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Corals have a "vegetarian side" that keeps them alive in nutrient-poor areas

Charles Darwin, during his monumental voyage on the HMS Beagle, was deeply intrigued by the mystery of how coral reefs thrived in nutrient-poor sections of the ocean. This scientific enigma, known as Darwin’s Paradox of Coral Reefs, has been an enduring question among marine biologists for decades.

A breakthrough discovery, led by the University of Southampton, has now unlocked this age-old mystery. 

It turns out that corals are not strictly carnivores, as once thought. Instead, they have a “vegetarian side,” actively farming and digesting their photosynthetic partners, the symbiont algae, to extract the nutrients they need.

Darwin’s Paradox of Coral Reefs

Professor Jörg Wiedenmann, head of the Coral Reef Laboratory at the University of Southampton, who led the study said: 

“The question as to why coral reefs thrive in parts of the oceans that are poor in nutrients is known as Darwin’s Paradox of Coral Reefs and has inspired the discovery of several important processes that can help to explain this phenomenon. We can now add the missing piece of the puzzle and help to solve the long-running mystery.”

“When Charles Darwin set sail on the HMS Beagle, he considered himself a geologist and during his voyage through tropical seas, quickly became interested in where and why coral reefs are formed.”

“Darwin correctly predicted how the subsidence of the Earth’s crust and the steady upward growth of corals interact to form vast reef structures. However, the biological mechanisms behind this vigorous growth remained unstudied.”

Symbiotic relationships

Contrary to their stony appearance, corals are vibrant colonies of individual polyps. These soft-bodied creatures, while resembling plants, are actually animals. 

Their symbiotic relationship with the microscopic algae residing within their cells is the critical to their survival. The algae generate carbon-rich compounds, providing the coral with essential energy. 

The algae also efficiently harness dissolved inorganic nutrients from the surrounding seawater, which the coral host can’t directly absorb.

Major breakthrough

The breakthrough in this research came when scientists, in collaboration with Lancaster University, Tel Aviv University, and the University of Jerusalem, identified the mechanism of nutrient transfer from the algae to the corals. 

“Over the many years during which we propagated symbiotic corals in our experimental aquarium system, we had observed that they grew very well even when they were not fed. It could not be explained by the current state of knowledge how nutrients were exchanged by the two partners of the symbiosis, so we figured that we were missing a big piece of the picture and started to analyse the process systematically,” said co-lead author Dr. Cecilia D’Angelo.

“One would expect that animals die or stop growing if they don’t eat. However, the corals looked perfectly happy and grew rapidly if we kept them in water with elevated levels of dissolved inorganic nutrients,” noted Dr. Loreto Mardones-Velozo, who conducted key experiments.

How the research was conducted 

Using isotopic labeling, a unique method to track the movement of nutrients, the team traced the path of nitrogen from the surrounding water to the algae, and subsequently to the coral host. 

“We used isotopic labeling to ‘spike’ the nutrients supplied to the corals with nitrogen atoms that were heavier than normal,” explained Bastian Hambach, manager of the Stable Isotope Mass Spectrometry Laboratory. “These isotopes allowed us to trace the coral’s use of the nutrients using ultrasensitive detection methods.”

“With this technique, we could unambiguously demonstrate that the nitrogen atoms that sustained the growth of the coral tissue were derived from the dissolved inorganic nutrients that were fed to their symbionts in the experiment,’ explained paleoceanographer Paul Wilson.

Digesting excess parts of their symbionts

Professor Jörg Wiedenmann of the University of Southampton said the team used 10 different coral species to quantify how the symbiont population grew along with their hosts. 

“Using mathematical models of the symbiont growth, we could show that the corals digest the excess part of their symbiont population to harvest nutrients for their growth. Our data suggest that most symbiotic corals can supplement their nutrition through such a ‘vegetarian diet,” said Professor Wiedenmann. 

Field studies confirmed the findings

Notably, these findings were not confined to the laboratory. Field studies conducted around islands in the Indian Ocean supported the lab results, showcasing that the “vegetarian side” of corals could significantly boost their growth in natural environments. 

“The reefs around some of these islands are supplied with substantial amounts of nutrients that come from ‘guano,’ the excrements of the seabirds nesting on the islands. On other islands, the seabird colonies have been decimated by invasive rats,” said Professor Nick Graham, a marine ecologist at Lancaster University.

“Accordingly, the associated reefs receive less nutrients. We measured the growth of staghorn coral colonies around islands with and without dense seabird populations and found that growth was more than twice as fast on reefs that were supplied with seabird nutrients.”

“We calculate that about half of the nitrogen molecules in the tissue of the coral animals from islands with seabirds can be traced back to uptake by the symbionts and the subsequent translocation to the host.”

Study implications 

The discovery that corals have this vegetarian side also raises concerns for the future. Global warming could disrupt the nutrient balance essential for coral health, suggested Dr D’Angelo. 

“Warming surface waters are less likely to receive nutrients from deeper water layers. The reduced water productivity can result in less nutrients for the symbionts and in-turn less food for the coral animals.”

In essence, this research not only unravels a mystery dating back to Darwin but also serves as a reminder of the delicate balance in nature and the looming challenges it faces.


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