Marine Sponge Archaea Show Dietary Flexibility, Turning to Amino Acids as Well as Ammonia

Researchers from the University of Vienna, working with Australian collaborators, have discovered that the ammonia‑oxidizing archaea living inside marine sponges are not the narrow specialists previously assumed. Instead, these microbes display a broader metabolic repertoire, readily consuming amino acids alongside their classic ammonia substrate.

The study, led by microbiologists Bettina Glasl and Katharina Kitzinger, employed a combination of metagenomic sequencing, laboratory incubations, and isotope tracing to monitor the nutritional preferences of the sponge‑associated archaea. The experiments revealed that when amino acids were available, the archaea incorporated them into their metabolism without abandoning their core role in oxidizing ammonia.

This flexibility challenges the long‑standing view that ammonia‑oxidizing archaea (AOA) are strict chemolithoautotrophs that rely solely on inorganic nitrogen. In the complex microenvironment of a sponge, where dissolved organic matter can fluctuate, the ability to switch between inorganic and organic nitrogen sources may provide a survival advantage, allowing the symbionts to maintain activity under varying environmental conditions.

The findings have broader implications for understanding nitrogen cycling in marine ecosystems. Sponges host dense microbial consortia that contribute significantly to the transformation of nitrogen compounds in coastal waters. If a substantial portion of their AOA can also process organic nitrogen, the net effect on nitrogen fluxes could be more nuanced than current models predict.

Future research will aim to quantify how widespread this “flexitarian” behavior is among different sponge species and other marine habitats. Determining the genetic triggers that enable the switch between ammonia and amino acid utilization could also shed light on the evolutionary pressures shaping microbial symbioses. The study adds a new layer to the picture of marine nitrogen dynamics and underscores the importance of revisiting assumptions about the metabolic rigidity of key microbial players.