Saudi Arabia: Symbiosis Corals Control Nitrogen Cycle in Reefs

Corals are shown to recycle their own waste ammonium using a surprising source of glucose — a finding that reveals more about the relationship between corals and their symbiotic algae.

Thuwal/Saudi Arabia — Symbiosis between corals and algae provides the backbone for building coral reefs, particularly in nutrient-poor waters like the Red Sea. Algae and corals cooperate to share nutrient resources, but the precise metabolic interactions at play are still unclear.

Now, Kaust researchers have shown that the coral host uses organic carbon — in glucose sourced from its symbiotic algae — to recycle its own waste ammonium. Previous research had suggested that the algae alone may be responsible for ammonium (nitrogen) recycling. The team of researchers believes that, by controlling this nitrogen recycling mechanism, the coral host can in turn control algal growth by restricting or enabling nitrogen flow.

Molecular research on coral-algae symbiosis is relatively young. The first genetic sequencing study focusing on the coral model anemone Aiptasia was published in 2014. To explore the molecular mechanisms underlying Aiptasia’s symbiotic relationship with the algae Symbiodiniaceae, the team first integrated all published RNA-sequencing data on this relationship and conducted a meta-analysis. Meta-analysis is a statistical method originally developed for medical research, to calculate the precise effects of a specific medicine on patients with a specific disease by combining results from multiple trials.

Guoxin Cui at the Red Sea Research Center worked on the project under the supervision of Manuel Aranda. He explains that in their case, each gene could be seen as an individual ‘medicine,’ and they could calculate the effect of each gene on symbiosis by monitoring its expression changes across many experiments. Because the scientists use large datasets compiled from multiple studies, they are confident of the effect size they calculated for each gene. By focusing on those genes that are definitely associated with symbiosis, they could eliminate noise from unwanted parameters.

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Once the team had identified a set of high-confidence genes, they set up a metabolomics experiment, with the help of their colleagues at Kaust’s Core Labs, using symbiotic and nonsymbiotic (or bleached) Aiptasia. They placed the anemones in water and added bicarbonate containing labeled carbon-13 (13C) isotopes. The symbiotic algae absorbed the bicarbonate during photosynthesis, transferring the 13C signal to the host’s metabolites. The team could then follow the carbon isotope through the metabolic pathways of the anemones and determine which were enriched with 13C.

Cui hopes that these insights will help in understanding what goes wrong when the relationship is placed under stress, for example, because of shifting climates.

References: Cui, G., Liew, Y.J., Li, Y., Kharbatia, N., Zahran, N.I., Emwas, A.-H. Eguilus, V.M. & Aranda, M. Host-dependent nitrogen recycling as a mechanism of symbiont control in Aiptasia. PLOS Genetics 15, e1008189 (2019).

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