Imagine there is a race of highly intelligent merfolk - let's call them Atlanteans. Both the mussels and the Atlanteans lived together amicably around brine pools.
The brine pools, found about 200 meters deep across the ocean floors, contain high concentrations of methane and other nasty chemicals which can kill anything that fall in. Only extremophiles, with the exceptions of mussels and Altanteans, can spend quality time inside the pool.
I wonder: how can the Altanteans, whose tech level is on par with us in the modern times, draw useful and reliable energy from this brine pool?