But in the deepest oceans, sunlight isn’t an option. You could filter the meagre snow of organic matter raining down from above but to really thrive, you need a different source of energy. For Riftia’s bacteria, that’s sulphur, or rather the sulphides that spew out of the vents. The bacteria oxidise these chemicals and use the liberated energy to fix carbon. This is chemosynthesis: making your own food using chemical energy instead of light or solar energy. And rather than producing oxygen as a waste product, as photosynthetic plants do, these chemosynthesising bacteria churn out pure sulphur.
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