Exploration of the metabolic flexibility of glycogen accumulating organisms through metatranscriptome analysis and metabolic characterization

Microplastics enhance the denitrification of glycogen-accumulating organisms by regulating electronic transport in carbon-nitrogen coupling

The increasing presence of microplastics (MPs) in wastewater treatment systems profoundly impacts microbial metabolism and process performance. However, the effects of MPs on the denitrification process of glycogen-accumulating organisms (GAOs) remain unclear. Herein, various types and concentrations of MPs were introduced into the activate sludge of GAOs to assess their impact on denitrification processes and to investigate the underlying mechanisms. Our findings revealed that adding 100 μm PVC increased the denitrification efficiency of GAOs by 14.6 %, whereas adding 100 nm PVC decreased efficiency by 8.4 %. Additionally, 100 nm PVC inhibited polyhydroxybutyrate (PHB) degradation, while 100 μm PVC promoted it. Furthermore, 100 nm and 100 μm PVC differently influenced metabolic functions, including reactive oxygen species (ROS) levels, electron transport chain (ETC) activity, and intracellular nicotinamide adenine dinucleotide (NADH) content. Metatranscriptome analyses revealed differential expression of genes such as phaC, CS, nuoL, CYC1, and nisK, which are involved in carbon-nitrogen metabolism and oxidative phosphorylation. Consequently, 100 μm PVC enhanced the denitrification rate in GAOs by promoting PHB decomposition, increasing NADH electron-donating capacity, and ultimately enhancing the denitrification rate of GAOs. Our findings reveal a novel mechanism on regulating the carbon-nitrogen coupling in activated sludge under the different particle size of MPs.

Study on the mechanisms of efficient phosphorus recovery by a pilot-scale biofilm sequencing batch reactor under low carbon demand

To study the mechanism of a novel pilot-scale biofilm sequencing batch reactor (PS-BSBR) for efficient phosphorus recovery under low carbon demand. The phosphate uptake/release performance and carbon source utilization efficiency of PS-BSBR and a typical enhanced biological phosphate removal (EBPR) -A2O process were compared, and the detection methods of different phosphorus forms were improved. The results showed that phosphate uptake/release content of PS-BSBR were 3.07 times and 4.47 times of that of A2O process under high carbon source utilization efficiency, respectively. The PS-BSBR mainly used inorganic phosphorus (IP) in the form of non-apatite inorganic phosphorus (NAIP) in EPS (85-90%), which was dependent on the adsorption of biologically induced extracellular polymers (EPS). The A2O process was mainly based on the IP in the form of NAIP (60-70%) in the cell for phosphate uptake and release, that was, relying on the biological phosphorus metabolism in the cell of polyphosphate-accumulating organisms (PAOs). Macroomics sequencing revealed that PS-BSBR had a variety of PAOs and a high-abundance glycogen-accumulating organisms (GAOs). By up-regulating the expression of key genes related to cellular phosphorus metabolism and EPS secretion, PS-BSBR promoted the phosphorus metabolism of PAOs cells and the biologically induced phosphate adsorption and desorption, which were dominated by the synthesis and decomposition of EPS. Therefore, the phosphorus absorption and release performance of PS-BSBR process was significantly better than that of A2O process. This study could provide theoretical support and regulatory guidance for the application of PS-BSBR process in sewage phosphorus recovery under the consumption of low carbon sources.

Glycogen-accumulating organisms promote phosphate recovery from wastewater by pilot-scale biofilm sequencing batch reactor: Performance and mechanism

A high phosphate (P) recovery concentration was achieved in pilot-scale biofilm sequencing batch reactor (BSBR) with a low carbon source (C) cost. Especially, a high-abundance glycogen-accumulating organisms (GAOs) (13.93-31.72%) was detected that was accompanied by a high P recovery concentration of BSBR. High-abundance GAOs obtain additional C through various C compensation pathways (split tricarboxylic acid cycle (TCA cycle), glyoxylate shunt and gluconeogenesis), thus reducing the need to compete with polyphosphate-accumulating organisms (PAOs) for C and weakening the adverse effects on P recovery by PAO cells. Under the action of N-acyl homoserine lactones (AHLs)-mediated quorum sensing (QS), GAOs promoted the secretion of a large amount of extracellular polymeric substances (EPS), which helped to realize the P recovery of EPS-dominated biofilms (68.02%-96.89%). This study provides a low-carbon technology for the recovery of high concentration P from municipal wastewater, and improves the ecological theory of P recovery in collaboration with GAOs and PAOs.

Responses of SNEDPR-AGS system under long-term exposure of polyethylene terephthalate microplastics for treating low C/N wastewater: Granular effect and microbial structure

The removal of nutrients in sewage treatment plants can be significantly impacted by carbon limitations, especially for treating low carbon to nitrogen ratio (C/N) wastewater, which can markedly increase operational costs. Simultaneous nitrification, endogenous denitrification, and phosphorus removal combined with aerobic granular sludge (SNEDPR-AGS) has emerged as one of the optimal processes for treating low C/N wastewater owing to its high carbon utilization efficiency; however, the long-term effect of microplastics (MPs) on this system remains unclear. This study investigated the granular effect and microbial response of an SNEDPR-AGS system for treating low C/N wastewater under long-term exposure (180 d) to polyethylene terephthalate microplastics (PET-MPs). The results showed that the integrity of the AGS structure was disrupted significantly as the PET-MP concentration increased, with clear AGS cracks appearing on days 180, 124, and 74 after exposure to 1, 10, and 100 mg/L of PET-MPs, respectively. Additionally, the addition of PET-MPs also inhibited denitrification and phosphorus removal due to a decrease in the relative abundance of functional genes (napAB, nirK/nirS, ppk1, ppk2, and ppx). Notably, both chemometric and high-throughput sequencing results indicated that the metabolic form of the system would shift from a polyphosphate-accumulating metabolism to a glycogen-accumulating metabolism. The reason may be that PET-MP stress inhibited the relative abundance of functional genes related to carbon, glycogen, phosphorus, and energy metabolism pathways in Candidatus Accumulibacter and Dechloromonas, but promoted their relative abundance of Candidatus Competibacter. Flow cytometry and molecular docking simulations have also demonstrated the direct toxic effects of PET-MPs on the SNEDPR-AGS system. The biological enhancement and functional recovery of damaged SNEDPR-AGS systems must be further investigated in future studies.

Synergistic Integration of Anammox and Endogenous Denitrification Processes for the Simultaneous Carbon, Nitrogen, and Phosphorus Removal

The feasibility of a synergistic endogenous partial denitrification-phosphorus removal coupled anammox (SEPD-PR/A) system was investigated in a modified anaerobic baffled reactor (mABR) for synchronous carbon, nitrogen, and phosphorus removal. The mABR comprising four identical compartments (i.e., C1-C4) was inoculated with precultured denitrifying glycogen-accumulating organisms (DGAOs), denitrifying polyphosphate-accumulating organisms, and anammox bacteria. After 136 days of operation, the chemical oxygen demand (COD), total nitrogen, and phosphorus removal efficiencies reached 88.6 ± 1.0, 97.2 ± 1.5, and 89.1 ± 4.2%, respectively. Network-based analysis revealed that the biofilmed community demonstrated stable nutrient removal performance under oligotrophic conditions in C4. The metagenome-assembled genomes (MAGs) such as MAG106, MAG127, MAG52, and MAG37 annotated as denitrifying phosphorus-accumulating organisms (DPAOs) and MAG146 as a DGAO were dominated in C1 and C2 and contributed to 89.2% of COD consumption. MAG54 and MAG16 annotated as Candidatus_Brocadia (total relative abundance of 16.5% in C3 and 4.3% in C4) were responsible for 74.4% of the total nitrogen removal through the anammox-mediated pathway. Functional gene analysis based on metagenomic sequencing confirmed that different compartments of the mABR were capable of performing distinct functions with specific advantageous microbial groups, facilitating targeted nutrient removal. Additionally, under oligotrophic conditions, the activity of the anammox bacteria-related genes of hzs was higher compared to that of hdh. Thus, an innovative method for the treatment of low-strength municipal and nitrate-containing wastewaters without aeration was presented, mediated by an anammox process with less land area and excellent quality effluent.

Cultivation of phosphate-accumulating biofilm: Study of the effects of acyl-homoserine lactones (AHLs) and cyclic dimeric guanosine monophosphate (c-di-GMP) on the formation of biofilm and the enhancement of phosphate metabolism capacity

This study investigated the mechanisms of microbial growth and metabolism during biofilm cultivation in the biofilm sequencing batch reactor (BSBR) process for phosphate (P) enrichment. The results showed that the sludge discharge was key to biofilm growth, as it terminated the competition for carbon (C) source between the nascent biofilm and the activated sludge. For the tested reactor, after the sludge discharge on 18 d, P metabolism and C source utilization improved significantly, and the biofilm grew rapidly. The P concentration of the recovery liquid reached up to 157.08 mg/L, which was sufficient for further P recovery via mineralization. Meta-omics methods were used to analyze metabolic pathways and functional genes in microbial growth during biofilm cultivation. It appeared that the sludge discharge activated the key genes of P metabolism and inhibited the key genes of C metabolism, which strengthened the polyphosphate-accumulating metabolism (PAM) as a result. The sludge discharge not only changed the types of polyphosphate-accumulating organisms (PAOs) but also promoted the growth of dominant PAOs. Before the sludge discharge, the necessary metabolic abilities that were spread among different microorganisms gradually concentrated into a small number of PAOs, and after the sludge discharge, they further concentrated into Candidatus_Contendobacter (P3) and Candidatus_Accumulibacter (P17). The messenger molecule C-di-GMP, produced mostly by P3 and P17, facilitated P enrichment by regulating cellular P and C metabolism. The glycogen-accumulating organism (GAO) Candidatus_Competibacter secreted N-Acyl homoserine lactones (AHLs), which stimulated the secretion of protein in extracellular polymeric substances (EPS), thus promoting the adhesion of microorganisms to biofilm and improving P metabolism via EPS-based P adsorption. Under the combined action of the dominant GAOs and PAOs, AHLs and C-di-GMP mediated QS to promote biofilm development and P enrichment. The research provides theoretical support for the cultivation of biofilm and its wider application.

Mechanisms of endogenous and exogenous partial denitrification in response to different carbon/nitrogen ratios: Transcript levels, nitrous oxide production, electron transport

Nitrite as an important substrate for Anammox can be provided by partial denitrification (PD). In this study, endogenous partial denitrification (EdPD) and exogenous partial denitrification (ExPD) sludge were domesticated and their nitrite transformation rate reached 74.4% and 83.4%, respectively. The impact of four carbon/nitrogen (C/N) ratios (1.5, 3.0, 5.0 and 6.0) on nitrous oxide (N2O) emission and denitrification functional genes expression in both PD systems were investigated. Results showed that elevated C/N ratios enhanced most denitrification genes expression, but in EdPD, high nitrite levels suppressed nosZ genes expression (from 9.4% to 1.4%), leading to increased N2O emission (0 to 3.4%). EdPD also exhibited lower electron transfer system activity, resulting in slower nitrogen oxide conversion efficiency and more stable nitrite accumulation compared to ExPD. These findings offer insights for optimizing PD systems under varying water quality conditions.

Metagenomics, metatranscriptomics, and proteomics reveal the metabolic mechanism of biofilm sequencing batch reactor with higher phosphate enrichment capacity under low phosphorus load

The biofilm sequencing batch reactor (BSBR) process has higher phosphate recovery efficiency and enrichment multiple when the phosphorus load is lower, but the mechanism of phosphate enrichment at low phosphorus load remains unclear. In this study, we operated two BSBR operating under low and high phosphorus load (0.012 and 0.032 kg/(m3·d)) respectively, and used metagenomic, metatranscriptomic, and proteomics methods to analyze the community structure of the phosphorus accumulating organisms (PAOs) in the biofilm, the transcription and protein expression of key functional genes and enzymes, and the metabolism of intracellular polymers. Compared with at high phosphorus load, the BSBR at low phosphorus load have different PAOs and fewer types of PAOs, but in both cases the PAOs must have the PHA, PPX, Pst, and acs genes to become dominant. Some key differences in the metabolism of PAOs from the BSBR with different phosphorus load can be identified as follows. When the phosphorus load is low, the adenosine triphosphoric acid (ATP) and NAD(P)H in the anaerobic stage come from the TCA cycle and the second half of the EMP pathway. The key genes that are upregulated include GAPDH, PGK, ENO, ppdk in the EMP pathway, actP in acetate metabolism, phnB in polyhydroxybutyrate (PHB) synthesis, and aceA, mdh, sdhA, and IDH1 in the TCA cycle. In the meantime, the ccr gene in the PHV pathway is inhibited. As a result, the metabolism of the PAOs features low glycogen with high PHB, Pupt, Prel, and low PHV. That is, more ATP and NAD(P)H flow to phosphorus enrichment metabolism, thus allowing the highly efficient enrichment of phosphorus from low concentration phosphate thanks to the higher abundance of PAOs. The current results provide theoretical support and a new technical option for the enrichment and recovery of low concentrations of phosphate from wastewater by the BSBR process.