It just occurred to me that I haven't posted any pictures of the neat little critters we study. Here's a glamour shot of two members of the genus Alviniconcha. These crazy little guys are not like snails you may have met before. First, there's very little calcification of the shell – most of what you see is a tough outer chitinous layer. If you try and dry the shell out, this layer shrinks and literally explodes the thin calcified layer beneath into hundreds of little pieces. Bummer for collectors, but good for the snails: vent water has a lower pH than normal seawater, making precipitation of calcium carbonate more difficult. Think of these guys as a preview of ocean acidification.
Second, and in large part why we're out here, is that these snails are autotrophs – like plants, they make their own food. In association with their symbiotic bacteria, Alviniconcha takes in CO2 from the environment, then reduces it into sugar. Unlike plants, the snail/bacteria partnership doesn't use light to power this reaction; instead, they use chemical energy from the vents.
In most of the habitats with which we're familiar, the abundance of oxygen means that most of the chemicals floating around are fully oxidized – they've lost their extra electrons – and thus aren't good sources of energy. The exception to this rule is organic carbon, like sugar, fat, and protein. The carbon atoms in these molecules had their electrons restored to them at some point in the past by a photoautotroph, or someone who uses light (photo) to make their own (auto) food (troph). Now all that good reduced carbon is available for chemoheterotrophs like you and me.
At vents, though, there's a ton of extra energy being dumped into the system from the hot mantle. There's also not much free oxygen deep down in the crust. This means that, at vents, there's a lot more reduced chemical species kicking around to take advantage of – stuff like sulfur and hydrogen, which up on the surface, we usually see in its oxidized form, like in sulfate (SO4) and water (H2O). All those extra electrons open the door for chemoautotrophs like Alviniconcha, which oxidizes hydrogen sulfide (H2S) to sulfate (SO4), and uses the energy to make its own food.
Going back to the picture: you may have noticed some subtle differences in the 'hairs' covering the shells of those two snails above. Is this just random variation in the population, or might these represent two closely related species? If they're different species, why are they found at the same sites? What about their bacterial partners – do those differ, too? Their physiological capabilities?
Lots more to learn...