NEW STUDY: Archaea as “flexitarians”: ammonia-oxidising microbes also feed on amino acids

Ammonia-oxidising archaea living in marine sponges are not the strict specialists scientists once thought. These important symbionts of marine sponges, which remove toxic ammonia, a metabolic waste product, can also consume amino acids such as valine, leucine and isoleucine as part of their metabolism. The findings, published in Science Advances, provide the first direct experimental evidence that these symbiotic microbes are mixotrophs, able to use CO₂ and amino acids as carbon sources, shedding new light on their relationship with their hosts.

Ammonia-oxidising archaea were traditionally thought to be strict chemolithoautotrophs, deriving energy from ammonia and carbon from CO₂. Although genomic evidence had suggested that some symbiotic archaea might also take up amino acids, this had not been demonstrated experimentally.

The study was led by Bettina Glasl and Katharina Kitzinger from CeMESS, in collaboration with researchers from the Australian Institute of Marine Science (AIMS), and supported by the FWF Cluster of Excellence “Microbiomes Drive Planetary Health”. The researchers studied the coral reef sponge Ianthella basta and its archaeal symbiont Nitrosospongia ianthellae. Using a combination of chemical and microscopic imaging techniques, including NanoSIMS, they tracked labelled amino acids in individual archaeal cells, directly linking the identity of the microbes to their metabolic activity.

The findings also point to a possible role in communication between the archaea and their animal hosts. By consuming and producing branched-chain amino acids, these microbes may influence molecules involved in host signalling, potentially affecting the conserved mTOR pathway. The study highlights how a flexible microbial metabolism may shape one of the oldest and most ecologically important symbioses on our planet.