CAT 2016 Slot 2 VARC: Coral Reef Symbiosis

Coral reefs occupy less than one percent of the ocean floor yet support roughly a quarter of all known marine species, a disproportion that has puzzled naturalists since Darwin first noted it during the voyage of the Beagle. The puzzle sharpens once one appreciates that reefs typically flourish in nutrient-poor tropical waters — waters that, by the ordinary logic of ecology, ought to support only sparse life, since productivity elsewhere in the ocean tracks the availability of dissolved nitrogen and phosphorus fairly closely. Darwin's paradox, as it is now called, was not resolved until the twentieth century, and its resolution turns out to be a story about a partnership rather than a place.

The coral animal itself, a small soft-bodied polyp related to sea anemones, is an unremarkable predator, capturing plankton with stinging tentacles. What makes reef-building corals extraordinary is the community of single-celled algae, called zooxanthellae, that live embedded within the polyp's own tissue. The algae photosynthesize, using sunlight to fix carbon, and they pass a substantial share of the resulting sugars directly to their coral host — by some estimates, enough to meet nearly all of the coral's daily energy requirements. In return, the coral supplies the algae with a sheltered, sunlit position near the surface, along with a steady supply of carbon dioxide and nitrogenous waste that the algae recycle into new organic matter. This exchange, in effect, allows the reef to run largely on recycled nutrients rather than on external input, which explains how such prodigious biological productivity can be sustained in water that would otherwise be nearly barren.

The relationship, however, is more precarious than the word "symbiosis" — with its comfortable connotations of stable mutual benefit — might suggest. Zooxanthellae are sensitive to temperature in a way the coral polyp, on its own, is not. When water temperatures rise even one or two degrees above the local summer maximum for a sustained period, the photosynthetic machinery inside the algae begins to malfunction, producing reactive oxygen compounds that are toxic to both partners. The coral responds by expelling its algae en masse, a phenomenon visible to the naked eye as "bleaching," since the algae's pigments are responsible for most of a healthy coral's colour. A bleached coral is not dead — the white skeleton remains, and if temperatures moderate quickly enough, the coral can sometimes recruit new algae and recover. But a coral that has expelled its primary energy source is simultaneously starving and stressed, and prolonged or repeated bleaching events, of the kind that have become markedly more frequent since the 1980s, leave reefs too depleted to recover before the next thermal event arrives.

It is tempting, given this fragility, to regard the coral-algae partnership as a kind of evolutionary miscalculation — an arrangement so finely tuned to a narrow temperature band that it was destined to fail as soon as the band shifted. This temptation should be resisted. The partnership evolved over tens of millions of years within a climate that, for all its fluctuations, changed far more slowly than the current warming trend; there was no available evolutionary pathway toward broader thermal tolerance because the selective pressure to develop one had never previously existed at the required pace. The vulnerability of reefs today is therefore less a design flaw in the symbiosis itself than a measure of how far outside its historical operating conditions the ocean has been pushed in a geologically instantaneous span of time. Some coral species do host more heat-tolerant algal strains, and reef scientists have begun deliberately inoculating vulnerable corals with these hardier partners in the hope of buying reefs time — a project that, tellingly, treats the symbiosis not as fixed biology but as a relationship that might yet be renegotiated.