Evidence for manganese-catalyzed nitrogen cycling in salt marsh sediments
Newton, Jennifer Denise
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Fixed nitrogen is important as a nutrient for organic matter formation and as an electron donor (nitrification) and acceptor (denitrification) for energy generation, but it is scarcely available in aquatic systems. Nitrification oxidizes ammonium to nitrite and nitrate. Denitrification uses these fixed species to form dinitrogen gas. The classic understanding of the nitrogen cycle requires dissolved oxygen for nitrification and assumes denitrification reduces nitrate to dinitrogen through various intermediates in anaerobic conditions. The global nitrogen budget is imbalanced with more marine denitrification measrued than previously estimated in the classic nitrogen cycle, suggesting alternative anaerobic nitrification and denitrification pathways exist. One alternative denitrification pathway is anammox, which directly oxidizes ammonium to dinitrogen with nitrite as the electron acceptor. Other alternative pathways for both nitrification and denitrification involve redox metals as catalysts. Manganese-catalyzed anaerobic nitrification and denitrification are thermodynamically favorable at neutral pH. However, experimental evidence for these processes is still lacking. This investigation seeks to uncover evidence of manganese-catalyzed nitrification and denitrification in saltmarsh sediments. Batch reactors with anaerobic sediment slurries from a saltmarsh in coastal Georgia were incubated in the presence and absence of colloidal manganese oxides and isotope-labeled ammonium and nitrate to trace dinitrogen formation. Results show that denitrification is more prominent in the manganese-treated reactors and that the classic denitrification pathway may not be substantial in shallow saltmarsh sediments. These data indicate that anammox and/or manganese-coupled denitrification are major contributors to the removal of fixed nitrogen. Ammonium removal in the manganese-treated reactors is accompanied by a high nitrite production compared to the nitrogen-only treatment, indicating manganese-coupled denitrification exists and/or anammox is promoted in the presence of manganese. Primary productivity is generally high in saltmarshes, but oxygen penetrates less than a few millimeters in the sediment. These observations suggest that oxygenic nitrification does not fuel denitrification below the sediment-water interface. The data show that manganese may play a role in the formation of nitrite and nitrate in oxygen-limited sediments.