Soru

Hydrothermal vent ecosystems, discovered in the late 1970s along oceanic spreading centers, challenged conventional ecological paradigms that assumed all macroscopic life ultimately depends on photosynthetic primary production. Among the most enigmatic organisms inhabiting these extreme environments is the giant vestimentiferan tubeworm Riftia pachyptila, which thrives adjacent to hydrothermal effluents characterized by temperatures exceeding 350 degrees Celsius, intense hydrostatic pressure, and elevated concentrations of hydrogen sulfide—a compound profoundly toxic to aerobic cellular respiration in almost all metazoans.

The survival of Riftia pachyptila is made possible through an obligate endosymbiosis with chemolithoautotrophic sulfur-oxidizing bacteria harbored within a specialized, highly vascularized internal organ termed the trophosome. Lacking a mouth, digestive tract, and gut throughout its adult stage, the tubeworm relies entirely on its bacterial symbionts to synthesize organic carbon from dissolved inorganic carbon via the Calvin-Benson cycle. In return, the host tubeworm must continuously supply the bacteria with metabolic substrates: oxygen, inorganic carbon, and hydrogen sulfide.

This physiological imperative creates a formidable biochemical paradox. In typical aerobic organisms, hydrogen sulfide reversibly binds to cytochrome c oxidase, poisoning the mitochondrial electron transport chain. Furthermore, spontaneous abiotic oxidation of hydrogen sulfide in the presence of oxygen would rapidly deplete both compounds before they reach the trophosome. Riftia pachyptila resolves this dilemma through the evolution of unique, high-molecular-weight extracellular hemoglobins present in its vascular and coelomic fluids. Unlike vertebrate hemoglobins, which carry only oxygen, these giant multimeric hemoglobins possess distinct, non-overlapping binding sites for both oxygen and hydrogen sulfide.

Crucially, the hemoglobin's affinity for sulfide is exceptionally high—exceeding its affinity for oxygen—which ensures that free sulfide concentrations in the hemolymph remain virtually negligible. By sequestering sulfide at dedicated cysteine residues, the hemoglobin prevents the poison from interacting with host cellular respiration components while simultaneously shielding sulfide from spontaneous chemical reaction with co-transported oxygen. Upon reaching the trophosome, sulfide is selectively released to the symbionts, which oxidize it to sulfate to generate the adenosine triphosphate (ATP) and reducing power necessary for carbon fixation.

According to the passage, the extracellular hemoglobins of Riftia pachyptila protect the host tubeworm's cellular respiration from sulfide poisoning primarily by doing which of the following?