Accelerating denitrification and mitigating nitrite accumulation by multiple electron transfer pathways between Shewanella oneidensis MR-1 and denitrifying microbial community

Fe/GMP functional nanomaterial enhancing the denitrification efficiency by bi-signal regulation: Electron transfer and microbial community

A novel functional nanomaterial composed of guanosine monophosphate (GMP) and Fe enhanced denitrification efficiency by regulating electron transfer and microbial community. Fe/GMP enhanced nitrate (NO3-) degradation rates by 3.00-fold in serum vial batch experiments, with a rate constant of 17.39 mg/(L·h) in sequencing batch reactor. Fe/GMP-mediated interface promoted the secretion of redox-active substances in the extracellular polymeric substances to enhance the extracellular electron transfer. Specifically, Fe/GMP regulated electron transfer and metabolism activity by dynamic conversion of Fe3+/Fe2+ redox signal. Additionally, enzyme activity assays verified the optimized electron distribution function of Fe/GMP and thus enhanced intracellular electron transfer. High-throughput sequencing confirmed Fe/GMP selectively enriched microorganisms (especially Thauera 50.70 %). The tetraethylammonium stress experiment demonstrated Fe/GMP as an exogenous signaling molecule to restore microbial communication for microbial community regulation. The study proposes a multifaceted synergistic mechanism based on the repeater function of Fe/GMP in denitrification and offers insights for practical applications.

Electric syntrophy-driven modulation of Fe0-dependent microbial denitrification

In natural or engineered anaerobic environments, iron oxidation-driven microbial denitrification plays a critical role in the water or wastewater treatment. Herein, we report a previously unidentified metallic iron (Fe0)-dependent denitrification mode driven by the electro-syntrophic interaction between electroactive microorganism and denitrifier. In a model denitrifying consortium of Shewanella oneidensis and Pseudomonas aeruginosa, we find that P. aeruginosa can accept electrons for nitrate reduction via the constructed electron transfer system of Fe0-S. oneidensis-P. aeruginosa. In the electro-syntrophic consortium, the membrane-bound CymA-OmcA-MtrC protein complexes of S. oneidensis drive the generation, transfer and consumption of electrons, thus enabling modulation of microbial metabolic activity. Specially, using Fe0 as the sole electron donor, S. oneidensis can act as a bio-engine to harvest electrons and conserve energy from Fe0 biocorrosion. Electrons released by S. oneidensis are utilized by P. aeruginosa for accomplishing microbial denitrification. Metatranscriptomics analysis demonstrated that the direct electron cross-feeding process facilitates the expression of genes encoding for denitrification enzymes, intracellular electron transfer proteins, and quorum sensing of P. aeruginosa. The Fe0-dependent electronic syntrophy in this work could provide a metabolic window for the growth of denitrifiers that is a new insight into nitrate removal or global nitrogen cycle.

Removal of dibutyl phthalate (DBP) by bacterial extracellular polymeric substances (EPS) via enzyme catalysis and electron transmission

Phthalic acid esters (PAEs) showed high environmental risk due to the widely existence and toxicity. Microbial-excreted extracellular polymeric substances (EPS) showed potential of degrading organic compounds. In this study, the degradation ability and the mechanisms of EPS from two bacteria (PAEs degrader Gordonia sihwensis; electrochemically active strain Shewanella oneidensis MR-1) were investigated. Results showed that EPS of the two bacteria had different composition of C-type cytochromes, flavins, catalase, and α-glucosidase. The removal of dibutyl phthalate (DBP) by total EPS were 68% of G. sihwensis and 72% for S. oneidensis. For both bacteria, the degradation rates k of EPS were as TB-EPS > LB-EPS > S-EPS. The degradation mechanisms of EPS from the two bacteria showed difference with electrochemical active components mediated electron transmission for S. oneidensis MR-1 and enzymes catalysis for G. sihwensis. Results of this study illustrated the variation of the contribution of active components of EPS to degradation.

Redox mediator chlorophyll accelerates low-temperature biological denitrification with responses of extracellular polymers and changes in microbial community composition

Low temperatures limit the denitrification wastewater in activated sludge systems, but this can be mitigated by addition of redox mediators (RMs). Here, the effects of chlorophyll (Chl), 1,2-naphthoquinone-4-sulfonic acid (NQS), humic acid (HA), and riboflavin (RF), each tested at three concentrations, were compared for denitrification performance at low temperature, by monitoring the produced extracellular polymeric substances (EPS), and characterizing microbial communities and their metabolic potential. Chl increased the denitrification rate most, namely 4.12-fold compared to the control, followed by NQS (2.62-fold increase) and HA (1.35-fold increase), but RF had an inhibitory effect. Chl promoted the secretion of tryptophan-like and tyrosine-like proteins in the EPS and aided the conversion of protein from tightly bound EPS into loosely bound EPS, which improved the material transfer efficiency. NQS, HA, and RF also altered the EPS components. The four RMs affected the microbial community structure, whereby both conditionally abundant taxa (CAT) and conditionally rare or abundant taxa (CRAT) were key taxa. Among them, CRAT members interacted most with the other taxa. Chl promoted Flavobacterium enrichment in low-temperature activated sludge systems. In addition, Chl promoted the abundance of nitrate reduction genes narGHI and napAB and of nitrite reduction genes nirKS, norBC, and nosZ. Moreover, Chl increased abundance of genes involved in acetate metabolism and in the TCA cycle, thereby improving carbon source utilization. This study increases our understanding of the enhancement of low-temperature activated sludge by RMs, and demonstrates positive effects, in particular by Chl.

Response of nutrients removal efficiency, enzyme activities and microbial community to current and voltage in a bio-electrical anammox system

The effects of different current intensities and voltage levels on nutrient removal performance and microbial community evolution in a Bio-Electrical Anammox (BEA) membrane bioreactor (MBR) were evaluated. The nitrogen removal efficiency increased with the current intensity within the range of 64-83 mA, but this improvement was limited at the current further increased. The phosphorus removal in the BEA MBR was attributed to the release of Fe2+, which was closely associated with the applied current to the electrodes. Heme c concentration, enzyme activities, and specific anammox activity exhibited a decreasing trend, while the functional denitrification genes showed a positive correlation with rising voltage. The nitrogen removal efficiency of the BEA system initially increased and then decreased with the voltage rose from 1.5V to 3.5V, peaking at 2.0V of 94.02% ± 1.19%. Transmission electron microscopy and flow cytometry results indicated that accelerated cell apoptosis/lysis led to an irreversible collapse of the biological nitrogen removal system at 3.5V. Candidatus Brocadia was the predominant anammox bacteria in the BEA system. In contrast, closely related Candidatus Kuenenia and Chloroflexi bacteria were gradually eliminated in electrolytic environment. The abundances of Proteobacteria-affiliated denitrifiers were increased with the voltage rising since the organic matter released by the cell apoptosis/lysis was accelerated at a high voltage level.