So I asked my friend who knows things about marine microbiology and she was actually pretty optimistic about free floating chemoautotrophs as the base of a food

So I asked my friend who knows things about marine microbiology and she was actually pretty optimistic about free floating chemoautotrophs as the base of a food web. You just need enough chemicals dissolved in the water (which I guess is how they grow them in the lab). Some other ideas we discussed were bacteria growing on suspended particles and as endosymbiotes in larger animals like the aforementioned tube worms. So yeah, leviathans are on the table.