Effective Nitrogen Removal from Aquaculture Wastewater by the Euryhaline Aerobic Denitrifier <i>Marinobacter</i> sp. MAD1

Nitrate accumulation is a prevalent challenge in aquaculture systems. Although aerobic denitrifiers offer a promising solution, few strains can tolerate varying salinities, and their performance in real aquaculture wastewater remains limited. In this study, an euryhaline aerobic denitrifying bacterium, Marinobacter sp. MAD1, was isolated from a recirculating aquaculture system. MAD1 demonstrated both ammonium assimilation and aerobic denitrification capacities, functioning effectively over a broad range of salinities (0–3.5%). Transcriptomic analysis suggests that strain MAD1 upregulates genes involved in K + uptake and sulfur-containing amino acid metabolism, such as cysteine, to maintain cell growth and aerobic denitrification under hyposaline conditions. However, in seawater conditions, strain MAD1 survival relies on classical compatible solute accumulation. Bioaugmentation with culture MAD1 significantly improved nitrate removal compared to the control reactor under moderate (1.5%) and high salinity (3.5%), lowering effluent nitrate from 21.76 ± 3.14 to 3.58 ± 2.57 mg-N/L and 15.55 ± 2.69 to 4.51 ± 2.38 mg-N/L, respectively. Repeated amendment of strain MAD1 led to its predominance within the community, with relative abundances reaching 88.03% at 1.5% salinity and 81.09% at 3.5% salinity. Our findings highlight the potential of euryhaline aerobic denitrifiers for nitrogen management in aquaculture systems under variable salinity.