Tolerance of the Marine Anammox Candidatus Scalindua to High Nitrate Concentrations: Implications for Recirculating Aquaculture Systems

Recirculating aquaculture systems (RAS) hold significant potential for sustainable aquaculture by providing a stable, controlled environment that supports optimal fish growth and welfare. In RAS, ammonium (NH4+) is biologically converted into nitrate (NO3−) via nitrite (NO2−) by nitrifying bacteria. As a result, NO3− usually accumulates in RAS and must subsequently be removed through denitrification in full RAS, or by regular water exchanges in partial RAS. The marine anammox bacteria Candidatus Scalindua can directly convert toxic NH4+ and NO2− into harmless nitrogen gas (N2) and has previously been identified as a promising alternative to the complex denitrification process or unsustainable frequent water exchanges in marine RAS. In this study, we evaluated the impact of high NO3− levels typically encountered in RAS on the performance and abundance of Ca. Scalindua in a laboratory-scale bioreactor. The bacterial composition of the granules, including the relative abundance of key nitrogen-cycling taxa, was analyzed along with the functional profile (i.e., NH4+ and NO2− removal efficiencies). For this purpose, a bioreactor was inoculated and fed a synthetic feed, enriched in NH4+, NO2−, minerals and trace elements until stabilization (Phase 1, 52 days). NO3− concentrations were then gradually increased to 400 mg·L−1 NO3−-N (Phase 2, 52 days), after which the reactor was followed for another 262 days (Phase 3). The reactor maintained high removal efficiencies; 88.0 ± 8.6% for NH4+ and 97.4 ± 1.7% for NO2− in Phase 2, and 95.0 ± 6.5% for NH4+ and 98.6 ± 2.7% for NO2− in Phase 3. The relative abundance of Ca. Scalindua decreased from 22.7% to 10.2% by the end of Phase 3. This was likely due to slower growth of Ca. Scalindua compared to heterotrophic bacteria present in the granule, which could use NO3− as a nitrogen source. Fluorescence in situ hybridization confirmed the presence of a stable population of Ca. Scalindua, which maintained high and stable NH4+ and NO2− removal efficiencies. These findings support the potential of Ca. Scalindua as an alternative filtering technology in marine RAS. Future studies should investigate pilot-scale applications under real-world conditions.