Micolucci, Federico

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.

Recirculating aquaculture systems (RAS) are promising candidates for the sustainable development of the aquaculture industry. A current limitation of RAS is the production and potential accumulation of nitrogenous wastes, ammonium (NH4+), nitrite (NO2−) and nitrate (NO3−), which could affect fish health and welfare. In a previous experiment, we have demonstrated that the marine anammox bacteria Candidatus Scalindua was a promising candidate to treat the wastewater (WW) of marine, cold-water RAS. However, the activity of the bacteria was negatively impacted after a direct exposure to RAS WW. In the current study, we have further investigated the potential of Ca. Scalindua to treat marine RAS WW in a three-phase experiment. In the first phase (control, 83 days), Ca. Scalindua was fed a synthetic feed, enriched in NH4+, NO2− and trace element (TE) mix. Removal rates of 98.9% and 99.6% for NH4+ and NO2−, respectively, were achieved. In the second phase (116 days), we gradually increased the exposure of Ca. Scalindua to nitrogen-enriched RAS WW over a period of about 80 days. In the last phase (79 days), we investigated the needs of TE supplementation for the Ca. Scalindua after they were fully acclimated to 100% RAS WW. Our results show that the gradual exposure of Ca. Scalindua resulted in a successful acclimation to 100% RAS WW, with maintained high removal rates of both NH4+ and NO2− throughout the experiment. Despite a slight decrease in relative abundance (from 21.4% to 16.7%), Ca. Scalindua remained the dominant species in the granules throughout the whole experiment. We conclude that Ca. Scalindua can be successfully used to treat marine RAS WW, without the addition of TE, once given enough time to acclimate to its new substrate. Future studies need to determine the specific needs for optimal RAS WW treatment by Ca. Scalindua at pilot scale.