Acetate Uptake Rate Research Articles

Candidatus Thiothrix phosphatis SCUT-1: A novel polyphosphate-accumulating organism abundant in the enhanced biological phosphorus removal system

A novel coccus Thiothrix-related polyphosphate-accumulating organism (PAO) was enriched in an acetate-fed enhanced biological phosphorus removal (EBPR) system. High EBPR performance was achieved for an extended period (>100 days). A high-quality draft genome (completeness 97.2 %, contamination 3.26 %) was retrieved, representing a novel Thiothrix species (with similarity<93.2 % to known Thiothrix species), and was denoted as 'Candidatus Thiothrix phosphatis SCUT-1'. Its acetate uptake rate (6.20 mmol C/g VSS/h) surpassed most Ca. Accumulibacter and known glycogen-accumulating organisms (GAOs), conferring their predominance in the acetate-fed system. Metatranscriptomic analysis suggested that Ca. Thiothrix phosphatis SCUT-1 employed both low- and high-affinity pathways for acetate activation, and both the conventional (PhaABC) pathway and the fatty acid β-oxidation pathway for PHA synthesis; additionally, a much more efficient FAD-dependent malate: quinone oxidoreductase (MQO) were encoded and employed than the traditional malate dehydrogenase (MDH) to oxidize malate to oxaloacetate in the TCA and glyoxylate cycle, collectively contributing to a higher acetate utilization and processing rate of this microorganism. Batch tests further demonstrated the versatile ability of this PAO in using VFA (acetate, propionate, and butyrate), lactate, amino acids (aspartate and glutamate), and glucose as carbon sources for EBPR, showing a partially overlapped but unique ecological niche of this microorganism comparing to Ca. Accumulibacter and known GAOs. A metabolic model was built for Ca. Thiothrix phosphatis SCUT-1 using the above-mentioned carbon sources for EBPR. Overall, this study represents the first comprehensive characterization of the physiology and metabolic characteristics of representative coccus Thiothrix-related PAOs, which are expected to provide new insights into PAO microbiology in EBPR systems.

Autogenerative high-pressure anaerobic digestion modelling of volatile fatty acids: Effect of pressure variation and substrate composition on volumetric mass transfer coefficients, kinetic parameters, and process performance

Pressurised anaerobic digestion is a valuable process that allows the production of biogas with a high methane content, reducing the energy costs for the biogas upgrading and injection into the distribution grid. This technology has attracted scientific research attention in the last decade, and kinetic models have been formulated. In this work, a modified Anaerobic Model Digestion n.1 of an autogenerative high‑pressure anaerobic digestion of volatile fatty acids in batch configuration is proposed by evaluating the effect of increasing the autogenerative pressure in the reactor on the prediction of the dynamic performances and kinetic parameters. Experimental results from the literature were used for developing, calibrating, and validating the model. Results highlighted the dependence of the mass transfer coefficient of the main component of biogas on the increasing autogenerative pressure: the higher the pressure, the higher the volumetric mass transfer coefficient. At atmospheric pressure, the volumetric mass transfer coefficient is almost equal (about 0.065–0.067 1/s). By increasing the pressure, H2 showed the highest value (0.098 1/s at 20 bar), while for CH4 and CO2, comparable values were assessed (0.085 1/s at 20 bar). Results also highlighted the effect of the pressure variation on acetate, butyrate, and propionate Monod maximum specific uptake rate, half saturation value, the first order decay rate, and the Upper and Lower pH levels for acetate degraders. Moreover, hydrogen produced during the process was considered an additional substrate, for which the kinetic parameters were assessed. Finally, simulation findings are consistent with the literature data in terms of volatile fatty acid consumption.

Effect of biofilm thickness on the activity and community composition of phosphorus accumulating bacteria in a moving bed biofilm reactor

Can biofilms enhance the rates of phosphorus removal in wastewater treatment? In order to narrow the scientific gap on the effect of biofilm thickness on the activity and microbial community of phosphorus-accumulating bacteria, this study investigated biofilms of 30 to 1000 µm thickness in a moving bed biofilm reactor. Measurements on 5 different biofilm carriers showed that biomass-specific phosphorus release and uptake rates increased as a function of biofilm thickness for biofilms thinner than about 110 µm but were lower for thicker biofilms of about 550–1000 µm. The reduced phosphorus uptake and release rates in the thickest biofilms can result from substrate mass transfer limitations whereas the low activity in the thinnest biofilms can be related to a too high turnover rate in the biofilm due to heterotrophic growth. Additionally, the microbial ecology of the different biofilms confirms the observed phosphorus uptake and release rates. The results from the full-length 16S rRNA gene sequencing of the bacterial community showed that the thicker biofilms were characterized by higher relative abundance (40–58%) of potential phosphorus accumulating genera Zoogloea, Acinetobacter, Dechloromonas and Ca. Accumulibacter. In contrast, the thinner biofilms were dominated by the genus Ferribacterium (34–60%), which might be competing with phosphorus-accumulating bacteria as indicated by the relatively high acetate uptake rates in the thinner biofilms. It is concluded that there is an optimal biofilm thickness of 100–500 µm, at which the phosphorus accumulating bacteria have the highest activity.

Development of Hypertolerant Strain of Yarrowia lipolytica Accumulating Succinic Acid Using High Levels of Acetate.

Acetate is emerging as a promising feedstock for biorefineries as it can serve as an alternate carbon source for microbial cell factories. In this study, we expressed acetyl-CoA synthase in Yarrowia lipolytica PSA02004PP, and the recombinant strain grew on acetate as the sole carbon source and accumulated succinic acid or succinate (SA). Unlike traditional feedstocks, acetate is a toxic substrate for microorganisms; therefore, the recombinant strain was further subjected to adaptive laboratory evolution to alleviate toxicity and improve tolerance against acetate. At high acetate concentrations, the adapted strain Y. lipolytica ACS 5.0 grew rapidly and accumulated lipids and SA. Bioreactor cultivation of ACS 5.0 with 22.5 g/L acetate in a batch mode resulted in a maximum cell OD600 of 9.2, with lipid and SA accumulation being 0.84 and 5.1 g/L, respectively. However, its fed-batch cultivation yielded a cell OD600 of 23.5, SA titer of 6.5 g/L, and lipid production of 1.5 g/L with an acetate uptake rate of 0.2 g/L h, about 2.86 times higher than the parent strain. Cofermentation of acetate and glucose significantly enhanced the SA titer and lipid accumulation to 12.2 and 1.8 g/L, respectively, with marginal increment in cell growth (OD600: 26.7). Furthermore, metabolic flux analysis has drawn insights into utilizing acetate for the production of metabolites that are downstream to acetyl-CoA. To the best of our knowledge, this is the first report on SA production from acetate by Y. lipolytica and demonstrates a path for direct valorization of sugar-rich biomass hydrolysates with elevated acetate levels to SA.

Mutants lacking global regulators, fis and arcA, in Escherichia coli enhanced growth fitness under acetate metabolism by pathway reprogramming.

Global regulatory transcription factors play a significant role in controlling microbial metabolism under genetic and environmental perturbations. A system-level effect of carbon sources such as acetate on microbial metabolism under disrupted global regulators has not been well established. Acetate is one of the major substrates available in various nutrient niches such as the mammalian gut and a keto diet. A substantial amount of acetate gets secreted in aerobic metabolism. Therefore, investigating the study on acetate metabolism is highly significant. It is known that the global regulatorsfisandarcAregulate acetate uptake genes inE. coliunder glucose conditions. This study deciphered the growth and flux distribution ofE. colitranscription regulatory knockouts Δfis,ΔarcAand double deletion mutant, ΔarcAΔfisunder acetate using 13C-metabolic flux analysis (MFA), which has not been investigated before. We observed that the mutants exhibited an expeditious growth rate (~ 1.2-1.6-fold) with a proportionate increase in acetate uptake rates compared to the wild type. 13C-MFA displayed the distinct metabolic reprogramming of intracellular fluxes via the TCA cycle, anaplerotic pathway and gluconeogenesis, which conferred an advantage of a faster growth rate with better carbon usage in all the mutants. This resulted in higher metabolic fluxes through the TCA cycle (~ 18-90%), lower gluconeogenesis (~ 15-35%) and higher CO2 and ATP production with the proportional increase in growth rate. The study reveals a novel insight by stating the sub-optimality of the wild-type strain grown under acetate substrate aerobically. These mutant strains efficiently oxidize acetate, thus acting as potential candidates for the biosynthesis of isoprenoids, biofuels, vitamins and various pharmaceutical products.Key Points• Mutants exhibited a better balance between energy and precursor synthesis than WT.• Leveraged in the unravelling of regulatory control under various nutrient shifts.• Metabolic readjustment resulted in optimal biomass requirement and faster growth.

Behind the Scenes: Mechanisms Regulating Climatic Patterns of Dissolved Organic Carbon Uptake in Headwater Streams

AbstractLarge variability in dissolved organic carbon (DOC) uptake rates has been reported for headwater streams, but the causes of this variability are still not well understood. Here we assessed acetate uptake rates across 11 European streams comprising different ecoregions by using whole‐reach pulse acetate additions. We evaluated the main climatic and biogeochemical drivers of acetate uptake during two seasonal periods. Our results show a minor influence of sampling periods but a strong effect of climate and dissolved organic matter (DOM) composition on acetate uptake. In particular, mean annual precipitation explained half of the variability of the acetate uptake velocities (VfAcetate) across streams. Temperate streams presented the lowestVfAcetate, together with humic‐like DOM and the highest stream respiration rates. In contrast, higherVfAcetatewere found in semiarid streams, with protein‐like DOM, indicating a dominance of reactive, labile compounds. This, together with lower stream respiration rates and molar ratios of DOC to nitrate, suggests a strong C limitation in semiarid streams, likely due to reduced inputs from the catchment. Overall, this study highlights the interplay of climate and DOM composition and its relevance to understand the biogeochemical mechanisms controlling DOC uptake in streams.

Enhanced isobutanol production from acetate by combinatorial overexpression of acetyl-CoA synthetase and anaplerotic enzymes in engineered Escherichia coli.

Acetic acid is an abundant material that can be used as a carbon source by microorganisms. Despite its abundance, its toxicity and low energy content make it hard to utilize as a sole carbon source for biochemical production. To increase acetate utilization and isobutanol production with engineered Escherichia coli, the feasibility of utilizing acetate and metabolic engineering was investigated. The expression of acs, pckA, and maeB increased isobutanol production by up to 26%, and the addition of TCA cycle intermediates indicated that the intermediates can enhance isobutanol production. For isobutanol production from acetate, acetate uptake rates and the NADPH pool were not limiting factors compared to glucose as a carbon source. This work represents the first approach to produce isobutanol from acetate with pyruvate flux optimization to extend the applicability of acetate. This technique suggests a strategy for biochemical production utilizing acetate as the sole carbon source.

Understanding the impact of the cofactor swapping of isocitrate dehydrogenase over the growth phenotype of Escherichia coli on acetate by using constraint-based modeling

It has been proposed that NADP+-specificity of isocitrate dehydrogenase (ICDH) evolved as an adaptation of microorganisms to grow on acetate as the sole source of carbon and energy. In Escherichia coli, changing the cofactor specificity of ICDH from NADP+ to NAD+ (cofactor swapping) decreases the growth rate on acetate. However, the metabolic basis of this phenotype has not been analyzed. In this work, we used constraint-based modeling to investigate the effect of the cofactor swapping of ICDH in terms of energy production, response of alternative sources of NADPH, and partitioning of fluxes between ICDH and isocitrate lyase (ICL) -a crucial bifurcation when the bacterium grows on acetate-. We generated E. coli strains expressing NAD+-specific ICDH instead of the native enzyme, and bearing the deletion of the NADPH-producing transhydrogenase PntAB. We measured their growth rate and acetate uptake rate, modeled the distribution of metabolic fluxes by Flux Balance Analysis (FBA), and quantified the specific activities of NADPH-producing dehydrogenases in central pathways. The cofactor swapping of ICDH led to one-third decrease in biomass yield, irrespective of the presence of PntAB. According to our simulations, the diminution in growth rate observed upon cofactor swapping could be explained by one-half decrease in the total production of NADPH and a lower availability of carbon for biosynthesis because of a change in the partition at the isocitrate bifurcation. Together with an increased total ATP production, this scenario resulted in a 10-fold increment in the flux of ATP not used for growing purposes. PntAB was identified as the primary NADPH balancing response, with the dehydrogenases of the oxidative branch of the Pentose Phosphate Pathway and the malic enzyme playing a role in its absence. We propose that in the context of E. coli growing on acetate, the NADP+-specificity of ICDH is a trait that impacts not only NADPH production, but also the efficient allocation of carbon and energy.

A model-based tool for reactor configuration of thermophilic biogas plants fed with Waste Activated Sludge

The aim of this study consists in the individuation of the most appropriate configuration for the thermophilic anaerobic digestion of Waste Activated Sludge (WAS); to this purpose, the Anaerobic Digestion Model n°1 (ADM1) was adopted. To enhance the energy production, the sludge underwent a thermal pre-treatment. Semi-continuous tests were performed for 281 days, both with raw and pre-treated WAS. The disintegration constant (k_dis) and the maximum acetate uptake rate (km_ac), were calibrated; the latter parameter was increased up to 2.2 KgCOD Kg−1COD d−1 by the pre-treatment. To evaluate the performances of different plant configurations, the specific methane yield and digester productivity were estimated for varying Hydraulic Retention Times (HRTs) and substrate concentrations. The pre-treatment impact on the substrate resulted in the 20% enhancement of the specific methane production for plants run with HRT <10 days; the most proper input concentration consisted in 30 g of Volatile Solids (VS) d−1. This procedure enabled to estimate the most appropriate HRT range for a) preventing the biomass washout (6/7 days for lower and 8 days for higher input concentration) and b) maximizing the methane and energy production for unit of digester volume (10/12 days).

Sulfide effects on the anaerobic metabolism of polyphosphate-accumulating organisms

Sulfate rich wastewaters can be generated from industry, use of seawater in urban environments, or by saline water infiltration into the sewerage. Under anaerobic conditions sulfate can be converted to sulfide, which may affect micro-organisms performing biological nutrient removal. The objective of this study was to evaluate the effect of sulfide on the activity of polyphosphate-accumulating organisms (PAO) in the anaerobic stage of the enhanced biological phosphorus removal process (EBPR). In this regard, a highly enriched culture of PAO was exposed in short-term activity tests to a range of sulfide concentrations at different operational pH values. The PAO activity was mainly affected by un-dissociated H2S. The specific acetate uptake rate was inhibited by 50% at around 60mg H2S.L−1. With increasing H2S concentrations, higher phosphate release rate to acetate uptake rate ratios were observed, possibly due to increased energy requirements for cell detoxification. Mathematical expressions were developed, which satisfactorily described the sulfide effects on the acetate uptake rate and phosphate release rate. The results show that, dependent on the pH, EBPR might be negatively affected by total sulfide concentrations exceeding 275mg SO4.L−1 at pH 6.5 or 1200mg SO4.L−1 at pH 7.8mg SO4.L−1 or when freshwater is partially replaced by seawater more than 45% (pH 7.8) or 10% (pH 6.5) used as secondary quality water. The findings of this study imply that sulfide, which is commonly found in different type of wastewaters, affects the anaerobic metabolism of PAO and may play an important role in the process performance of treatment plants treating wastewaters with high sulfide content.

Enrichment of a mixed microbial culture for polyhydroxyalkanoates production: Effect of pH and N and P concentrations.

Polyhydroxyalkanoates (PHA) are biopolymers that can be an alternative against conventional plastics. The study reported herein evaluated the enrichment of a mixed microbial culture (MMC) operated under feast/famine regime and different pHs in a sequencing batch reactor (SBR) using acetate as sole carbon source to produce polyhydroxyalkanoates (PHAs). The enrichment step was evaluated at controlled pH of 7.5 and also without pH control (averaged value of 9.0). The acetate uptake rate (-qS) of both enrichments at the end of the experimental period exhibited similar behaviour being about 0.18CmolAcCmolX-1h-1 and 0.19CmolAcCmolX-1h-1 for SBR-A and SBR-B, respectively. However, the PHA-storing capacity of the biomass enriched without pH control was better, exhibiting a maximum PHA content of 36% (gPHAg-1 VSS) with a PHA production rate (qPHA) of 0.16CmolPHACmolX-1h-1. Batch experiments were performed to evaluate PHA-storing capacity of the enriched culture at different pHs and nutrients concentrations. In the pH experiments (without nutrient limitation), it was found that in the absence of controlled pH, the enriched biomass exhibited a PHA content of 44% gPHAg-1 VSS with -qS and PHA to substrate yield (YPHA/Ac) of 0.57CmolAcCmolX-1h-1 and 0.33CmolPHACmolAc-1, respectively. Regarding the experiments at variable nutrients concentration (pH ranging 8.8 to 9.2), the results indicate that the PHA content in the enriched biomass is significantly higher being around 51% gPHAg-1 VSS under nitrogen limitation. This work demonstrated the feasibility of the enrichment of a MMC with PHA storage ability without pH control. Results also suggest that better PHAs contents and substrate uptake rates are obtained without controlling the pH in the accumulation step. Finally, this work also highlights the importance of understanding the role of nutrients concentration during the accumulation step.

Metabolic Response of "Candidatus Accumulibacter Phosphatis" Clade II C to Changes in Influent P/C Ratio.

The objective of this study was to investigate the ability of a culture highly enriched with the polyphosphate-accumulating organism, “Candidatus Accumulibacter phosphatis” clade IIC, to adjust their metabolism to different phosphate availabilities. For this purpose the biomass was cultivated in a sequencing batch reactor with acetate and exposed to different phosphate/carbon influent ratios during six experimental phases. Activity tests were conducted to determine the anaerobic kinetic and stoichiometric parameters as well as the composition of the microbial community. Increasing influent phosphate concentrations led to increased poly-phosphate content and decreased glycogen content of the biomass. In response to higher biomass poly-phosphate content, the biomass showed higher specific phosphate release rates. Together with the phosphate release rates, acetate uptake rates also increased up to an optimal poly-phosphate/glycogen ratio of 0.3 P-mol/C-mol. At higher poly-phosphate/glycogen ratios (obtained at influent P/C ratios above 0.051 P-mol/C-mol), the acetate uptake rates started to decrease. The stoichiometry of the anaerobic conversions clearly demonstrated a metabolic shift from a glycogen dominated to a poly-phosphate dominated metabolism as the biomass poly-phosphate content increased. FISH and DGGE analyses confirmed that no significant changes occurred in the microbial community, suggesting that the changes in the biomass activity were due to different metabolic behavior, allowing the organisms to proliferate under conditions with fluctuating phosphate levels.

Biomethane potential of food waste: modeling the effects of mild thermal pretreatment and digestion temperature

ABSTRACTIn order to enhance anaerobic biodegradability of food waste (FW), thermal pretreatment was applied. The effectiveness in terms of biodegradability extent and process rate improvement was investigated. To this aim, Biomethane Potential tests were carried out under mesophilic and thermophilic conditions. The IWA anaerobic digestion Model 1 (ADM1), a powerful tool for modeling the anaerobic digestion (AD) of different substrates, was implemented to predict the methane production. Disintegration constant (k_dis) and maximum acetate uptake rate (km_ac) were identified as the most sensitive parameters and were calibrated over the observed methane production. Pretreatment improvement was more evident in enhancing parameters related to the process rate, such as solubilization extent (+153%) and disintegration constant (+18%), rather than increasing substrate biodegradability. Thermophilic conditions proved to be effective in speeding up the whole AD process, since all the kinetics were significantly improved (disintegration rate increased up to fivefold). Furthermore, it was demonstrated that, after k_dis and km_ac calibration, default thermophilic ADM1 parameters can be suitable to model FW digestion.

Low oxygen levels contribute to improve photohydrogen production in mixotrophic non-stressed Chlamydomonas cultures.

BackgroundCurrently, hydrogen fuel is derived mainly from fossil fuels, but there is an increasing interest in clean and sustainable technologies for hydrogen production. In this context, the ability of some photosynthetic microorganisms, particularly cyanobacteria and microalgae, to produce hydrogen is a promising alternative for renewable, clean-energy production. Among a diverse array of photosynthetic microorganisms able to produce hydrogen, the green algae Chlamydomonas reinhardtii is the model organism widely used to study hydrogen production. Despite the well-known fact that acetate-containing medium enhances hydrogen production in this algae, little is known about the precise role of acetate during this process.ResultsWe have examined several physiological aspects related to acetate assimilation in the context of hydrogen production metabolism. Measurements of oxygen and CO2 levels, acetate uptake, and cell growth were performed under different light conditions, and oxygenic regimes. We show that oxygen and light intensity levels control acetate assimilation and modulate hydrogen production. We also demonstrate that the determination of the contribution of the PSII-dependent hydrogen production pathway in mixotrophic cultures, using the photosynthetic inhibitor DCMU, can lead to dissimilar results when used under various oxygenic regimes. The level of inhibition of DCMU in hydrogen production under low light seems to be linked to the acetate uptake rates. Moreover, we highlight the importance of releasing the hydrogen partial pressure to avoid an inherent inhibitory factor on the hydrogen production.ConclusionLow levels of oxygen allow for low acetate uptake rates, and paradoxically, lead to efficient and sustained production of hydrogen. Our data suggest that acetate plays an important role in the hydrogen production process, during non-stressed conditions, other than establishing anaerobiosis, and independent of starch accumulation. Potential metabolic pathways involved in hydrogen production in mixotrophic cultures are discussed. Mixotrophic nutrient-replete cultures under low light are shown to be an alternative for the simultaneous production of hydrogen and biomass.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0341-9) contains supplementary material, which is available to authorized users.

Hydrologic and biogeochemical influences on carbon processing in the riparian zone of a subarctic stream

Small streams and their adjacent riparian zones contribute disproportionately to the processing and transport of C compared to the catchment area and, thereby, may alter the flux of dissolved organic C (DOC) from aquatic ecosystems. We examined the factors influencing C uptake and CO2 production in the riparian zone of a boreal stream. We tested the hypothesis that enhanced water velocity increases the supply of DOC and nutrients to microorganisms and, therefore, enhances DOC mineralization in the riparian zone. We measured in situ DOC mineralization rate in the riparian zone of a headwater stream at locations that varied with respect to the water velocity of shallow ground water. We added acetate as a tracer to measure C uptake in shallow ground water and monitored CO2 production during tracer experiments. We assessed relationships of these measures of C dynamics with the time that the tracer remained in shallow ground water surrounding the injection site (plume residence time), pre-injection DOC concentration, nutrient availability, and aromaticity of dissolved organic matter. Acetate uptake rate varied 2 orders of magnitude from −0.002 to −0.19/min, whereas CO2 production rate varied 5-fold from 0.01 to 0.06/min. Acetate uptake rate was negatively correlated with mean plume residence time, indicating that rapid water velocity increased C decomposition in the riparian zone. Moreover, Damköhler numbers (Da) were generally <1, indicating that substrate uptake was slower than hydrologic transport. Neither acetate nor CO2 rates were correlated with pre-injection riparian groundwater chemistry. Spatial heterogeneity of water velocity in the subsurface has a critical influence on the processing and subsequent export of C and nutrients from the riparian zone to streams.

Metabolic modelling of full-scale enhanced biological phosphorus removal sludge

This study investigates, for the first time, the application of metabolic models incorporating polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs) towards describing the biochemical transformations of full-scale enhanced biological phosphorus removal (EBPR) activated sludge from wastewater treatment plants (WWTPs). For this purpose, it was required to modify previous metabolic models applied to lab-scale systems by incorporating the anaerobic utilisation of the TCA cycle and the aerobic maintenance processes based on sequential utilisation of polyhydroxyalkanoates, followed by glycogen and polyphosphate. The abundance of the PAO and GAO populations quantified by fluorescence in situ hybridisation served as the initial conditions of each biomass fraction, whereby the models were able to describe accurately the experimental data. The kinetic rates were found to change among the four different WWTPs studied or even in the same plant during different seasons, either suggesting the presence of additional PAO or GAO organisms, or varying microbial activities for the same organisms. Nevertheless, these variations in kinetic rates were largely found to be proportional to the difference in acetate uptake rate, suggesting a viable means of calibrating the metabolic model. The application of the metabolic model to full-scale sludge also revealed that different Accumulibacter clades likely possess different acetate uptake mechanisms, as a correlation was observed between the energetic requirement for acetate transport across the cell membrane with the diversity of Accumulibacter present. Using the model as a predictive tool, it was shown that lower acetate concentrations in the feed as well as longer aerobic retention times favour the dominance of the TCA metabolism over glycolysis, which could explain why the anaerobic TCA pathway seems to be more relevant in full-scale WWTPs than in lab-scale systems.

The effect of substrate competition on the metabolism of polyphosphate accumulating organisms (PAOs)

The type of carbon source present in the wastewater is one factor that affects the competition between polyphosphate accumulating organisms (PAO) and glycogen accumulating organisms (GAO) and therefore, the efficiency of the enhanced biological phosphorus removal (EBPR) process. This study investigated the impact of the carbon source composition on the anaerobic and aerobic kinetics of PAOs and the EBPR performance of an 85% PAO enrichment. When both acetate (HAc) and propionate (HPr) were present, propionate was depleted more quickly, with a constant uptake rate of 0.18 ± 0.02 C-mol/(C-mol biomass·h), while the acetate uptake rate decreased with an increase in propionate concentration, due to the substrate competition between acetate and propionate. The metabolic model for PAOs was modified to incorporate the anaerobic substrate competition effect. The aerobic rates for phosphorus (P) uptake, glycogen production and polyhydroxyalkanoates (PHA) degradation were within the same range for all tests, indicating that these rates are essentially independent of the acetate and propionate concentration, simplifying the calibration procedure for metabolic models. The metabolic model applied to describe the anaerobic and aerobic activity agreed well with the experimental data of HAc, HPr, P, PHA and biomass growth. The low glycogen consumption observed suggest that some reducing equivalents were generated anaerobically through the TCA cycle. The results of this work suggest that the propionate uptake kinetics by PAOs can provide them an advantage over GAOs in EBPR systems, even when the propionate fraction of the influent is relatively low.

Impact of salinity on the anaerobic metabolism of phosphate-accumulating organisms (PAO) and glycogen-accumulating organisms (GAO).

The use of saline water as secondary quality water in urban environments for sanitation is a promising alternative towards mitigating fresh water scarcity. However, this alternative will increase the salinity in the wastewater generated that may affect the biological wastewater treatment processes, such as biological phosphorus removal. In addition to the production of saline wastewater by the direct use of saline water in urban environments, saline wastewater is also generated by some industries. Intrusion of saline water into the sewers is another source of salinity entering the wastewater treatment plant. In this study, the short-term effects of salinity on the anaerobic metabolism of phosphate-accumulating organisms (PAO) and glycogen-accumulating organisms (GAO) were investigated to assess the impact of salinity on enhanced biological phosphorus removal. Hereto, PAO and GAO cultures enriched at a relatively low salinity level (0.02 % W/V) were exposed to salinity concentrations of up to 6 % (as NaCl) in anaerobic batch tests. It was demonstrated that both PAO and GAO are affected by higher salinity levels, with PAO being the more sensitive organisms to the increasing salinity. The maximum acetate uptake rate of PAO decreased by 71 % when the salinity increased from 0 to 1 %, while that of GAO decreased by 41 % for the same salinity increase. Regarding the stoichiometry of PAO, a decrease in the P-release/HAc uptake ratio accompanied with an increase in the glycogen consumption/HAc uptake ratio was observed for PAO when the salinity increased from 0 to 2 % salinity, indicating a metabolic shift from a poly-P-dependent to a glycogen-dependent metabolism. The anaerobic maintenance requirements of PAO and GAO increased as the salinity concentrations risen up to 4 % salinity.

Quantifying bacterial biomass fixed onto biological activated carbon (PAC and GAC) used in drinking water treatment

Hybrid processes coupling the use of powder activated carbon (PAC) with membrane filtration for drinking water production are emerging as promising alternatives to conventional technologies due to their enhanced control of dissolved contaminants. The quantification of biomass colonizing PAC is crucial for modeling, designing control strategies and improving the overall performance of these processes. The aim of this study was to examine the applicability of several common methods developed for colonized granular activated carbon (GAC) to PAC. Six analytical methods (based on the measurement of proteins, polysaccharides, heterotrophic plate counts, potential glucose respiration, bacterial ATP and a potential acetate uptake rate, which is proposed herein) were compared. The results showed that the rates of glucose respiration and of acetate consumption could be used interchangeably. Proteins were also an interesting alternative for on-site measurements. It was concluded that biological PAC-based processes sustained a level of heterotrophic activity similar to or greater than that observed in GAC biofilters.

Analysis of Enhanced Current-Generating Mechanism of Geobacter sulfurreducens Strain via Model-Driven Metabolism Simulation

Microbial fuel cells (MFCs) are a class of ideal technologies that function via anaerobic respiration of electricigens, which bring current generation and environmental restoration together. An in-depth understanding of microbial metabolism is of great importance in engineering microbes to further improve their respiration. We employed flux balance analysis and selected Fe(iii) as a substitute for the electrode to simulate current-generating metabolism of Geobacter sulfurreducens PCA with a fixed acetate uptake rate. Simulation results indicated the fluxes of reactions directing acetate towards dissimilation to generate electrons increased under the suboptimal growth condition, resulting in an increase in the respiration rate and a decrease in the growth rate. The results revealed the competitive relationship between oxidative respiration and cell growth during the metabolism of microbe current generation. The results helped us quantitatively understand why microbes growing slowly have the potential to make good use of fuel in MFCs. At the same time, slow growth does not necessarily result in speedy respiration. Alternative respirations may exist under the same growth state due to redundant pathways in the metabolic network. The big difference between the maximum and minimum respiration mainly results from the total formate secretion. With iterative flux variability analysis, a relatively ideal model of variant of G. sulfurreducens PCA was reconstructed by deleting several enzymes in the wild model, which could reach simultaneous suboptimal growth and maximum respiration. Under this ideal condition, flux towards extracellular electron transfer rather than for biosynthesis is beneficial for the conversion of organic matter to electricity without large accumulations of biomass and electricigens may maximize utilization of limited fuel. Our simulations will provide an insight into the enhanced current-generating mechanism and identify theoretical range of respiration rates for guiding strain improvement in MFCs.