Polyhydroxyvalerate Research Articles

Metabolic shifts in glycogen-accumulating organisms: Exploring carbon and phosphorus dynamics under two distinct phosphorus limitation modes

Glycogen-accumulating organisms (GAOs) are recognized as competitors of phosphorus-accumulating organisms (PAOs) in enhanced biological phosphorus removal (EBPR) systems. However, different types of GAOs exhibit unique metabolic traits in different environments. To characterize the metabolic activity of GAOs, two distinct modes were constructed. Long-term operation with the mode of anaerobic post-drainage demonstrated the successful establishment of a rapid and enduring GAOs system dominated by Candidatus Contendobacter community. Acetate was completely consumed within 60 min of the anaerobic phase. Concurrently, glycogen levels decreased by 0.062 g Glycogen/g SS, while intracellular carbon storage of poly-hydroxybutyrate (PHB) and poly-hydroxyvalerate (PHV) increased by 0.027 g PHB/g SS and 0.007 g PHV/g SS, respectively. Various concentrations of phosphorus loads illustrated the resistance of the system to high phosphorus loads, while maintaining glycogen accumulating metabolism (GAM). Under complete phosphorus limitation, no correlation between glycogen and acetate was observed during the initial 140 d. The addition of 1 mg/L phosphate to the influent preserved the typical GAM characteristics. Moreover, the pathway involved in glycogen conversion to trehalose may be pivotal in enhancing anaerobic glycogen degradation. The complete tricarboxylic acid cycle ensured efficient energy production, thereby sustaining the foundational metabolism. Notably, sucC and mdh were critical regulators of energy flux. This study provides a distinctive metabolic profile of a robust and resilient GAOs model with Ca. Contendobacter.

Effect of oxic and anoxic conditions on intracellular storage of polyhydroxyalkanoate and polyphosphate in Magnetospirillum magneticum strain AMB-1.

Magnetotactic bacteria (MTB) are microorganisms widely inhabiting the oxic-anoxic interface of aquatic environments. Beside biomineralizing magnetic nanocrystals, MTBs are able to sequester various chemical elements (e.g., carbon and phosphorus) for the biogenesis of intracellular granules, like polyhydroxyalkanoate (PHA) and polyphosphate (polyP), making them potentially important in biogeochemical cycling. Yet, the environmental controls of intracellular storage of carbon and phosphorus in MTB remain poorly understood. Here, we investigated the influence of oxic, anoxic and transient oxic-anoxic conditions on intracellular storage of PHA and polyP in Magnetospirillum magneticum strain AMB-1. In the incubations with oxygen, transmission electron microscopy revealed intercellular granules highly rich in carbon and phosphorus, which were further interpreted as PHA and polyP based on chemical and Energy-Dispersive X-ray spectroscopy analysis. Oxygen had a strong effect on PHA and polyP storage in AMB-1 cells, as PHA and polyP granules accounted for up to 47 ± 23% and 5.1 ± 1.7% of the cytoplasmic space, respectively, during continuous oxic conditions, while granules disappeared in anoxic incubations. Poly 3-hydroxybutyrate (PHB) and poly 3-hydroxyvalerate (PHV) accounted for 0.59 ± 0.66% and 0.0033 ± 0.0088% of dry cell weight, respectively, in anoxic incubations, while the values increased by a factor of 7 and 37 after oxygen was introduced. The results highlight a tight link between oxygen, carbon and phosphorus metabolisms in MTB, where favorable oxic growth conditions can lead to metabolic induction of polyP and PHA granule biogenesis.

The synthesis of polyhydroxyalkanoates from low carbon wastewater under anaerobic-microaerobic process: effects of pH and nitrogen and phosphorus limitation

Polyhydroxyalkanoate (PHA) is a new type of bio-polyester which is expected to replace traditional petroleum-based plastics. The experiment was carried out based on anaerobic-microaerobic process. Firstly, the PHA accumulation capacity of activated sludge under different pH conditions was discussed, and then the batch test of nitrogen and phosphorus limitation was carried out under the condition of optimal pH. Moreover, the content of PHA monomer under nutrient restriction was also studied. The experimental results showed that when pH was not controlled (7.5~8.5), C:N and C:P weight ratio was equal to 150, the maximum content of PHA accounted for 50.39% and 36.07% of the dry cell weight, respectively. Besides, it was found that increasing the C:N weight ratio was beneficial to increasing the proportion of polyhydroxyvalerate (PHV) monomer in PHA.

Characterization of bioplastics produced by haloarchaeon Haloarcula sp strain NRS20 using cost-effective carbon sources

As good models for developing techniques, Haloarchaea are using as cell factories to produce a considerable concentration of bioplastics, polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), and polyhydroxyvalerate (PHV). In this study, low-cost carbon sources by Sudan Black staining was applied for screening haloarchaea a hypersaline environment (southern coast of Jeddah, Saudi Arabia). The growth of the selected isolate and PHB-production under different carbon sources, temperature, pH values and NaCl concentrations were investigated. The biopolymer was extracted and quantitatively measured. The biopolymer was qualitatively identified by Fourier-transform infra-red analysis (FTIR) and High Performance Liquid Chromatography (HPLC). The potential Haloarcula sp strain NRS20 (MZ520352) could significantly accumulate PHB under nutrient-limiting conditions using different carbon sources including starch, carboxymethyl cellulose (CMC), sucrose, glucose and glycerol with 23.83%, 14%, 11%, 12% and 8% of PHB/CDW respectively under 25% NaCl (w/v), pH 7, at 37 °C. The results of FTIR pattern indicated that the significant peak at 1709.22 cm−1 confirmed the presence of the ester carbonyl-group (C=O) which is typical of PHB. HPLC analysis indicated that produced PHB was detected at 7.5 min with intensity exceeding the standard PHB at 8.0 min. Few potential species of haloarchaea were reported for economical PHB-production, here, Haloarcula sp strain NRS20 showed high content of PHB, exhibited a promising PHB-producer using inexpensive sources of carbon.

Towards PHA Production from Wastes: The Bioconversion Potential of Different Activated Sludge and Food Industry Wastes into VFAs Through Acidogenic Fermentation

Acidogenic fermentation of wastes produces volatile fatty acid (VFA)-rich streams that can be used as low-cost carbon sources for polyhydroxyalkanoate (PHA) production. In this study, an inoculum collected from an anaerobic reactor of a municipal WWTP was conditioned to suppress methanogenic activity. The heat-shock conditioning method of the inoculum proved to be more efficient than acid and alkaline conditioning methods for methanogens inhibition. Then, the pre-conditioned inoculum was used to determine the acidogenic potential of different wastes: three waste activated sludge (WAS) samples generated at different sludge retention times (SRTs, 2, 7 and 14 days), olive mill wastewater (OMW), glycerol, apple pomace (AP) and winterization oil cake (WOC). Batch tests were performed in quintuplicate at 37 °C and pH 7. A higher degree of acidification was observed for high-rate activated sludge (2 days of SRT) (69%), followed by olive mill wastewater (OMW) (43%), while the lowest was for glycerol (16%). The results for the winterization oil cake (WOC) samples interestingly elucidated a high content of propionic acid with a high odd-to-even ratio (0.86) after fermentation. Feeding the VFA profile obtained from WOC into a PHA production system led to a significant production of 0.64 g PHA g−1 C with 30% polyhydrobutyrate (PHB) to 69% polyhydroxyvalerate (PHV) as monomeric units of HB-co-HV, decoupling the need for a related carbon source for co-polymer production.Graphic Abstract

Haloarchaea as Cell Factories to Produce Bioplastics.

Plastic pollution is a worldwide concern causing the death of animals (mainly aquatic fauna) and environmental deterioration. Plastic recycling is, in most cases, difficult or even impossible. For this reason, new research lines are emerging to identify highly biodegradable bioplastics or plastic formulations that are more environmentally friendly than current ones. In this context, microbes, capable of synthesizing bioplastics, were revealed to be good models to design strategies in which microorganisms can be used as cell factories. Recently, special interest has been paid to haloarchaea due to the capability of some species to produce significant concentrations of polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), and polyhydroxyvalerate (PHV) when growing under a specific nutritional status. The growth of those microorganisms at the pilot or industrial scale offers several advantages compared to that of other microbes that are bioplastic producers. This review summarizes the state of the art of bioplastic production and the most recent findings regarding the production of bioplastics by halophilic microorganisms with special emphasis on haloarchaea. Some protocols to produce/analyze bioplastics are highlighted here to shed light on the potential use of haloarchaea at the industrial scale to produce valuable products, thus minimizing environmental pollution by plastics made from petroleum.

Polyhydroxyalkanoate (PHA) production with acid or alkali pretreated sludge acidogenic liquid as carbon source: Substrate metabolism and monomer composition

The organic acid rich acidogenic liquid produced from waste sludge anaerobic fermentation was used as a potential feedstock for polyhydroxyalkanoates (PHA) production, which is a cost-effective and environmentally sustainable PHA production system. To analyze the effect of substrate on monomer composition of PHA, different acidogenic liquid generated from waste sludge fermentation in varied initial pH were used as external carbon source. The maximal PHA content accounted for 60.3 % of the dry cell with initial fermentation pH 10.0 sludge acidogenic liquid and the recovered polymer composition was 98.3 % polyhydroxybutyrate (PHB) and 1.7 % polyhydroxyvalerate (PHV) by mass. The utilization efficiency of soluble chemical oxygen demand (SCOD), protein, NH4+-N and volatile fatty acids (VFAs) was 95.4 %, 90.5 %, 98.0 %, 99.3 %, respectively. The tyrosine-like protein, tryptophan-like protein, fulvic acid-like organics and soluble microbial by-products were primary used for PHA production, while the humic acid-like organics used little. Furthermore, mass balance analysis was introduced to further illustrate the technology reliability.

Topological indices of polyhydroxybutyrate and polycaprolactone

Polyhydroxyalkanoates (PHAs) are the biopolymers that are prepared and synthesized by different microorganisms. Several physical properties of these polymers are similar to petroleum-based plastics like polypropylene, showing potential of these natural polymers to be alternate for petroleum-based plastics. The most common PHAs in different microorganisms are: Polyhydroxybutyrate (PHB), Polyhydroxyvalerate (PHV) and their copolymer (PHBV). In this study we have determined Zagreb, general Randić, atom bond connectivity and geometric arithmetic topological indices for these three polymers, which can be helpful for correlating several physical properties of these polymers with the topological indices.

Potential of waste activated sludge to accumulate polyhydroxyalkanoates and glycogen using industrial wastewater/liquid wastes as substrates.

Recovery of the organics in industrial wastewaters/liquid wastes as polyhydroxyalkanoate (PHA) and/or glycogen (GLG) in waste activated sludge is a useful strategy to not only improve the resource value of waste activated sludge but also reduce the energy and cost of waste disposal and wastewater treatment. This study aimed to evaluate the potential of activated sludge to accumulate PHA and GLG using complex substrates (actual and simulated industrial wastewaters/liquid wastes) in addition to various simple organic substrates (organic acids, saccharides, and glycerol). The 24 h PHA and GLG accumulation experiments resulted in the accumulation of up to 25.5%, 6.0% and 14.1% of polyhydroxybutyrate (PHB), polyhydroxyvalerate (PHV) and GLG, respectively, from simple substrates, and up to 9.8%, 0.1% and 14.6%, respectively, from complex substrates. The results indicated that activated sludge can accumulate PHA and GLG even from complex wastewater substrates, although the accumulated PHA and GLG levels were not sufficiently high. The results also indicated that the PHA and GLG accumulation abilities of activated sludge from complex substrates can be drastically enhanced by a short-term acclimation to the corresponding substrate. This study will present the practical implications for value-added resource production through the combined use of waste activated sludge and industrial wastewaters.

Removal of micropollutants during biological phosphorus removal: Impact of redox conditions in MBBR

Further biological polishing of micropollutants in WWTP effluents is limited by the lack of available carbon for cometabolic degradation. Metabolism of polyhydroxyalkanoates (PHAs) stored intracellularly during enhanced biological phosphorus removal (EBPR) could serve as carbon source for post-denitrification and micropollutant cometabolism.The removal of nine micropollutants (i.e., pharmaceuticals and corrosion inhibitors) was investigated by using Moving Bed Biofilm Reactors (MBBRs), selecting phosphorus (PAO) or glycogen (GAO) accumulating organisms under different redox conditions. Three laboratory-scale MBBRs were operated in sequencing-batch mode under cyclical anaerobic and aerobic/anoxic conditions for phosphorus removal. Batch experiments were performed to evaluate the biodegradation potential of micropollutants along with the utilization of internally stored PHA. Experiments showed that aerobic PAO were able to efficiently remove most of the targeted micropollutants. The removal of benzotriazole, 5‑methyl‑1H‑benzotriazole, carbamazepine, ketoprofen and diclofenac occurred simultaneously to phosphorus uptake and terminated when phosphorus was no longer available. Denitrifying PAO and aerobic GAO exhibited lower removal of micropollutants than aerobic PAO. Degradation profiles of stored PHA suggested a diverse utilization of the different fractions of PHA for phosphorus and micropollutant removal, with PHV (poly 3‑hydroxyvalerate) most likely used for the cometabolism of targeted micropollutants.

Production of polyhydroxyalkanoates by activated sludge: Correlation with extracellular polymeric substances and characteristics of activated sludge

To understand PHAs accumulation process well in the stand point of the sludge characteristics, polyhydroxyalkanoates (PHAs) accumulation by activated sludge was investigated in correlation with sludge properties, such as particle size, mixed liquor suspended solids (MLSS) and extracellular polymeric substances (EPS) under different carbon sources (e.g., acetate, propionate and mixed carbons) at 2, 4, 6 and 8 g/L. The efficiency of PHAs synthesis from sludge with sodium acetate as carbon source was superior to that with sodium propionate as carbon source and mixed carbon sources in the sequencing biological reactors, while EPS and sludge characteristics showed similar trends for each carbon source. Various carbon sources did not induce significant differences in the correlations. Polyhydroxybutyrate (PHB) showed good correlation with MLSS (R = 0.782) and sludge floc size (R = −0.58). In the reactors with carbon sources of propionate and mixed carbons, polyhydroxyvalerate (PHV) and PHBV correlated with MLSS (R = 0.480 and 0.489, respectively), sludge floc size (R = −0.921 and −0.666, respectively) and concentrations of polysaccharides (R = 0.726, 0.557, respectively). According to the correlations, improving the concentration of sludge, promoting the production of extracellular polymeric substances (EPS) and decreasing the sludge floc size were supposed to be effective approaches for further accumulation of the PHAs. This study is the first to systematically establish the correlations among EPS, characteristics of activated sludge and PHAs accumulation, can provide a theoretical foundation for the engineering design and the operational parameters selection of PHAs production by activated sludge.

Removal and Transformation of Pollutants in a Two-Line Denitrifying Phosphorus Removal Process Treating Low C/N Municipal Wastewater: Influence of Hydraulic Retention Time

A two-line denitrifying phosphorus removal process (2L-DPR) was established treating low C/N municipal wastewater efficiently in our previous studies, while hydraulic retention time (HRT) is one of the most important factors determining the substrate loading, contact time for biomass, and pollutants and further affect performance of the whole system. Removal and transformation mechanism of organic carbon (C), nitrogen (N), and phosphorus (P) were investigated together with mass balance under various HRTs (6, 9, and 18 h) in the established 2L-DPR process. The results showed that in anaerobic units, the concentration of the main storage products in activated sludge such as poly-hydroxyvalerate (PHV) and poly-hydroxybutyrate (PHB) at HRT of 9 h was higher than that under other HRTs. The highest TN and TP removal efficiency was also achieved under the HRT of 9 h with removal rates of 55.9% and 84.6% respectively. Increasing HRT from 6 to 9 h greatly enhanced TN removal in anoxic and aerobic units; however, HRTs had little influence on COD removal with effluent concentration of 48.6, 49.1, and 48.9 mg/L, respectively. HRT affected phosphorus up-taken in anoxic and aerobic units rather than on the release of phosphorus processes in anaerobic units.

Analysis and optimization of process parameters for production of polyhydroxyalkanoates along with wastewater treatment by Serratia sp. ISTVKR1

A previously reported biodegrading bacterial strain Serratia sp. ISTVKR1 was studied for polyhydroxyalkanoate (PHA) production along with wastewater contaminant removal. Nile red fluorescence, GC–MS, FT-IR, NMR and TEM confirmed the accumulation of homopolymer poly-3-hydroxyvalerate (PHV) within the bacterial cells. Analysis of culture after 72h of bacterial treatment showed maximum COD removal (8.4-fold), non-detection of organic contaminants such as 1H-Cyclopropa [a] naphthalene (R.T.=10.12) using GC–MS and increased proportion of elements like Cr, Mn, Fe, Ni, Cu, Cd and Pb in the bacterial cell pellets by SEM-EDX analysis. Optimization of process parameters for enhanced PHA production along with wastewater treatment done using Response Surface Methodology (RSM) showed 5% and 0.74% increase in the PHA production (0.3368±0.13gL−1) and % COD reduction (88.93±2.41) of wastewater, respectively. The study, thus established the production of PHA along with wastewater contaminant removal by Serratia sp. ISTVKR1.

Effects of matrix types on formation and transformation of energy-accumulating substances in enhanced biological phosphorus removal (EBPR)

Enhanced biological phosphorus removal (EBPR) has been widely used in wastewater treatment. In this study, a laboratory investigation of activated sludge in A/O-SBR reactor was conducted to probe the effects of the matrix types on EBPR polyphosphate, intracellular polysaccharide, polyhydroxyalkanoates (PHA) formation and transformation. There is a decrease in anaerobic condition and an increase in aerobic condition for the intracellular glycogen of sodium propionate matrix and sodium acetate matrix. While the intracellular glycogen of glucose matrix shows a decreasing tendency in both anaerobic and aerobic reaction process. Sodium acetate matrix is beneficial to the formation of polyhydroxybutyrate (PHB), but the content of PHB is relatively small. PHB and poly-3-hydroxyvalerate (PHV) contents in PHA are quite similar in both anaerobic and aerobic reactions with a PHB/PHV ratio of 0.83-1.45. The synthesis of PHV and PHB is mainly in the initial anaerobic stage (0 h - 1 h). Glucose matrix is helpful to the formation of PHV. The content of polymphosphorus shows an increasing tendency in both anaerobic and aerobic stages, suggesting that glucose matrix acclimation of the reactor favors the formation of polymphosphorus.

Effects of matrix types on formation and transformation of energy-accumulating substances in enhanced biological phosphorus removal (EBPR).

Enhanced biological phosphorus removal (EBPR) has been widely used in wastewater treatment. In this study, a laboratory investigation of activated sludge in A/O-SBR reactor was conducted to probe the effects of the matrix types on EBPR polyphosphate, intracellular polysaccharide, polyhydroxyalkanoates (PHA) formation and transformation. There is a decrease in anaerobic condition and an increase in aerobic condition for the intracellular glycogen of sodium propionate matrix and sodium acetate matrix. While the intracellular glycogen of glucose matrix shows a decreasing tendency in both anaerobic and aerobic reaction process. Sodium acetate matrix is beneficial to the formation of polyhydroxybutyrate (PHB), but the content of PHB is relatively small. PHB and poly-3-hydroxyvalerate (PHV) contents in PHA are quite similar in both anaerobic and aerobic reactions with a PHB/PHV ratio of 0.83-1.45. The synthesis of PHV and PHB is mainly in the initial anaerobic stage (0 h - 1 h). Glucose matrix is helpful to the formation of PHV. The content of polymphosphorus shows an increasing tendency in both anaerobic and aerobic stages, suggesting that glucose matrix acclimation of the reactor favors the formation of polymphosphorus.

Effects of different ratios of glucose to acetate on phosphorus removal and microbial community of enhanced biological phosphorus removal (EBPR) system.

In this study, the effects of different ratios of glucose to acetate on enhanced biological phosphorus removal (EBPR) were investigated with regard to the changes of intercellular polyhydroxyalkanoates (PHAs) and glycogen, as well as microbial community. The experiments were carried out in five sequencing batch reactors (SBRs) fed with glucose and/or acetate as carbon sources at the ratios of 0:100%, 25:75%, 50:50%, 75:25%, and 100:0%. The experimental results showed that a highest phosphorus removal efficiency of 96.3% was obtained with a mixture of glucose and acetate at the ratio of 50:50%, which should be attributed to more glycogen and polyhydroxyvalerate (PHV) transformation in this reactor during the anaerobic condition. PCR-denaturing gradient gel electrophoresis (DGGE) analysis of sludge samples taken from different anaerobic/aerobic (A/O) SBRs revealed that microbial community structures were distinctively different with a low similarity between each other.

Enhanced primary sludge sonication by heat insulation to reclaim carbon source for biological phosphorous removal

Ultrasound pretreatment is a potent step to disintegrate primary sludge (PS). The supernatant of sonicated PS is recycled as an alternative carbon source for biological phosphorus removal. In this study, we investigated the role of temperature on PS disintegration during sonication. We found that a temperature of 60°C yielded a dissolution rate of about 2% soluble chemical oxygen demand (SCOD) as compared to 7% SCOD using sonication at the specific energy (SE) of 7359kJ/kg TS. Using the SE of 6000kJ/kg TS with heat insulation during sonication, the SCOD dissolution rate of PS was similar to the result at the SE of 7051kJ/kg TS without heat insulation. Upon treatment with sonication, the PS released low concentrations of Cu and Zn into the supernatant. The phosphorus-accumulating organisms (PAOs) used the supernatant of sonicated PS as the carbon source. Supplementation with the diluted sonicated PS supernatant (SCOD≈1000mg/L) in anaerobic phase resulted in the release of phosphorus (36mg/L) and the production of polyhydroxyalkanoates (PHAs) (0.36g PHA/g SS). Compared with sodium acetate, higher polyhydroxyvalerate (PHV) faction in the polyhydroxyalkanoates (PHAs) was observed in the biomass when incubated with sonicated PS as the carbon source. This work provides a simple pathway to conserve energy and to enhance efficiencies of ultrasonic pretreatment and the recovery of carbon source from the sludge for improving the phosphorus removal in the ENR system.

Effect of carbon source type on intracellular stored polymers during endogenous denitritation (ED) treating landfill leachate

Glycogen accumulating organisms (GAOs) capable of storing organic compounds as polyhydroxyalkanoate (PHA) have been used for endogenous denitritation (ED), but the effect of carbon sources type on nitrogen removal performance of GAOs treating landfill leachate is unclear. In this study, a successful ED system treating landfill leachate (COD/NH4+-N (C/N): 4) without external carbon source addition was applied. The mature leachate with C/N of 1 was used as the feeding base solution, with acetate, propionate, and glucose examined as the carbon sources, and their effects on yields and compositions of PHA produced by GAOs were determined and associated with nitrogen removal performance. In the case of sole carbon source, acetate was much easier to be stored than propionate and glucose, which led to a higher nitrogen removal efficiency. Glucose had the lowest amount of PHA storage and led to the lowest performance. In the case of composite carbon sources (two scenarios: acetate + propionate; acetate + propionate + glucose), GAOs stored sufficient PHA and exhibited similar nitrogen removal efficiencies. Moreover, type of carbon source influenced the compositions of PHA. The polyhydroxybutyrate (PHB) fraction in PHA was far more than polyhydroxyvalerate (PHV) in all tests. PHV was synthesized only when acetate existed in carbon source. The microbial diversity analysis revealed that Proteobacteria was the most abundant phylum. Among the 108 genera detected in this ED system, the genera responsible for denitritation were Thauera, Paracoccus, Ottowia and Comamonadaceae_unclassified, accounting for 46.21% of total bacteria. Especially, Paracoccus and Comamonadaceae_unclassified transformed the carbon source into PHA for denitritation, and carried out endogenous denitritation.

Effect of polyhydroxyalkanoates on dark fermentative hydrogen production from waste activated sludge

Polyhydroxyalkanoates (PHA), an intracellular energy and carbon storage polymer, can be accumulated in activated sludge in substantial quantities under wastewater dynamic treatment (i.e., substrate feast-famine) conditions. However, its influence on hydrogen production has never been investigated before. This study therefore evaluated the influences of PHA level and composition in waste activated sludge (WAS) on hydrogen production. The results showed that with the increase of sludge PHA content from 25 to 178 mg per gram volatile suspended solids (VSS) hydrogen production from WAS alkaline anaerobic fermentation increased from 26.5 to 58.7 mL/g VSS. The composition of PHA was also found to affect hydrogen production. When the dominant composition shifted from polyhydroxybutyrate (PHB) to polyhydroxyvalerate (PHV), the amount of generated hydrogen decreased from 51.2 to 41.1 mL/g VSS even under the same PHA level (around 130 mg/g VSS). The mechanism studies exhibited that the increased PHA content accelerated both the cell solubilization and the hydrolysis process of solubilized substrates. Compared with the PHB-dominant sludge, the increased PHV fraction not only slowed the hydrolysis process but also caused more propionic acid production, with less theoretical hydrogen generation in this fermentation type. It was also found that the increased PHA content enhanced the soluble protein conversion of non-PHA biomass. Further investigations with enzyme analyses showed that both the key hydrolytic enzyme activities and hydrogen-forming enzyme activities were in the sequence of the PHB-dominant sludge > the PHV-dominant sludge > the low PHA sludge, which was in accord with the observed order of hydrogen yield.

Assessing the influence of CH4 concentration during culture enrichment on the biodegradation kinetics and population structure

Methanotrophic communities were enriched in three stirred tank reactors continuously supplied with CH4-laden air at 20, 2 and 0.2 gCH4 m−3 in order to evaluate the influence of CH4 concentration on the biodegradation kinetics, population structure and potential polyhydroxyalkanoate production under sequential nitrogen limitations. The population structure of the enriched cultures, dominated by type I methanotrophs, was influenced by CH4 concentration. No significant correlation between CH4 concentration and the maximum specific degradation rate (qmax) or the half-saturation constant (KS) was recorded, microorganisms enriched at 2 gCH4 m−3 presenting the highest qmax and those enriched at 20 and 0.2 gCH4 m−3 exhibiting the lowest KS. Maximum polyhydroxybutyrate (PHB) contents of 1.0% and 12.6% (w/w) were achieved at 20 and 2 g CH4 m−3, respectively. Polyhydroxyvalerate (PHV) was also detected at PHV:PHB ratios of up to 12:1 and 4:1 in the communities enriched at 20 and 0.2 gCH4 m−3, respectively.