Many of the creatures that cluster around deep-sea hydrothermal vents have entered a paradoxical pact with a toxin. Surviving at depths that no sunbeam will ever penetrate, the invertebrates provide refuge to chemoautotrophic bacteria, and in return, the bacteria supply their host with nutrients. While some hosts deliver methane to their lodgers, others must run the gauntlet of delivering reduced sulphur as sustenance to their guests. `Understanding which form of sulphur is transported through the host's tissue was a key question in understanding the ecology of these creatures' explains Audrey Pruski. Working with Aline Fiala-Médioni, she began the quest to discover how some symbiotic hosts deal with the deadly delivery(p. 2923). But before Pruski could discover how symbiotic hosts handle sulphur, she needed to identify the creatures that harbour sulphur-oxidizing bacteria. She began searching for a biomarker.
Knowing that Riftia pachyptila, the sulphur dependent tubeworm,seemed to have high levels of sulphur amino acids in its tissues,Fiala-Médioni suspected that the amino acids could set thiotrophic symbioses apart from the others. Gathering more than 30 species of symbiotic mussels, clams and tubeworms, Pruski began painstakingly analysing the free amino acids carried in the symbioses' tissues. Time and again, the only amino acid that turned up exclusively in creatures that host sulphur oxidising bacteria, was thiotaurine. Pruski had found the thiotrophic biomarker. But when she began searching the literature for clues about the unusual compound's metabolic role, virtually nothing was know; except that thiotaurine is a key component in beauty products!
But Pruski remembers that everyone was sceptical. Without a clear role for the unusual amino acid in the symbioses' lives, many believed that the amino acid was just an artefact. She needed to find convincing evidence that thiotaurine might function in the symbioses' metabolic trade. Would the animals synthesise thiotaurine in response to hydrogen sulphide exposure?
This time Pruski restricted her hunt to three sulphur-based symbioses: two bivalves, the vent clam Calyptogena magnifica and the mussel Bathymodiolus thermophilus, and Riftia. As soon as the animals were brought to the surface she gathered samples of tissues that housed their symbiotic partners, and exposed the tissue to sulphide; far from straightforward when the research vessel was pitching about in a storm.
Back on dry land, Pruski compared the levels of thiotaurine from the symbioses' tissues, before and after the sulphide treatment. As the sulphide exposures increased, all three symbioses produced higher levels of thiotaurine. Thiotaurine wasn't an artefact. And when she monitored the level of various thiotaurine precursor compounds in the animals' tissues, she realised that the bivalves had come up with a different thiotaurine synthesis route from Riftia.
But why have the symbioses developed an alternative sulphur transport system when they also have well-described protein transport systems? Pruski suspects that thiotaurine functions in concert with the transport proteins to protect the hosts from sulphur's darker side. But the tiny amino acid also has an advantage over the better-known proteins; it's small enough to cross several membrane barriers, transporting sulphur from the host's blood to the chemoautotrophic symbionts. Thiotaurine could mark the symbiotic paradox's turning point; sulphur's transition from toxin to life source.