The role of coupled DNRA-Anammox during nitrate removal in a highly saline lake

N Valiente,F Jirsa,T Hein,W Wanek,J Prommer,P Bonin,J.J Gómez-Alday

doi:10.1016/j.scitotenv.2021.150726

Abstract

Nitrate (NO3−) removal from aquatic ecosystems involves several microbially mediated processes, including denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox), controlled by slight changes in environmental gradients. In addition, some of these processes (i.e. denitrification) may involve the production of undesirable compounds such as nitrous oxide (N2O), an important greenhouse gas. Saline lakes are prone to the accumulation of anthropogenic contaminants, making them highly vulnerable environments to NO3− pollution. The aim of this paper was to investigate the effect of light and oxygen on the different NO3− removal pathways under highly saline conditions. For this purpose, mesocosm experiments were performed using lacustrine, undisturbed, organic-rich sediments from the Pétrola Lake (Spain), a highly saline waterbody subject to anthropogenic NO3− pollution. The revised 15N-isotope pairing technique (15N-IPT) was used to determine NO3− sink processes. Our results demonstrate for the first time the coexistence of denitrification, DNRA, and anammox processes in a highly saline lake, and how their contribution was determined by environmental conditions (oxygen and light). DNRA, and especially denitrification to N2O, were the dominant nitrogen (N) removal pathways when oxygen and/or light were present (up to 82%). In contrast, anoxia and darkness promoted NO3− reduction by DNRA (52%), combined with N loss by anammox (28%). Our results highlight the role of coupled DNRA-anammox, which has not yet been investigated in lacustrine sediments. We conclude that anoxia and darkness favored DNRA and anammox processes over denitrification and therefore to restrict N2O emissions to the atmosphere.

Highlights

Nitrogen (N) is an essential component of all living organisms and its availability controls the function of aquatic ecosystems
The contribution of anammox to total N loss ranged from 8% (OD) to 28% (AD) (Fig. 2). This range corresponds with studies performed in continental shelf sediments (Song et al, 2013) (28%), intertidal sediments (Hsu and Kao, 2013) (12%), and is close to the global mean value, including inland waters (Trimmer and Engström, 2011) (23%). These findings provide a better understanding of the contribution of dissimilatory nitrate reduction to ammonium (DNRA) and anammox to inorganic N removal in inland waters in general, and in particular for saline lakes
Our findings provide the first evidence for the coexistence of denitrification, DNRA, and anammox in a highly saline lake

Summary

Introduction

Nitrogen (N) is an essential component of all living organisms and its availability controls the function of aquatic ecosystems. Microbial processes are controlling the Earth's N cycle for ~2.7 billion years. The microbial transformation of dissolved inorganic N to gaseous N forms is a pivotal sink that regulates the flux of N into the biosphere, being able to mitigate the effects of excessive anthropogenic inputs. Microbial processes in the inorganic N cycle have been widely studied in aquatic ecosystems, in both, water and sediments. Among inorganic N species, nitrate (NO3−) is a widespread compound, being responsible for water degradation due to excessive fertilizer use in agriculture (Spalding and Exner, 1993). NO3− accumulation can increase primary production in surface waters and, as a consequence, can trigger oxygen deficiency and promote eutrophication of surface waterbodies (Vitousek et al, 1997)

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