Coral Reefs and Calcium Carbonate Architecture
Symbiodiniaceae Dinoflagellates, Aragonite Saturation, and Wave Energy Dissipation
Sailing aboard HMS Beagle in 1842, Charles Darwin marveled that coral reefs thrive as crowded oases inside clear tropical waters with almost zero nutrients. Jasper de Goeij solved Darwin's Paradox in 2013 by discovering the sponge loop. Crevice sponges filter dissolved sugar leaked by corals and shed it as cellular detritus for reef animals.
A stony coral polyp is a carnivorous animal armed with stinging tentacles, yet it gets 90 percent of its energy from solar power. Packed inside its cells at two million per square centimeter are photosynthetic Symbiodiniaceae dinoflagellates. These algae disable host digestive lysosomes and export glucose and glycerol straight into animal cytoplasm.
Thermal coral bleaching is a defensive expulsion triggered by photosynthetic overload. When summer water rises 1 to 2 degrees Celsius above normal maximums, the photosynthetic electron-transport chain inside the symbiont algae jams under sunlight. The stalled machinery leaks toxic oxygen radicals into the host cell, forcing the polyp to eject its algae.
Corals do not passively precipitate limestone out of seawater. Beneath its bottom tissue, the polyp seals off a microscopic calcifying pocket and runs a $\text{Ca}^{2+}/\text{H}^+$-ATPase pump that swaps two protons out for each calcium ion brought in. Raising internal pH by nearly a unit converts bicarbonate into carbonate, crystallizing aragonite tenfold faster.
Because the sea has absorbed 30 percent of industrial carbon dioxide emissions, surface pH has dropped from 8.2 to 8.1—a 26 percent rise in acidity. Excess protons bind with carbonate ions to form bicarbonate, lowering the aragonite saturation state ($\Omega_{\text{arag}}$). Corals must spend extra energy pumping protons out to keep skeletons dense.
Healthy coral reefs act as self-repairing breakwaters that dissipate 97 percent of open-ocean wave energy before it hits the beach. As deep swells shoal onto the shallow reef crest, branching Acropora tables generate intense hydraulic drag. Roughly 86 percent of wave energy breaks right on the crest, shielding coasts from cyclone surge.
In 1842, Darwin deduced that ring-shaped Pacific coral atolls began as fringing reefs around volcanic islands that slowly sank while upward-growing corals kept pace. Critics doubted him for a century until geologists drilled Enewetak Atoll in 1952. Beneath 1,405 meters of shallow-water reef limestone accumulated over 50 million years, the drill struck volcanic basalt.
Because corals are cemented to rock, over 130 species on Australia's Great Barrier Reef synchronize spawning to a single night each spring. Guided by warming water, lunar phase, and cryptochrome blue-light receptors tuned to twilight, colonies across hundreds of kilometers release billions of buoyant egg-sperm bundles simultaneously—discovered by Peter Harrison in 1984.
Much of the white sand on tropical beaches was excreted by parrotfish (family Scaridae). Equipped with fluorapatite beak teeth stiffer than copper and a grinding throat mill, parrotfish chew dead coral rock to harvest algae. A single bumphead parrotfish (Bolbometopon muricatum) grinds and defecates up to five metric tons of coral sand per year.
Space on a reef crest is so scarce that neighboring corals wage nightly warfare for millimeters of rock. When a slow-growing massive coral touches a fast-growing branching rival, it transforms feeding tentacles within days into sweeper tentacles five times normal length. Packed with stinging capsules, sweepers burn the neighbor back to bare rock.
At reef cleaning stations, two-gram cleaner wrasses (Labroides dimidiatus) pick 1,200 parasites a day from the open jaws of predatory groupers. Redouan Bshary showed that cleaners prefer to cheat by biting the client's nutritious mucus. Groupers enforce honesty through punishment: chasing a cheating cleaner or switching stations on their next visit.
The loudest biological sound on a reef comes from a four-centimeter pistol shrimp (Alpheus heterochaelis) snapping its claw shut at 30 meters per second. Detlef Lohse and Barbara Schmitz proved in 2000 that the snap shoots a fast water jet that opens a vacuum cavitation bubble. Imploding at 218 decibels, the bubble heats trapped gas to 4,700 Kelvin and stuns prey.