Effects of particles on potential denitrification in the coastal waters of the Beibu Gulf in China

Although the influence of suspended particulate matter (SPM) on denitrifying activity has been identified in river waters recently through metabolic incubations and community gene analysis, the regulations of SPM to denitrification in marine systems are still poorly understood. In the present study, the effects of suspended particle properties (including concentration, composition and size) on potential denitrification were explored in the coastal Beibu Gulf water columns based on a series of ¹⁵N-labeled incubations under artificial anaerobic condition. A gradient of oxygen (O₂) concentrations was also manipulated in the incubated seawaters to test the sensitivity of denitrification to O₂ exposure. According to our experiments, potential denitrification was the dominant pathway for N₂ production with major contribution (>60%) recovered from the particle-associated (PA) fraction. The Highest rate occurred in the benthic nepheloid layer, where high particle content induced by sediment resuspension were observed, suggesting that resuspended particles may act as a hot spot for marine nitrogen (N) loss. Both content and lability of particulate organic carbon (POC) were tightly related to the denitrification rates, with denitrification enhanced by autochthonous POC fractions more significantly. The PA denitrification was higher on small particles (1.2-10μm) compared to the large ones (>10μm), probably due to larger specific surface area and higher specific POC content in small particles. O₂ suppressed denitrifying activity for both bulk water samples and PA fractions. Although in situ denitrifying activity should be minor or neglected because of high-O₂ inhibition, the novel findings of particle effects on anaerobic denitrification can still be applied to hypoxic marine environments. Our research also implies that resuspended particles from sediment may act as a hot spot for N loss, and therefore to be a first step toward future studies in high particle loaded marine regimes.