Nitrogen removal in marine environments: recent findings and future research challenges

Abstract

International audienceRespiratory reduction of nitrate (denitrification) is recognized as the most important process converting biologically available (fixed) nitrogen to N2. In current N cycle models, a major proportion of global marine denitrification (50–70%) is assumed to take place on the sea floor, particularly in organic rich continental margin sediments. Recent observations indicate that present conceptual views of denitrification and pathways of nitrate reduction and N2 formation are incomplete. Alternative N cycle pathways, particularly in sediments, include anaerobic ammonium oxidation to nitrite, nitrate and N2 by Mn-oxides, and anaerobic ammonium oxidation coupled to nitrite reduction and subsequent N2 mobilization. The discovery of new links and feedback mechanisms between the redox cycles of, e.g., C, N, S, Mn and Fe casts doubt on the present general understanding of the global N cycle. Recent models of the oceanic N budget indicate that total inputs are significantly smaller than estimated fixed N removal. The occurrence of alternative N reaction pathways further exacerbates the apparent imbalance as they introduce additional routes of N removal. In this contribution, we give a brief historical background of the conceptual understanding of N cycling in marine ecosystems, emphasizing pathways of aerobic and anaerobic N mineralization in marine sediments, and the implications of recently recognized metabolic pathways for N removal in marine environments

Seasonal oxygen, nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

“…The wide significance of DNRA in estuarine and coastal sediments has been recognized only very recently (Gardner et al 2006;Dong et al 2011), and it has been suggested that its importance was largely underestimated in previous budgets (Burgin and Hamilton 2007). Finally, anammox (the anaerobic oxidation of ammonium coupled to nitrite reduction) was demonstrated to play a role together with denitrification in the removal of fixed N 2 in coastal zones and estuaries, even though anammox accounts for B30 % of the N 2 production in such ecosystems Hulth et al 2005). However, the relevance of anammox in systems affected by variable degrees of eutrophication and hypoxia is still debated.…”

Denitrification in the water column of the central Baltic Sea

“…The nitrogen part of the eutrophication is to some degree counteracted by removal of fixed nitrogen in the anoxic parts of sediments and water columns (Voss et al, 2005). This removal is known to be mediated by denitrification and/or anammox, which occur only at very low or zero oxygen concentrations (Hulth et al, 2005;Kuypers et al, 2005;Jensen et al, 2008;Thamdrup and Dalsgaard, 2008). Both processes require NO À 3 or NO À 2 (hereafter collectively called NO À x ) as substrate, which may be supplied from either nitrification or from land and in either case NO À…”

Deep Nitrate Deficit Observed in the Highly Oxygenated East/Japan Sea and Its Possible Cause

“…Denitrification occurs under low oxygen conditions in the environment because nitrate is used as an electron acceptor instead of oxygen (Hulth et al 2005;Brandes et al 2007;Naqvi et al 2010). Denitrification removes nitrate from the water deviating the nitrate to phosphate (N/P) ratio to below the traditional Redfield ratio of 16 (Hupe and Karstensen 2000).…”