Diseased mushroom coral in the sea off Thailand | Photo: Georg Heiss

What makes corals sick?

In the Journal of Microbial Ecology, researchers from the Leibniz Centre for Tropical Marine Research (ZMT) report on the influence of environmental pollution on bacteria in reefs and on the causes of coral disease.

Corals can bleach when the water temperature rises, this condition is well known. But also a number of other diseases with names that sound familiar can attack corals: consumption, necrosis, smallpox... Although they have increased considerably in recent decades there is still little knowledge about triggers and mechanisms of most coral diseases.

Researchers at the Leibniz Centre for Tropical Marine Research (ZMT) have discovered an important cause of coral diseases. Bacteria, the microscopically small inhabitants of the reefs, play a decisive role in this process.

Many different bacterial strains usually live quite harmoniously together with corals. Some bacteria can even be of great benefit to for example stony corals, where they play a role in the settlement of their larvae. But this community can lose its balance.

One reason for this is the eutrophication of the oceans. More and more waste water from coastal towns, hotels and resorts, agriculture and aquaculture is being washed into the sea on densely populated tropical coasts. The waste water contains nitrogen and phosphates, among other things, which lead to strong algae growth. Algae compete with corals for the space in the reef and overgrow them, if the supply of nutrients in the normally rather clear and nutrient-poor water of the reefs becomes abundant.

Algae release certain sugars into the water, such as glucose or galactose, which can also be found in high concentrations in waste water. The sea around the corals is then enriched with these dissolved carbons - a feast for certain types of bacteria that proliferate unabated. A team led by microbiologists Astrid Gärdes and Amy Cardenas from ZMT examined the role of these bacteria as potential pathogens in corals.

In the laboratory, they exposed corals and algae to different concentrations of sugars and after two days they analysed the bacterial communities in the water of the test tanks. Using the most modern methods, metagenomics and metatranscriptomics, they succeeded in identifying the bacterial species and to specify the genes that were active in the bacteria.

What they found was surprising: In water with higher sugar concentrations the composition of the bacterial species changed fundamentally. "We found many more pathogenic bacteria that are otherwise only rarely present in coral reefs, because they can process the sugars efficiently," explains Dr. Astrid Gärdes, head of the Marine Microbiology group at the ZMT. "On the other hand, many of the harmless bacteria now turned on genes that produce a cocktail of harmful substances and thus become pathogenic."

Such bacterial toxins affect corals, but their immune system usually takes care of these pests. However, if algae are on the increase in reefs, the corals are often weakened as the algae also release toxins to harm them. They can then no longer withstand bacterial infection.

Therefore, in reefs dominated by algae, a high number of corals are often found which, for example, suffer from white smallpox, white or black band disease. "A hot spot for coral disease is the Caribbean," says Astrid Gärdes. Here, not only waste water but also excessive overfishing lead to the proliferation of algae, as many of the herbivorous fish fall victim to hobby and professional fishermen."

Publication:

Cárdenas A., Neave MJ., Haroon MF., Pogoreutz C., Rädecker N., Wild C., Gärdes A., Voolstra CR. (2017) Excess labile carbon promotes the expression of virulence factors in coral reef bacterioplankton. The ISME Journal.