This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Symbiotic ammonia-oxidizing archaea in marine sponges are not strict specialists but true "flexitarians." A team led by microbiologists Bettina Glasl and Katharina Kitzinger from the University of Vienna, in collaboration with partners from Australia, has shown that, unlike their free-living relatives studied thus far, these microbes dine not only on ammonia but also on amino acids such as valine, leucine and isoleucine—and presumably do so to communicate with their animal host. The findings, published in the journal Science Advances , shed new light on one of the oldest animal-microbe symbiotic relationships on Earth and point to a previously underestimated role played by archaea.
Marine sponges are among the oldest animals. Their simple bodies are teeming with microbes. This partnership, also called symbiosis, is vital for the sponge's survival in the oceans.
Ammonia-oxidizing archaea are key symbionts in marine sponges. They are renowned as the sponge's microscopically sized sanitation crew because they remove toxic ammonia, a metabolic waste product. Until now, it was assumed that they are strict "chemolithoautotrophs": They derive energy from inorganic compounds, in this case ammonia, and use CO2 as their carbon source.
Hints that symbiotic ammonia-oxidizing archaea might be more flexible came from their genomes. Unlike many free-living ammonia-oxidizing archaea, they carry genes encoding transporters for branched-chain amino acids. The new study provides the first direct experimental evidence that symbiotic ammonia-oxidizing archaea are mixotrophs, capable of using CO2 and amino acids as carbon sources.
The researchers studied the coral reef sponge Ianthella basta, also called elephant ear sponge, and its ammonia-oxidizing archaeal symbiont Nitrosospongia ianthellae. Using a cutting-edge combination of advanced chemical and microscopic imaging techniques, including NanoSIMS, they were able to observe the dietary habits of individual symbiont cells in their sponge host. "The real challenge was to prove which specific cells were consuming the amino acids within the sponge," says Kitzinger, co-lead author and specialist in single-cell analysis.
"We could literally see the labeled amino acids being incorporated into individual symbiont cells, linking a cell's identity to its function rather than relying on genome predictions alone." The discovery has implications beyond the microbes' diet. As they can both consume and produce branched-chain amino acids, symbiotic ammonia-oxidizing archaea may influence the availability of these essential amino acids within their ancient animal host. "We used to think of these symbionts primarily as a waste disposal service for the sponge," says Glasl, co-lead author and expert in sponge–microbiome interactions.
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