Question

Passage:
In the late 1970s, the discovery of hydrothermal vent ecosystems along the Galápagos Rift challenged prevailing biological assumptions regarding the energetic foundations of multicellular life. Deprived of solar radiation, these benthic communities rely not on photosynthesis, but on chemoautotrophy driven by lithotrophic microorganisms. Among the most conspicuous organisms inhabiting these vents is the vestimentiferan tube worm Riftia pachyptila, which can attain lengths exceeding two meters despite entirely lacking a functional digestive tract, mouth, or anus in its adult form.

Instead, R. pachyptila derives its metabolic sustenance through an obligate symbiotic relationship with intracellular sulfur-oxidizing bacteria housed within a highly vascularized internal organ termed the trophosome. The endosymbionts synthesize organic carbon by oxidizing hydrogen sulfide (H₂S) venting from the ocean crust, a biochemical pathway that requires a continuous supply of both sulfide and dissolved oxygen. For R. pachyptila, this metabolic reliance poses a profound physiological obstacle: in most metazoans, hydrogen sulfide binds irreversibly to cytochrome c oxidase in mitochondria, fatally arresting cellular respiration. Furthermore, in the presence of oxygen, sulfide spontaneously oxidizes abiotically into toxic sulfur derivatives unless rapidly transported and compartmentalized.

To circumvent both sulfide toxicity and spontaneous oxidation, R. pachyptila utilizes specialized extracellular hemoglobins of exceptionally high molecular mass. Unlike typical vertebrate hemoglobins, which are tetrameric and transport oxygen exclusively, R. pachyptila vascular fluids contain two distinct classes of giant hemoglobins: a 3.5-million-Dalton hollow sphere found in vascular serum and a 400,000-Dalton molecule present in coelomic fluid. Biochemical assays demonstrated that these hemoglobins possess distinct, non-competing binding sites: oxygen binds conventionally to heme groups, whereas sulfide binds reversibly to free cysteine residues located on specific globin and linker chains.

This dual-binding mechanism prevents sulfide from poisoning the worm’s own respiratory chain while sequestering the volatile compound during transit from the branchial plume to the trophosome. Notably, the affinity of the worm's hemoglobin for hydrogen sulfide is several orders of magnitude higher than its affinity for oxygen, ensuring efficient uptake even in ambient microenvironments where sulfide concentrations fluctuate drastically. Once delivered to the trophosomal bacteriocytes, sulfide is dissociated and metabolized by the symbionts, demonstrating an unprecedented evolutionary adaptation to an environment characterized by extreme chemical toxicity.

According to the passage, Riftia pachyptila prevents hydrogen sulfide from fatally disrupting its own cellular respiration by doing which of the following?