Corals turn to algae for stored food when times get tough

Researchers at EPFL present new evidence for the crucial role of algae in the survival of their coral hosts. Ultra-high resolution images reveal that the algae temporarily store nutrients as crystals, building up reserves for when supplies run low.

The relationship between corals and the microscopic algae they harbor is a classic example of biological symbiosis – the mutually beneficial interaction of two species. But crucial details regarding their relationship have remained elusive until now. Using state-of-the-art imaging techniques, Anders Meibom and his team of researchers in the Laboratory for Biological Geochemistry have found new evidence on the vital role algae play in helping corals survive in environments where nutrients are scarce. Their findings were published in the journal mBio on May 16, 2013.

“Coral reefs are the jungles of our oceans – hotspots of biodiversity that easily outcompete all other marine ecosystems,” says Christophe Kopp, first-author of the publication. Coral bleaching occurs when the colorful algae abandon their coral host because of environmental strains like rising sea temperatures. On their own, corals struggle to survive in tropical waters where nutrients are scarce, and persistent starvation can have irreversible effects. While it is well known that algae help corals to assimilate certain nutrients, such as nitrogen from seawater, how this occurs, and to what extent the corals can get by on their own, are less clear.

To study how nitrogen-rich nutrients are taken up and processed by the corals and the algae that inhabit them, Meibom's research group teamed up with the Aquarium Tropicale Porte Dorée in Paris to run a series of experiments. There, they fed the corals nitrogen-rich compounds labeled with a heavy nitrogen isotope that they could later trace in the lab. Every few minutes, they extracted bits of coral, which they fixed and analyzed with a state-of-the-art isotopic imaging instrument, a so-called NanoSIMS.

Next, they assembled a timeline of how the nitrogen is processed by the corals and their resident algae by lining up the images of the samples extracted at different times. A combination of electron microscopy and mass spectrometry allowed them to study with unprecedented precision into which cellular compartments the heavier nitrogen isotopes had been incorporated.

Crystal food banks

The research revealed that the corals depend strongly on the algae to extract sufficient nutrients from the water. This was particularly true when the corals were exposed to nitrate, a compound that they are unable to process and assimilate on their own.

But most interestingly, the scientists observed that the algae act as tiny food banks. Their images revealed that the algae temporarily store the nitrogen in the form of uric acid crystals – a fact they later confirmed using crystallographic analysis. This way, the algae can stock up on nutrients when supply is abundant and draw on them when supply drops, leaching some out to their coral host.

Because coral reefs are at the foundation of immense economic activity, both as tourist magnets and as the habitats of some of the most productive fish populations, understanding their fate as the environment they inhabit changes is not only of ecological, but also of economic importance.

The research was performed in close collaboration with EPFL's Interdisciplinary Centre For Electron Microscopy (CIME), the Institute of Earth Science at the University of Lausanne, as well as the Aquarium Tropicale Porte Dorée and the Muséum d'Histoire Naturelle in Paris. The work is funded by an ERC Advanced grant and by a grant from the Swiss National Science Foundation.