Question

In the late twentieth century, the discovery of deep-sea hydrothermal vent ecosystems challenged the fundamental ecological paradigm that solar radiation is the indispensable primary energy source for all complex metazoan life. Operating in total darkness under extreme hydrostatic pressures, these ecosystems rely instead on chemosynthesis driven by lithotrophic bacteria. Among the most prominent organisms inhabiting Eastern Pacific vent fields is the giant tube worm, *Riftia pachyptila*. Lacking a mouth, gut, or functional digestive tract, *R. pachyptila* relies entirely for its nutritional requirements on obligate endosymbiotic bacteria housed within a specialized, highly vascularized internal organ called the trophosome.

To sustain these sulfur-oxidizing symbionts, the host worm must assimilate large quantities of hydrogen sulfide ($\text{H}_2\text{S}$) alongside oxygen ($\text{O}_2$) and inorganic carbon from vent fluids. However, free hydrogen sulfide presents a severe physiological paradox: it binds irreversibly to cytochrome c oxidase, the terminal enzyme complex of the mitochondrial respiratory chain, thereby halting aerobic ATP synthesis in eukaryotic cells. *R. pachyptila* circumvents this toxic inhibition through a unique biochemical adaptation involving its extracellular hemoglobins.

Unlike mammalian hemoglobin, which is rapidly poisoned by sulfide ions, *R. pachyptila* possesses giant extracellular hemoglobins dissolved directly in its vascular blood and coelomic fluid. Structural analysis reveals that these multi-subunit protein complexes feature specialized cysteine residues and free zinc-binding sites capable of reversibly binding hydrogen sulfide at high affinity simultaneously with oxygen, without allowing sulfide to bind to the heme iron sites. Consequently, sulfide is rendered biologically inert during transit through the worm's circulatory system, effectively preventing metabolic poisoning of the host's peripheral tissues while ensuring a continuous, high-volume flux of reduced sulfur compounds directly to the trophosomal endosymbionts.

According to the passage, which of the following explicitly explains how *Riftia pachyptila* prevents hydrogen sulfide from inhibiting its own cellular respiration?