Symbiotic relationships between species are a popular evolutionary strategy, a reality anyone with a dog can verify. But the bacterial strains Thioploca and Anammox are taking it to a whole other level: in exchange for being Thioploca's toilet, Anammox gets to ride its elevator-like “sulfer braids” for free. It's among the more ingeniuos quid-pro-quos in the world, and it's necessitated by the tough conditions both bacteria have to confront in their habitat.
These are strains that live in the anoxic seabeds off the west coast of California and South America. Without the option of oxygen as a metabolic aid, Thioploca relies on less conventional sources—nitrate and sulfide—to fuel itself. It also had to evolve a way to be mobile so that it could scavenge those rare resources from the muddy sediment.
A snot-tunnel is the obvious answer. By secreting mucus into hollow chains, which can grow up to 7 centimeters, the Thioploca filament creates both a protective bunker and an elevator through the mud. It collects nitrate towards the top of the chain, where the seabed is, and encounters the sulfide on the way down. Reacting the two chemicals together is its specialty, and it can power itself off this process alone.