Isabel Rigutto (Radboud University)

Summary of presentation:
Coastal ecosystems experience increased nitrogen-eutrophication due to anthropogenic activities, which can lead to deoxygenation and the accumulation of reduced compounds such as ammonium, methane and sulfide. This can impact the nitrogen cycle, in which the extremely potent greenhouse gas nitrous oxide (N₂O) is an intermediate. We therefore explored the effect of oxygen and sulfide on microbial N₂O dynamics in coastal surface sediments. Sediments were collected in March and August 2024 from the eutrophic marine Lake Grevelingen, which has oxygenated bottom waters in spring, while in summer its bottom waters are euxinic and sulfide accumulates in the surface sediments. We employed a complementary array of methods, including microsensor and porewater depth profiling, microbial meta-omics analyses, and batch incubations. In spring, the surface sediment was characterized by net N₂O consumption, with Flavobacteriia clade II nosZ sequences exhibiting both the highest relative abundance and transcriptional activity. Batch incubations with ¹⁵N-labeled substrates revealed potential for nitrification and for N₂O production through incomplete denitrification. Yet, in situ N₂O production in spring was likely substrate-limited as oxygen and nitrate (NO₃-) were rapidly depleted in the surface sediments. We assessed the effect of sulfide accumulation in summer on the sedimentary N₂O cycling potential by sulfide additions to batch incubations. N₂O consumption was enhanced at sulfide concentrations up to 1 mM, but 4 mM of sulfide was inhibitory to all steps of denitrification. Combined, this study shows that eutrophic coastal sediments maintain potential for microbial N₂O production and consumption during oxygen and nitrate/nitrite (NOx) limitation, and at sulfide concentrations in the low mM range. Hence, restoration of eutrophic ecosystems can stimulate N₂O production through increased availability of oxygen and NOx, which might impact the source or sink status of the sediment depending on how well N₂O reduction can keep up with the increased N₂O production.