On the deep seafloor, animals are relatively uncommon, their abundance limited by the slow descent of organic matter from surface oceans. However, where hydrothermal springs punctuate the deep seafloor, animal populations can be remarkably dense (Fig. 26.9). Why are animals so abundant around hydrothermal vents?

The many animals around deep-sea hydrothermal vents subsist on a thriving community of microorganisms. These microorganisms gain carbon by reducing CO₂ to form carbohydrates. However, they obtain the energy to fuel this process not from sunlight but from chemical reactions. Hence, these microorganisms are called chemoautotrophs. They oxidize molecules and ions such as H₂, H₂S, and Fe²⁺ to generate the ATP and reducing power required to incorporate CO₂ into organic molecules. In chemoautotrophic metabolism, the electron acceptor is most commonly O₂ or nitrate (NO₃–). Thus, chemoautotrophy requires access to both oxidized (O₂ and nitrate) and reduced (H₂, H₂S, and Fe²⁺) substances.

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Oxidized and reduced molecules react readily with each other and so are rarely present in the same environment. For this reason, chemoautotrophs tend to live along the interface between an environment where oxygen is present and one in which oxygen is absent. Chemoautotrophic prokaryotes therefore thrive where reduced gases from Earth’s interior meet oxygen-rich seawater.

Quick Check 4 How could the biological carbon cycle have worked on the primitive Earth, where oxygen gas was essentially absent from the atmosphere and oceans?