A polyhydroxybutyrate-co-3-hydroxyvalerate solid-phase denitrification process for treating nitrate-rich recirculating mariculture systems water: Nitrogen metabolism and salt tolerance mechanism

Mariculture water faces nitrate accumulation challenges, and solid-phase denitrification (SPD) shows great potential in nitrate removal for treating mariculture water. Salinity influences both mariculture organisms and SPD performance. This study investigated the impacts of different salinities (1.5 %, 2.5 %, and 3.5 %) and influent nitrate (NO 3 - -N) concentrations (50, 100, and 200 mg/L) on nitrogen removal performance and microbial communities of SPD process using polyhydroxybutyrate-co-3-hydroxyvalerate (PHBV) as the carbon source. The PHBV-SPD process achieved efficient nitrogen removal at different salinities with nitrate removal volumetric rates ( r V ) ranging from 33.9 to 48.2 mg/(L∙d). Increasing influent NO 3 - -N concentration did not significantly enhance r V , and a zero-order kinetic model effectively described the NO 3 - -N removal in the PHBV-SPD reactors. PHBV degradation was the primary limiting factor for nitrate removal rate, as indicated by relatively low DOC concentrations and the improved performance observed upon ethanol addition to the PHBV-SPD reactors. Salinity significantly affected nitrite accumulation, with higher nitrite concentrations observed at 1.5 % salinity compared to 2.5 % and 3.5 %, particularly under high nitrate concentrations. Halophilic bacteria (e.g., Phenylobacterium , Pseudomonas , and unclassified_o_Rhizobiaceae ) contributed to nitrate removal. Notably, both denitrification and assimilatory nitrate reduction (ANR) play important roles in nitrogen metabolism, since ANR contributed to the salt tolerance mechanism through compatible solutes synthesis and accumulation, particularly glutamate synthesis. This study advanced the understanding of the interplay between nitrogen metabolism and salt tolerance mechanism and provided practical guidance for the application of PHBV-SPD in mariculture water treatment under varying salinities and nitrate concentrations. • Polyhydroxybutyrate-co-3-hydroxyvalerate-based solid-phase denitrification (PHBV-SPD) removed nitrate efficiently. • Salinity slightly affected NO 3 - -N removal but strongly impacted NO 2 - -N accumulation. • Salinities and nitrate concentrations shaped microbial communities in PHBV-SPD. • Denitrification and assimilatory nitrate reduction (ANR) remove nitrate jointly. • High salinity enhances ANR’s role in nitrogen metabolism.