Since fire doesn't burn under water aquatic alien sophonts will use alternative technologies.
The question is, which alternatives to fire can they use? I am interested in all aspects of fire, (heat, light and quick method of chemical reaction with oxygen), although they may incorporate different methods i.e. for heat, a different method is available, for light, some other possibility and some other choice for burning things in oxygen, if dry fire (like we have on the surface) is not available.
I'll deal with each aspect in turn.
There are a few options here. Firstly if the sophonts live in an active tectonic area they could use a hydrothermal vent. They would have to find , or create, a chimney type vent or the heat wouldn't be concentrated enough but they could then transport the heat around by piping the hot liquid from the vent in heavily insulated pipes to where it is needed. Think of it as a really hot version of a modern central heating system. Pros - Renewable. Once set up requires minimal input. Could possibly filter metal sulphide particles from the liquid which might be useful. Cons - Geographically limited. Hard to set up originally. Potentially difficult to maintain. Can contain toxic chemicals. Can't control the heat level, it is always on.
Secondly They could use chemical reactions. For example they could use a group one metal reaction with water which creates, depending on the metal, something ranging from a fizzing lump of metal, Lithium (Li), to an explosion similar in strength to a depth charge, Francium(Fr). Please do try this at home. Small amounts of Potassium(K) added carefully to water should create a burst of heat, or possibly just an explosion. Placing the potassium in a sealed container will help prevent it floating away, moving around too much and will reduce the risks presented by the production of potassium hydroxide and hydrogen. Alternatively, petrol, gunpowder and some metal oxides and metals still burn underwater although they usually burn in a fairly uncontrolled way and often explode. Pros - Can be done anywhere. Many create metals or hydrogen as a by-product which could be useful. Cons - Requires raw materials such as oil or iron oxide which are quite hard to mine. Potassium is difficult and dangerous to store underwater, if it gets wet it will explode. Hard to control the reaction, once it has started you can't really stop it.
I think the best option is probably bacteria. Most bacteria produce heat when they respire and this can create a decent warming effect. Try touching a well established compost heap, it will feel warm. I don't think that they will be able to produce enough heat for cooking but they could certainly keep a house warm if you lined the walls with bacteria. Pros - Portable. Quick to manufacture more. Once a colony has started it will keep growing. If you live near the surface the bacteria can photosynthesise so will require no feeding. Will oxidise the room. May help suppress pathogens. May be able to be harvested for food. Some bacteria produce light as well as heat so that solves two problems in one. Cons If you live in the deep ocean they will need a supply of minerals. Limited heat output. Can't be turned off.
Some bacteria produce light. See heat for more details. Further to this, many fish and jellyfish, particularly deep sea ones, also produce light so could be used. For instance, a group of bioluminescent fish could be trapped in a glass bowl topped with netting and used as a light source. Pros - Portable. Gives you a family pet. Cons - Need feeding. Might try and escape.
Many reactions still work underwater, for example thermite reactions will still work to oxidise things. Magnesium will oxidise in water as will some other, similar metals. I think displacement reactions should still work so many things can still be oxidised underwater using fairly unreactive metals oxides.Overall although the simplest reaction has gone (burning) almost everything can still react with oxygen using other, slower or more difficult reactions.
You don't have to count fire out just because you're underwater. Oxygen bubbles can be collected from marine vegetation and held in an upturned bowl. A primitive underwater people could create a basic heat source by carving a deep indentation into a stone, holding it up upside down and filling it with oxygen and some flammable substance (fish oil?). This dome of oxygen and fuel would just need a spark and you would get fire. Once you understand the basic principle, you could build bigger domes for holding larger quantities of oxygen, big enough to hold kilns and forges.
I've thought about this for a while, and the best answer is to develop a technology tree that is completely independent from fire and fire-like effects.
This is a pretty radical concept, since fire was man's first tool, and indeed man's first step away from being an ape. But mer-folk, or what have you, would not realistically be able to develop a chemical or biochemical process similar to fire as their first technology. Fire was available to early man, who watched lightning strike and forests burn. Merfolk would hardly have many opportunities to watch sodium or lithium react with seawater.
No, the underwater technology tree would start with some other root, and branch out in ways completely alien to what we have seen here on the terrestrial Earth. Mer-folk would not cook things, dry out hides, smelt metals, bake pottery or any other ancient technology that had its root in fire.
Depends what you want to use the fire for. These creatures would have no need of it to cook food, as most ocean plants can be eaten raw as well as many species of fish. If they needed light, they could potentially allow bio-luminescent creatures to grow in their settlements that would naturally light the area. Electricity for heat could be created by setting up underwater geothermal power-stations near cracked portions of the crust.
It has been theorized that humans learned about fire from lighting strikes, which set trees and such on fire. The first humans probably collected fire into bowls or other objects made of rock.
Then we learned to use it for different purposes.
Next we learned to create it.
It would work the same way underwater. Someone mentioned vents, but there is also magma. Whoever the creatures are, they would:
Very often we forget we live in an atmosphere that reacts with different chemicals, because we have learned to control those reactions to the point we forget about them. In our atmosphere fire actually needs oxygen to survive, so our ancestors probably had to learn if they put it in an open container of wood it would simply burn the container up.
If they put it in an enclosed container or stone with a lid it would go out.
A stone container that had a safe opening would be best.
A being who evolved living in the water would be as adapted to his environment to the extent making decisions on how to keep fire active under the water would be as natural to them as keeping it active in an oxygen atmosphere is to us.
Don't always assume the problems your sentient creatures face as they evolve under the water are all that different than our ancestors did on land and look at our own solutions for correlations that may work before you go on a hunt for unorthodox solutions.
Some sort of enclosed container to put magma in seems like a good idea. Maybe a partially dry cave with dried drift wood to create a source of flame would be a good place to take your magma bowl to start a fire. Your beings would almost have to be amphibians for this to work well. A lot of fish even are capable of coming temporarily to the surface, so amphibians might not be an absolute necessity.
If you think this would be terribly inconvenient you might be right, but think back on how inconvenient gaining and keeping fire was for our ancestors. Quest for Fire is a 1981 movie you might want to look at.
Bicomponent chemical reactions should be used.
If they use something, which reacts on contact with water (eg. lithium or francium) it will be viewed as an extremely dangerous, unstable substance, and could only be produced is special 'dry rooms' by acquatics wearing water filled environmental suits.
The other possibility would be to use something, which only reacts with water, if heated (ignited), the thermal conductivity of water will make difficult to maintain the 'chain reaction'.
So it looks like, that expect special cases, they should use a hypergolic mix, whose components don't react with water, but react with each other, when mixed.
Or they could use nuclear energy. Water is a good moderator, fantastic coolant, and good radiation shield, so they won't have a problem by building a reactor. It will give a lot of heat and nice, bluish cherenkov light.
