The reef knot

[Global] The bond between coral and algae, on which an entire ecosystem rests, must be protected.

Over the last 600 million years, a succession of “reefs” have appeared and disappeared in the oceans in myriad forms. The true history of corals is a bit murky because of the several extinction events on Earth that they have either been wiped out by or emerged from. However, what we call coral reefs today are the result of something the coral animal first began to do about 230 million years ago, when the climate was changing, and the waters of the Tethys sea were warmer than before.

Until this time, corals were only eating small fish and bits of plankton. A coral colony is made up of hundreds of little vase-shaped animals called polyps that use their tentacles to net bits of food and digest it in their stomachs. But, in this changed environment, corals began to do something different.

They started capturing free-floating algae — millions of them — and keeping them inside their tissues.

With these chloroplast-rich protists in their midst, corals could harvest the energy of the sun directly — using the algae as their photosynthetic go-between.

Earlier limited by what their tentacles could snag, corals now began to farm the algae for most of their food. In turn, the algal cells got a place to live, and protection from predators, but the real benefits were for the coral host. They now had an enormous amount of energy, and began to proliferate, growing in tall minarets and pyramids.

This is also when they got their colour — from pigments in the algae — and the reef canvas exploded in remarkable hues.

Over time, an ecosystem began to take shape around these expanding reefs. Fish were drawn to them for food and protection. Eels made homes in crevices, and crabs and snails burrowed between coral branches. Herbivores like parrotfish kept the reef clean of algae, allowing baby coral space to grow.

A rich tapestry of relationships began to form on reefs. Today, they are one of the most biologically diverse systems, supporting over 25 per cent of all marine life. Nearly 500 million people depend on them for their food and livelihood.

But the relationship on which all of this rests — the bond between coral and algae — is a fragile thing. Warming ocean waters and polluted seas disrupt the process of algal photosynthesis by the algae. Beyond a threshold temperature (around 30°C) toxic amounts of oxygen begin to build up inside the coral tissue, which neither algae nor coral are able to regulate. Eventually, the coral expels its algal boarders in a process called ‘bleaching’. With the algae goes colour, and entire reefs turn a ghostly white, lingering in a state of coma.

Some coral species have learnt to adapt, switching between algal types to ones that can tolerate warmer water. If waters cool down fast enough, corals can sometimes regain algae and recover. But if sea temperatures keep rising, as they are now, corals remain bleached, becoming physically weaker and more susceptible to disease. The islands and atolls they fortify become exposed to the elements, compromising the physical integrity of the land and its people.

The future of reefs will depend on whether we choose to lose an experience or somehow reclaim it.

View original article at: The reef knot





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