Symbiotic Ammonia Oxidation in the Marine Environment

Ammonia oxidation is a fundamental step in the marine nitrogen cycle, catalyzing the conversion of ammonia to nitrite or nitric oxide and generating reductive power for the autotrophic growth of microorganisms. The ecology, diversity, and properties of ammonia-oxidizing microbes in the ocean's plankton have been extensively studied, but these microbes can also live in association or symbiosis with marine hosts such as sponges, corals, jellyfish, bivalves, and crustaceans. Sequencing-based studies have revealed that ammonia-oxidizing archaea of the family Nitrosopumilaceae are prevalent in various marine hosts, although other ammonia-oxidizing taxa are also found and coexist within the same host. Ammonia oxidation rates are highly variable between host species, even between closely related taxa. Limited knowledge is available on the metabolic interactions that ammonia-oxidizing microbes have, but theoretical considerations indicate that they could make significant contributions to carbon fixation for their hosts. Additionally, ammonia-oxidizing microbes appear to also have undergone specific genomic adaptations to their host environment, and the hosts may also enable ammonia oxidation to occur in habitats where planktonic counterparts might be limited. This review identifies key knowledge gaps and highlights the need for further research to fully understand the ecological significance of symbiotic ammonia oxidation in marine ecosystems.