Archaea as Flexitarians: Ammonia-Oxidizing Microbes Feed on Amino Acids
- Symbiotic ammonia-oxidizing archaea residing within marine sponges are not strict specialists as long believed, but instead function as true flexitarians that consume amino acids alongside ammonia, according to...
- For decades, the scientific community assumed that ammonia-oxidizing archaea maintained a rigid diet.
- However, genomic analyses previously hinted at greater metabolic flexibility when researchers spotted genes encoding transporters for branched-chain amino acids.
Symbiotic ammonia-oxidizing archaea residing within marine sponges are not strict specialists as long believed, but instead function as true flexitarians that consume amino acids alongside ammonia, according to a study published in the journal Science Advances. Led by researchers at the University of Vienna and the Australian Institute of Marine Science, the findings reveal that microbes such as Nitrosospongia ianthellae utilize branched-chain amino acids like valine, leucine, and isoleucine, pointing to a previously unknown mechanism of communication between ancient animal hosts and their microscopic symbionts.
Challenging Decades of Microbiological Dogma
For decades, the scientific community assumed that ammonia-oxidizing archaea maintained a rigid diet. According to reports from the University of Vienna, these microbes were thought to feed exclusively on ammonia and simple nitrogenous compounds like cyanate and urea to fix carbon dioxide. This classification placed them strictly among chemolithoautotrophs, organisms that derive energy from inorganic compounds.
However, genomic analyses previously hinted at greater metabolic flexibility when researchers spotted genes encoding transporters for branched-chain amino acids. The new study supplies the first direct experimental evidence that these symbiotic archaea operate as mixotrophs. They successfully harness both carbon dioxide and amino acids as carbon sources, upending long-held assumptions about their ecological role in marine environments.
Observing Single-Cell Activity in Marine Sponges
To uncover these dietary habits, researchers focused on the coral reef sponge Ianthella basta, also known as the elephant ear sponge, and its archaeal symbiont Nitrosospongia ianthellae. Marine sponges represent some of the oldest animals on Earth, hosting dense microbial populations vital for survival in ocean habitats. Ammonia-oxidizing archaea act as a microscopic sanitation crew by removing toxic ammonia, which serves as a metabolic waste product for the sponge host.
Investigating individual microbes presented a significant technical hurdle. According to co-lead author Katharina Kitzinger from the University of Vienna, the primary challenge involved proving precisely which cells consumed the amino acids inside the sponge. Utilizing advanced chemical and microscopic imaging techniques, including NanoSIMS, the research team watched labeled amino acids incorporate directly into individual symbiont cells. This methodology linked cellular identity directly to specific metabolic functions rather than relying solely on genome predictions.
Implications for Host-Microbe Communication
We used to think of these symbionts primarily as a waste disposal service for the sponge. By producing and consuming these amino acids, the symbionts can potentially modulate important signalling molecules for the sponge. This could be a form of communication between microbes and animal hosts with deep evolutionary roots.
Because these symbiotic archaea both consume and produce branched-chain amino acids, they actively influence the overall availability of these essential compounds within their animal hosts. Co-lead author Bettina Glasl noted that this metabolic interplay extends far beyond simple waste management. By modulating signaling molecules, the microbes establish an interactive communication channel with their hosts.