilicone liquids. I don't image that there are huge oceans of the stuff down there but imagine some lakes of it harboring life so alien we would have to completely rewrite our biochemistry books! – Jim2B Mar 5 '16 at 19:02
• What do you think the distance from the sun needs to be for a planet that can support life with lipids in liquid hydrogen? – Stephanie Mar 5 '16 at 22:25
• According to my handy-dandy planets spreadsheet, no planet in our Solar System quite makes it. Triton (largest moon of Neptune) comes closest. Required Temperature is 20 K - 33 K (that's 20 - 33 above absolute zero). Triton's average temperature is about 35 K. If Triton had a slightly higher albedo (reflectivity), it would qualify. Possibly the bright heart shaped region on Pluto could reach this temperature but Pluto's average temperature is too warm. – Jim2B Mar 6 '16 at 0:49
• I guess an Earth sized planet at 40 AU would work, though I imagine the atmosphere would be very thick and have a lot of helium, the lipids in liquid hydrogen are the hardest to work with since the melting point is very close to the boiling point – Stephanie Mar 6 '16 at 1:22

Nothing. The only element that is chemically similar and abundant enough is silicon but it suffers from a serious flaw: It's too big. It doesn't like forming long chain molecules. Look in nature, you don't find big blocks of silicon. Rather, you find Si - O - Si - O type structures. For rocks, fine--but when you try to stick the normal structures of life on there you now have hydrogen and oxygen stuck to silicon when they would prefer to be stuck to each other. The result is at best unstable, at worst a high explosive.

• You assume different lifeforms must drink hydrogen and breathe oxygen, but silicon lifeforms could as well breathe and drink something else or not breathe and/or not drink at all. – Markus von Broady Mar 5 '16 at 21:30
• @MarkusvonBroady I don't care what they breathe or drink. I'm talking about what happens when you replace carbon with silicon in large organic molecules. – Loren Pechtel Mar 5 '16 at 22:13
• "you now have hydrogen and oxygen stuck to silicon" - my point is that this probably is a wrong assumption you made - alien lifeforms might have something else than oxygen and hydrogen in their bodies. – Markus von Broady Mar 5 '16 at 22:18
• The oxygen is necessary to get the long chains of silicon--they don't bond well into long chains without it. Sure, you could substitute things farther down the table but that won't change the basic problem. – Loren Pechtel Mar 5 '16 at 22:19
• Not only that but Si doesn't have the same rich hybrid bonding set as C. – JDługosz Mar 6 '16 at 15:05

Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy.

• Arsenic doesn't replace carbon, though. – Loren Pechtel Mar 5 '16 at 21:11
• Right: arsenic replaces phosphorus, not carbon. – JDługosz Mar 6 '16 at 15:06

Other elements that could be the basis for biochemistry are germanium, tin, lead, silicon, boron, sulfur, and phosphorus.

Specific polymers and chemicals that form possible biochemistries are:

• Boranes (B, H)
• Silicones (Si, O)
• Polydimethylsiloxane (Si, O, CH3)
• Phenylsilicone (Si, O, C6H5)
• Polymeric Diphenyllead Oxide (Pb, O, C6H5)
• Polymeric Diphenylltin (Sn, C6H5)
• Butylpolystannoxane Polymer (Sn, O, OH, C4H5)
• Polysilazane (Si, N, H, CH3)
• Polymeric Phosphonitrilic Chloride (N, P, Cl)
• Dimethyl Polyborophane (H, B, P, CH3)
• Polymeric Silyl Orthoborate (Si, O, B, CH3)
• Dimethylated Polygermane Organopolymer (H, C, Ge, CH3)

All information in this post came from Xenology1. The info in parentheses is the ions and elements in the polymer.