Reading — Test 11

Reading passage

Among the most productive ecosystems in the ocean are coral reefs, structures built by tiny animals called polyps that secrete calcium carbonate skeletons over successive generations. What makes reef-building corals remarkable is not the polyps alone but a partnership each polyp maintains with microscopic algae known as zooxanthellae, which live within the polyp's own tissue. This relationship, a form of symbiosis in which both partners benefit, underlies nearly every aspect of reef biology, from growth rate to coloration to vulnerability under environmental stress. The mechanics of the partnership are straightforward in outline, if intricate in detail. Zooxanthellae, single-celled dinoflagellates equipped with chlorophyll, reside in the cells lining a polyp's gut cavity, sheltered from predators and grazers that might otherwise consume free-floating algae. In exchange for this protection, the algae conduct photosynthesis using sunlight that penetrates the shallow, clear waters coral reefs typically inhabit, and they pass a substantial share of the resulting sugars, amino acids, and other organic compounds directly to the host polyp. Researchers have estimated that in some coral species, photosynthetic products supplied by zooxanthellae can account for the majority of a polyp's daily energy requirements, a subsidy that allows corals to thrive in waters where dissolved nutrients are otherwise scarce. The polyp, in turn, supplies the algae with carbon dioxide and nitrogenous waste products, compounds that would otherwise need to be scavenged from the surrounding water at greater metabolic cost. This exchange of metabolic byproducts for fixed carbon is what enables corals to construct reefs of such scale in nutrient-poor tropical seas, environments sometimes described, somewhat paradoxically, as marine deserts. The symbiosis also explains the vivid coloration for which reefs are celebrated. Coral tissue itself is largely translucent; the golden-brown, green, or reddish hues visible on a healthy reef derive chiefly from pigments within the zooxanthellae and, in some species, from fluorescent proteins the coral produces that may help regulate the intensity of light reaching the algae. When environmental conditions shift beyond a coral's tolerance range, most commonly when water temperatures rise even a degree or two above the typical seasonal maximum, the partnership can break down. The polyp expels its algal residents, a process termed bleaching because the loss of pigmented zooxanthellae leaves the white calcium carbonate skeleton visible through the now-transparent tissue. A bleached coral is not necessarily dead; if favorable conditions return within a matter of weeks, the polyp may reacquire algae from the surrounding water and recover. Prolonged bleaching, however, deprives the coral of its principal energy source, and starvation frequently follows. The precision of this symbiosis carries an evolutionary cost: dependence. Because reef-building corals derive so much of their energy from a single algal partner, the health of an entire reef system is tied to a partnership that can be disrupted by comparatively small changes in ocean temperature, acidity, or turbidity. Some coral species host several genetically distinct lineages of zooxanthellae simultaneously, and evidence suggests that corals harboring a more heat-tolerant lineage are more likely to survive warming events than those hosting only heat-sensitive lineages. This variability has led some researchers to propose that assisted evolution, deliberately introducing more thermally tolerant algal strains into vulnerable coral populations, might help reefs withstand rising ocean temperatures, though such interventions remain experimental and their long-term ecological consequences are not yet fully understood. Beyond its significance for individual colonies, coral symbiosis underwrites the extraordinary biodiversity for which reefs are known. The energy zooxanthellae supply allows corals to grow quickly enough to build the complex three-dimensional structures that shelter an estimated quarter of all marine species at some stage of their life cycle, despite reefs occupying less than one percent of the ocean floor. Losing this partnership on a large scale, as has occurred during several mass bleaching events in recent decades, therefore threatens not merely the corals themselves but the wider community of fish, invertebrates, and other organisms that depend on reef structure for food and shelter.