Algae are one of the organic resources in Oxygen not included. In addition, it is an important source of oxygen in early game.
Oxygen diffuser converts algae to oxygen at constant proportion of 91%, using 120w of power and generating 1.5 kDTU/s of thermal energy. Oxygen diffuser will stop working and warn of overpressure if there is more than 1800 g of gas in the surrounding tile. To prevent this, build it closer to your main shaft or in wide areas. If there are germs in algae - they too will be in the resulting oxygen. Place Buddy Bud near – it will protect your base from airborne germs.
As opposite, algae terrarium produces oxygen at 133% mass proportion but much slower and need 180 kg/c of clean water.
By default, algae are not a renewable resource. So at mid game you supposed to use electrolyzer technology. Better to say, the game forces you to make oxygen from the water when algae and slime sources are ending.
However, if the electrolyzer is not your choice or you do not have water/steam geysers on the map (which is impossible), there is one way to produce algae. See if it's worth it.
You need to make Puft farm. Puft consumes polluted oxygen and produces slime, which can then be distilled into algae and polluted water. Happy tame Puft consumes 30 kg of polluted oxygen per cycle, excreting 15 kg of slime. Ok, regardless of total oxygen gain now we have another question – where to get a lot of free polluted oxygen. One Puft demands 50g/s. Thus, for a continuously running one algae distiller, you need 24 happy Pufts and 1.2 kg/s of polluted oxygen.
Average polluted oxygen vent emitting gas at 15-180 g/s. 10 times less than needed. Not an option.
What about Morbs? 20kg/3c = 11g/s polluted oxygen. 4.55:1 Morbs to Puft ratio. 110 Morbs for one Algae distiller. This may even be a good gaming strategy, if not for the fall in total FPS because of pathfinding algorithms for that insane crowd of critters. Nevertheless, 10:1 Morbs to dupes ratio (including a 10% loss on a deodorizer) would be a better idea.
Polluted water can be evaporated to polluted oxygen in 1:1 mass ratio, but this is a very slow process and takes 25 tiles of water surface to sustain consumption of single dupe.
There is one tip. Build a liquid reservoir, fill it with polluted water and then destroy. This results in dropping 5000 kg bottle of polluted water, which releases gas at a high rate about 200 g/s, occupying one tile. However, it is impossible to automate this process.
Using polluted water in a scheme such as “P.water-> P.oxygen -> Puft -> Slime -> Algae -> Oxygen” is not a wise idea, since this chain has 25% conversion rate. This means only a waste of time and oxygen. It’s better to just clean up P.oxygen with deodorizer if you think what to do with P.oxygen vent. Filtration medium will become a renewable resource when you reach the surface.
Anyway, the electrolyzer is the best choice for mid to end game.