The corals that make up the reefs are incredibly fragile organisms that rely on a delicate relationship with tiny colourful algae that populate their insides to survive.
Insight into coral food source could help save the reefs
Coral reefs are the jungles of our oceans. They are hot spots of biodiversity, breeding grounds for thousands of species and, as divers will attest, some of nature's most fascinating spectacles.
They form the backbone of some of the world's most productive fishing grounds and attract millions of tourists every year, so they are also of enormous economic importance.
But the corals that make up the reefs are incredibly fragile organisms that rely on a delicate relationship with tiny colourful algae that populate their insides to survive.
When this relationship is pushed off balance by rising sea temperatures or pollution, the algae abandon their coral hosts, depriving them of vitally important food sources and their lush colours in a process known as coral bleaching.
The coral have evolved to coexist in a mutually beneficial relationship with the algae, which can use photosynthesis to extract food from the ocean, while corals cannot. Left to themselves, corals struggle to extract enough nutrients from the tropical waters in which they live.
That much we have known for decades, but several mysteries have remained unsolved. How do the algae take up nutrients from the surrounding water? How do they pass them on to the corals? And how efficiently can corals capture nutrients - nitrogen in particular - when they are on their own?
These issues have baffled scientists for so long mostly because of the small scales involved. But advances in imaging technology have given us hope that we could finally address them. All we needed to proceed were corals.
Without an ocean nearby, we ran a series of experiments in a fish tank at the Tropical Aquarium in Paris and at the Interuniversity Marine Institute on the Red Sea.
For up to six hours, we fed the corals with nitrogen-rich compounds containing "heavy" nitrogen atoms labelled with an extra proton so that we would later be able to detect them in the lab.
Following the feeding, the corals and their resident algae continued processing these compounds, breaking them down and metabolising the labelled nitrogen before incorporating it into their tissues. Over the course of several days, we extracted coral samples from the aquarium, which we fixed and prepared for analysis.
Back in the lab, with a combination of state-of-the-art imaging techniques, we were able to locate precisely where the heavy nitrogen atoms were at the time the samples had been extracted.
By lining up the images chronologically, we could track the heavy nitrogen as it was incorporated by both the corals themselves and their resident algae.
Confirming prevailing knowledge, the algae outperformed the corals in taking up nitrogen from the compounds we provided.
But the real surprise came when Christophe Kopp, the lead author of our study, zoomed in on the algae. The heavy nitrogen appeared to be concentrated in pockets showing up as small bright dots in our images.
Under closer inspection, it turned out that the algae were storing the nitrogen as uric acid crystals, tightly packaged in one of the most compact forms available.
Then, over the next four days, this nitrogen was slowly transferred out of the algae and into the corals in a process that seemed capable of lasting for weeks.
The ability to adapt to an uneven supply of nutrients could be critical for corals to thrive. Near the shore in particular, the composition of ocean water is far from constant. After rainstorms, for instance, rivers flush water rich in nutrients - and often contaminated by pollution - from the continent into the oceans. But between these events, tropical ocean water is often devoid of nutrients.
Despite their fragility, corals and their resident algae appear to have evolved to withstand such fluctuations, with the algae acting as microscopic silos, storing harvested nutrients for the corals to draw upon when supply is low.
Anders Meibom is professor in biological geochemistry at the Ecole Polytechnique Fédérale de Lausanne