Digital ACT Reading Practice Question #1623

The primary purpose of the passage is to explain: Passage Excerpt (Natural Science): Deep within the abyssal benthic zone, hydrothermal vent communities flourish in total absence of sunlight, reliant entirely upon chemolithoautotrophy. Specialized tubeworms of the genus Riftia lack a digestive tract, mouth, and gut; instead, they house dense colonies of endosymbiotic sulfur-oxidizing bacteria in a vascularized organ called the trophosome. These bacteria oxidize toxic hydrogen sulfide escaping from subterranean magma plumes, fixing dissolved inorganic carbon into organic metabolites that nourish the host worm. The ecological success of this mutualism hinges on hemoglobin with unprecedented affinity for sulfide and oxygen simultaneously, preventing sulfide from binding cytochrome c oxidase and paralyzing cellular respiration.