Dubilier found the answer in 2001, when she realised that they have two different symbionts: one big, one small, and both mingling beneath their skin.13 The small bacterium grabs sulphates, which are plentiful in Elba sediments, and converts them into sulphides. The big bacterium then oxidises the sulphides to power chemosynthesis, much like Riftia’s microbes. In the process, it produces sulphates that its smaller neighbour can reuse. The two microbes feed each other in a cycle of sulphur, which then feeds the worm – a symbiosis à trois. By adding the small sulphate-grabbing bacteria to their
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