Soru

Passage:
In 1977, oceanographic explorations of the Galápagos Rift revealed dense biological communities thriving around deep-sea hydrothermal vents at depths exceeding 2,500 meters. Completely isolated from solar radiation, these ecosystems confounded classical ecological models, which posited that all macroscopic life ultimately depends on photosynthetic primary production. Central to these vent ecosystems was the giant tube worm, Riftia pachyptila, an organism that reaches lengths of up to two meters despite possessing neither a mouth, gut, nor functional digestive tract at maturity.

Subsequent physiological and histological investigations revealed that Riftia sustains itself through an obligate mutualism with chemoautotrophic, sulfur-oxidizing bacteria housed intracellularly within a specialized, highly vascularized internal organ termed the trophosome. Rather than utilizing light energy to fix inorganic carbon into carbohydrates, these endosymbionts oxidize reduced sulfur compounds—specifically hydrogen sulfide ($H_2S$)—emanating from the geothermal vent fluids. The energy liberated by this oxidation reaction drives the Calvin-Benson cycle, synthesizing organic molecules that are subsequently translocated to nourish the host tube worm.

However, this metabolic arrangement imposes severe physiological challenges on the host organism. Hydrogen sulfide is notoriously cytotoxic to aerobic organisms because it readily binds to cytochrome c oxidase, thereby halting mitochondrial respiration. Moreover, hydrothermal vent fluids are rapidly diluted by cold, oxygenated ambient seawater, requiring Riftia to concurrently extract both dissolved oxygen and toxic hydrogen sulfide from a turbulent and chemically fluctuating boundary layer. The worm accomplishes this dual uptake across its highly vascularized branchial plume, which projects directly into the venting fluid-seawater interface.

To transport these mutually reactive substances to the trophosome without precipitating systemic metabolic poisoning, Riftia produces an extraordinary class of extracellular hemoglobins. Unlike human hemoglobin, which contains four heme subunits and is contained within erythrocytes, Riftia possesses massive, free-floating vascular hemoglobins composed of up to 144 polypeptide chains. Crucially, biochemical analysis demonstrated that these multi-subunit hemoglobins possess distinct, non-competing binding sites: oxygen binds reversibly to the conventional iron-porphyrin active sites, whereas hydrogen sulfide binds reversibly with exceptionally high affinity to specific free cysteine residues located on the globin linker chains. This structural separation prevents the sulfide from oxidizing the heme-bound iron or reacting destructively with bound oxygen during vascular transport. Consequently, the host’s respiratory pigments simultaneously protect host tissue from sulfide toxicity while delivering both essential metabolic substrates directly to the trophosomal symbionts in sufficient quantities to fuel continuous autotrophic carbon fixation.

Based on the passage, which of the following explains why Riftia pachyptila's extracellular hemoglobin is capable of transporting hydrogen sulfide without interfering with its oxygen-carrying capacity?