A new study proposes a solution to a question that perplexed Darwin: How do corals thrive in barren seascapes?

After Charles Darwin happened across “vast rings of coral-rock” while voyaging through the southern Pacific Ocean on the Beagle, he wrote that upon seeing them, “everyone must be struck with astonishment.” These encounters inspired in the naturalist a lifelong fascination with coral reefs and one persistent question: How do vibrant corals flourish in often-barren ocean landscapes?

Known as the Darwin Paradox, this mystery has continued to puzzle generations of oceanographers. A new study published today in Nature offers a solution. According to its authors, corals make up for nutrient scarcity by harvesting and feeding on their resident algae, like hungry farmers. It’s “a really, really beautiful study,” says coral biologist Mónica Medina of Pennsylvania State University, who was not involved in the work.

Researchers have long known that corals maintain a mutually beneficial relationship with the single-celled algae that call the animals’ tissue home. There, sheltered from the harsh conditions of the open ocean, the algae feed on the corals’ waste products. In return, the algae convert sunlight into energy-rich food molecules that nourish themselves and their hosts. Corals also feed on drifting zooplankton to capture other essential nutrients.

But these food sources alone can’t account for the world’s vast and prolific coral reefs. “There are lots of studies that have examined [coral] nutrient transport and tried to do the math, but it never really quite made perfect sense,” says Virginia Weis, a marine physiologist at Oregon State University who was not involved in the study. In particular, the useful forms of nutrients that corals need in order to grow—including nitrogen and phosphorus—tend to be in short supply around reefs.

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However, sea creatures living in the vicinity excrete plenty of inorganic nitrogen and phosphorus, which the coral’s algal residents can readily consume. Jörg Wiedenmann, head of the coral reef laboratory at the University of Southampton, wondered whether there was a connection. Could the algae somehow be passing these nutrients along to their coral hosts?

To find out, Wiedenmann and colleagues placed coral samples in tanks without any food. To half of the tanks, they added inorganic nutrients that only the coral’s resident algae could enjoy. Corals in the tanks containing no nutrients at all stopped growing and had lost about half of their algal partners after just 50 days, giving them a bleached and stunted appearance. But in the tanks where the algae had been fed, the coral grew robustly. Scientists knew algae provided corals with energy via photosynthesis, but these results suggested they were also sustaining coral growth by converting inorganic nitrogen and phosphorus into coral chow. Yet how the algae transferred those nutrients remained a mystery.

Wiedenmann wondered whether the corals were eating their algal partners. The team calculated the expected growth of the coral’s algal population given the input of nutrients, then compared its prediction with the actual number of algal cells in the tanks. They found far fewer algae than expected. What’s more, the amount of nitrogen and phosphorus contained within the unaccounted-for cells aligned with the nutrients required for the coral to grow as big as it did. Wiedenmann was convinced: The corals were preying on the algae.

Looking for evidence of this phenomenon in nature, the team observed the reef growth in the Indian Ocean’s Chagos archipelago. Here, seabirds live in high density on some islands but not others, and the birds’ droppings contain inorganic nutrients that algae can eat directly, but that coral cannot. Over a period of 3 years, the researchers found that reefs near islands with lots of birds—and, therefore, plenty of algae food—grew twice as fast as those near islands where birds were scarce. A unique form of nitrogen that is plentiful in bird droppings, but not zooplankton, also reappeared in corals around bird-dense islands. To the researchers, this was further evidence that the algae were indeed shuttling nutrients from the birds to their coral hosts.

The study nicely combines laboratory and fieldwork to crack a long-standing mystery, Medina says. It could also help scientists better understand the devastating effects of coral bleaching, she adds, in which the relationship between corals and their algal residents breaks down.

Source:

Molly Rains at Science

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