One of the many frequent claims that crop up in both popular and scientific reports about deep-sea hydrothermal vents is that chemosynthetic ecosystems are independent of photosynthetic ecosystems. Even high quality scientific reporting have been guilty of making this claim: “These animals live completely independent of sunlight” (NOAA Ocean Explorer).

But are chemosynthetic systems totally independent of the sun?

Recall the basic equation for chemosynthesis:

CO₂ + O₂ + 4H₂S -> CH₂0 + 4S + 3H₂O

Now, this is a basic chemosynthesis reaction using hydrogen sulfide as the electron donor, this is not the only possible chemical pathway for chemosynthesis. Notice that one of the required molecules is O₂. Where did that oxygen come from? Photosynthesis!

So while chemosynthetic ecosystems are not directly dependent on the sun for energy, a critical part of the chemosynthetic pathways requires a byproduct of photosynthesis, therefore, it is inaccurate to state that “These animals live completely independent of sunlight”

~Southern Fried Scientist