Physiological ecology of sulfide metabolism in hydrothermal vent and cold seep vesicomyid clams and vestimentiferan tube worms. 1995. American Zoologist 35: 102-111.
The deep-sea hydrothermal vent worm Riftia pachyptila must supply its intracellular symbiotic bacteria with constant supplies of sulfide and oxygen. How does this occur, since the two tend to react spontaneously together?
The host worm must prevent spontaneous oxidation of sulfide because sulfide, along with oxygen, is used to fuel bacterial metabolism and provide nutrition for the worm. R. pachyptila transports both molecules by binding them to hemoglobin present in the blood. This hemoglobin (three different forms are known in Riftia) is unique in that it binds both sulfide and oxygen with a high affinity.
The spontaneous oxidation of the two chemicals does not occur because each binds to a different site on the hemoglobin molecule. The authors found that in R. pachyptila, sulfide doesn't form sulf-hemoglobin when bound to hemoglobin. This phenomenon is unique only to this animal. Instead, R. pachyptila binds sulfide by breaking disulfide bridges that are not important to the protein structure of the molecule.
There may be small fluctuations in the concentration of sulfide in the external environment, but Riftia still manages to supply the symbionts with a steady supply of sulfide due to its large blood volumes. Sulfide, which can be toxic, interacts minimally with the worm's tissues because it is kept at low levels due to rapid oxidation by bacteria. These things, along with oxygen and sulfide having different binding sites on hemoglobin molecules, ensures a steady supply of oxygen and sulfide reaches the symbionts and prevents spontaneous oxidation of sulfide.
(250 words)
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