Evidence for Assimilatory Nitrate Reduction as a Previously Overlooked Pathway of Reactive Nitrogen Transformation in Estuarine Suspended Particulate Matter

Abstract

Suspended particulate matter (SPM) contributes to the loss of reactive nitrogen (Nr) in estuarine ecosystems. Although denitrification and anaerobic ammonium oxidation in SPM compensate for the current imbalance of global nitrogen (N) inputs and sinks, it is largely unclear whether other pathways for Nr transformation exist in SPM. Here, we combined stable isotope measurements with metagenomics and metatranscriptomics to verify the occurrence of dissimilatory nitrate reduction to ammonium (DNRA) in the SPM of the Pearl River Estuary (PRE). Surprisingly, the conventional functional genes of DNRA (nirBD) were abundant and highly expressed in SPM, which was inconsistent with a low potential rate. Through taxonomic and comparative genomic analyses, we demonstrated that nitrite reductase (NirBD) in conjunction with assimilatory nitrate reductase (NasA) performed assimilatory nitrate reduction (ANR) in SPM, and diverse alpha-and gamma-proteobacterial lineages were identified as key active heterotrophic ANR bacteria. Moreover, ANR was predicted to have a relative higher occurrence than denitrification and DNRA in a survey of Nr transformation pathways in SPM across the PRE spanning 65 km. Collectively, this study characterizes a previously overlooked pathway of Nr transformation mediated by heterotrophic ANR bacteria in SPM and has important implications for our understanding of N cycling in estuaries.

Current status and emerging frontiers in enzyme engineering: An industrial perspective

“…Screening and identification of novel enzymes are performed by function-based or sequence-based. Hu et al proved that suspended particulate-matter that are present in nitrate-contaminated estuarine river contained genes of dissimilatory nitrate reductase ( nirBD ) using metagenomics libraries [ 96 ]. They were able to prove that diverse alpha-and gamma-proteobacteria that are present in the suspended particulate matters played a crucial role in the conversion of nitrate to ammonia and therefore regarded as key active heterotrophic bacteria.…”

“…They were able to prove that diverse alpha-and gamma-proteobacteria that are present in the suspended particulate matters played a crucial role in the conversion of nitrate to ammonia and therefore regarded as key active heterotrophic bacteria. This conversion helps to balance the ecosystem in the nitrate-contaminated estuarine river [ 96 ]. Finally, metagenomics libraries could also assist to screen newly synthesized Escherichia coli that are assembled from different DNA gene circuits which are comprised of transporters, promoters with terminators.…”

Thiomicrorhabdus marina sp.nov., an obligate chemolithoautotroph isolated from tidal zone sediment, and genome insight into the genus Thiomicrorhabdus

“…In a previous study, it was found that nirBDencoded nitrite reductase can not only catalyze fermentation-modedependent DNRA but also regulate ANR (Luque-Almagro et al, 2011). Nitrite reductase (NirBD) and assimilatory nitrate reductase (NasA) can work together to enable ANR to proceed properly (Hu et al, 2022). In the microbial nitrogen cycle, ANR is a form of anabolic metabolism in which nitrate is first reduced to nitrite by NasA, then nitrite is converted to ammonia (Reitzer, 2003).…”

“…The reactive nitrogen transformation pathway via ANR sometimes occurs at more than the DNRA and denitrification (Hu et al, 2022).…”

In Situ/Operando Methods for Understanding Electrocatalytic Nitrate Reduction Reaction

“…[25][26][27][28] Gastrointestinal microorganisms reduce nitrate to nitrite, and oxidize ferrous ions in hemoglobin to ferric ions, thus rendering the oxygen transport mechanism of red blood cells ineffective. [29][30][31][32] Impaired oxygen transport can lead to cyanosis, fatigue, shortness of breath, cerebral hypoxia, and even death. [33][34][35][36][37] Therefore, it is necessary to develop some efficient methods to treat nitrate to reduce the impact on human beings.…”