Deep-sea vents are suffused with chemical energy, including hydrogen (H2), hydrogen sulfide (H2S), methane (CH4), and oxygen (O2), courtesy of carousing reactions with oceanic crustal rocks. This comes about because billions of years of photosynthetic activity has created a thin veneer of highly oxidized material over a mass of chemically reduced rock. Earth is essentially a battery.
Hydrothermal circulation taps this battery: fluids circulating through seafloor crust are charged with reduced (electron-laden) compounds, emerging as jets into the oxidizing environment of the ocean. The carry of compounds from the reduced rock mass (the anode) to the ocean (the cathode) is a flow of electrons. As with any battery, this electron flow is a usable energy source.
Mussels evolutionarily managed the neat trick of harboring bacteria in their gills that convert these charged ingredients into metabolic sustenance for the mollusks. Some of the mussel microbes specialize in methane, others hydrogen sulfide, while some like their energy straight with a chaser: H2 and O2.
A hydrogen-consuming symbiosis is particularly surprising. Hydrogen consumption by life forms is rare, as its natural production in reliable quantities occurs in only a relatively few places, such as in vent systems hosted by peridotite (mantle-like) rock. In being highly reactive, hydrogen is a uniquely rich energy source.