Corals can survive in nutrient-poor waters thanks to the help of microscopic algae, according to a study published in Nature. This phenomenon has fascinated researchers since Charles Darwin. The study shows that corals “farm” their microscopic algae inside their cells, allowing corals to tap into a pool of nutrients that was considered unavailable to them. In practical terms, they eat algae to get the nutrition they need.
“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,” said Professor Jörg Wiedenmann, Head of the Coral Reef Laboratory at the University of Southampton, who led the study. “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.”
Stony corals are soft-bodied creatures that look like plants but are classed as animals. Individual polyps live together as a colony and secret limestone skeletons, which form the three-dimensional framework known as reefs. Corals rely on microscopic algae that live inside their cells. The photosynthetic algae produce large amounts of carbon-rich compounds, such as sugars, which they share with the host coral.
The algae can also extract inorganic nutrients from the water, including nitrate and phosphate. Even in water with low nutrient levels, these compounds can still be found as excretion products of sponges and other organisms. The coral, however, cannot use inorganic nutrients directly. This has been a mystery for years: How exactly can corals use these nutrients to grow?
Now, a team from the University of Southampton is keen to unravel the mystery. Using labelled nitrogen to track the movement between the coral and the algae, the researchers discovered that corals actually digest some of their symbiont population to access the nitrogen and phosphorous that corals extract from the water. “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,” said Professor Paul Wilson, paleoceanographer at the University of Southampton.
This mechanism allows corals to grow quickly even if they don’t receive additional food. “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 Dr. Cecilia D’Angelo, Associate Professor of Coral Biology at Southampton and one of the lead authors.
“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,” added Dr Loreto Mardones-Velozo, researcher in the Coral Reef Laboratory.
The team analysed corals growing in different areas, some with seabirds on them and some without. “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. Accordingly, the associated reefs receive less nutrients,” said Professor Nick Graham, Marine Ecologist from Lancaster University. “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.”
The authors warn that climate change and water pollution — which alter the amount of nutrients accessible to corals — can damage these animals. Both too much and too little can alter their natural supply routes. “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,” said Dr D’Angelo from the University of Southampton. As this study shows, coral can endure periods of starvation by feeding off their symbionts, but a more prolonged nutrient depletion caused by climate change can be extremely detrimental in the long term.
Wiedenmann, C. D’Angelo, M.L. Mardones et al. (2023). Reef-building corals farm and feed on their photosynthetic symbionts. Nature: https://doi.org/10.1038/s41586-023-06442-5.