Response of Nitrate Processing to Bio-labile Dissolved Organic Matter Supply Under Variable Oxygen Conditions in a Sandy Beach Seepage Face

Abstract

Supply of bio-labile dissolved organic matter (DOM) has been assumed to be a key factor for the intensity of nitrate (NO3–) removal in permeable coastal sediments. In the present study, a series of flow through reactor experiments were conducted using glucose as a N-free bio-labile DOM source to permeable sediments from a sandy beach seepage face to identify its effect on benthic NO3– removal. The results revealed a shift from the dominance of NO3– production to removal processes when NO3– input concentration increased from 10 to 80 μM under oxic conditions. Sediment microbiota information suggests that nitrification (e.g., Nitrosomonas and Nitrososphaera) and denitrification (e.g., Marinobacter and Bacillus) were dominant pathways for benthic NO3– production and removal in the studied sediment. Compared with the active response of sediment microbiota to NO3– additions, the supply of glucose (approximately 300 μM final concentration added) did not significantly change the NO3– removal efficiency under aerobic conditions (dissolved oxygen saturation approximately 100%). Similarly, an insignificant increase of NO3– removal rate after glucose amendment of the circulating water was obtained when dissolved oxygen (DO) saturation decreased to approximately 70% in the input solution. When DO at the input solution was decreased to 30% saturation (sub-oxic conditions), the removal rate of NO3– in the group amended with glucose increased, suggesting that glucose stimulated denitrifiers. These results revealed that NO3– removal relied mainly on the anaerobic environment at particle surfaces, with a dependence on the sedimentary organic matter as an electron supplier under bulk aerobic conditions, while the bio-labile DOM was consumed mainly by aerobic respiration instead of stimulating NO3– reduction. However, the respiration triggered by the over-supply of bio-labile DOM reduced the DO in the porewater, likely depressing the activity of aerobic reactions in the permeable sediment. At this point, the benthic microbiota, especially potential denitrifiers, shifted to anaerobic reactions as the key to support nitrogen metabolism. The glucose amendment benefited NO3– reduction at this point, under sub-oxic conditions.

Exploring feedback mechanisms for nitrogen and organic carbon cycling in tropical coastal zones

“…In cohesive marine sediments, anammox shows a negative correlation with the labile organic carbon content (Rich et al, 2020). In permeable sediments, DO penetration depths that can be orders of magnitude higher than in cohesive sediments, favoring the creation of anoxic microniches thus placing oxic and anoxic DIN processing in close proximity (e.g., Jiang et al, 2021b). The injection of NO 3 into permeable sediment porewaters has been identified to enhance denitrification rates, although porewater DOC content, even the labile fractions, seems not to be the preferred carbon source when SOC is present (e.g., Jiang et al, 2018a;Jiang et al, 2021b).…”

“…Atmosphere: Atmospheric transport is a key fertilization process to support surface ocean carbon fixation (Ibańhez et al, 2022). In addition, aerosols are also a pool for organic carbon, containing substance from terrestrial volatilization (Jiang et al, 2021b) and emission of sea spray driven by waves (Rinaldi et al, 2020). Organic fractions could be attached on the surface of colloidal particles, while the residence time is still limited due to the exposure to ultraviolet rays, varying from several hours to weeks.…”

Microbiota for Nitrogen Removal in Wastewater Treatments and Marine Environments: Advocating Communication and Interactive Research

“…Nevertheless, N accumulation is still frequently observed in urban and rural areas and a significant quantity of reactive N produced from anthropogenic activities enters into marine systems via surface loadings, submarine groundwater discharge and atmospheric deposition (Jiang et al, 2021a). In marine environments, N excess is rapidly removed via biogeochemical reactions, such as biological assimilation in the euphotic zone, aerobic/anaerobic denitrification and dissolved oxygen (DO) dependent Anammox in both water parcels and sediments (Jiang et al, 2021b). In fact, the microbial strains for N removal used in wastewater treatments could be frequently observed in marine environments, e.g., Thiosphaera pantotropha in denitrification or Candidatus Brocadia sinica in Anammox.…”