Estuarine and coastal environments are assumed to contribute to nitrous oxide (N 2 O) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying N 2 O production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate N 2 O production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the N 2 O molecule increased due to increasing nitrate concentration, suggesting the changes in N 2 O production pathways. Enhanced N 2 O production under high nitrate concentration was not mediated by bacterial denitrification, but instead was mainly regulated by fungal denitrification. Elevated nitrate concentration increased the contribution of fungal denitrification to N 2 O production by 11−25%, whereas it decreased bacterial N 2 O production by 16−33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13−28% of N 2 O production. Elevated nitrate concentration significantly mediated nirK-type denitrifiers structure and abundance, which are the keystone taxa driving N 2 O production. Collectively, these results suggest that increasing nitrate concentration can shift N 2 O production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced N 2 O production and have widespread implications for N 2 O projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.
In the context of an increasing atmospheric carbon dioxide (CO2) level, acidification of estuarine and coastal waters is greatly exacerbated by land-derived nutrient inputs, coastal upwelling, and complex biogeochemical processes. A deeper understanding of how nitrifiers respond to intensifying acidification is thus crucial to predict the response of estuarine and coastal ecosystems and their contribution to global climate change. Here, we show that acidification can significantly decrease nitrification rate but stimulate generation of byproduct nitrous oxide (N2O) in estuarine and coastal waters. By varying CO2 concentration and pH independently, an expected beneficial effect of elevated CO2 on activity of nitrifiers (“CO2-fertilization” effect) is excluded under acidification. Metatranscriptome data further demonstrate that nitrifiers could significantly up-regulate gene expressions associated with intracellular pH homeostasis to cope with acidification stress. This study highlights the molecular underpinnings of acidification effects on nitrification and associated greenhouse gas N2O emission, and helps predict the response and evolution of estuarine and coastal ecosystems under climate change and human activities.
Hypoxia, defined as dissolved oxygen (DO) in water <2 mg L −1 , has occurred worldwide in estuarine and coastal environments during the past five decades (Breitburg et al., 2018;Zhang et al., 2010). Estuarine and coastal hypoxia not only alters regional biogeochemical processes and also affects biodiversity and fisheries, which have attracted considerable attention globally (
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