rRNA and Poly-β-Hydroxybutyrate Dynamics in Bioreactors Subjected to Feast and Famine Cycles

Feast and famine cycles are common in activated sludge wastewater treatment systems, and they select for bacteria that accumulate storage compounds, such as poly-β-hydroxybutyrate (PHB). Previous studies have shown that variations in influent substrate concentrations force bacteria to accumulate high levels of rRNA compared to the levels in bacteria grown in chemostats. Therefore, it can be hypothesized that bacteria accumulate more rRNA when they are subjected to feast and famine cycles. However, PHB-accumulating bacteria can form biomass (grow) throughout a feast and famine cycle and thus have a lower peak biomass formation rate during the cycle. Consequently, PHB-accumulating bacteria may accumulate less rRNA when they are subjected to feast and famine cycles than bacteria that are not capable of PHB accumulation. These hypotheses were tested with Wautersia eutropha H16 (wild type) and W. eutropha PHB-4 (a mutant not capable of accumulating PHB) grown in chemostat and semibatch reactors. For both strains, the cellular RNA level was higher when the organism was grown in semibatch reactors than when it was grown in chemostats, and the specific biomass formation rates during the feast phase were linearly related to the cellular RNA levels for cultures. Although the two strains exhibited maximum uptake rates when they were grown in semibatch reactors, the wild-type strain responded much more rapidly to the addition of fresh medium than the mutant responded. Furthermore, the chemostat-grown mutant culture was unable to exhibit maximum substrate uptake rates when it was subjected to pulse-wise addition of fresh medium. These data show that the ability to accumulate PHB does not prevent bacteria from accumulating high levels of rRNA when they are subjected to feast and famine cycles. Our results also demonstrate that the ability to accumulate PHB makes the bacteria more responsive to sudden increases in substrate concentrations, which explains their ecological advantage.     

Adaptive responses of Ralstonia eutropha to feast and famine conditions analysed by flow cytometry

Results obtained by flow cytometry allow conclusions to be drawn about how the physiological states of Ralstsonia eutropha JMP134 are connected with survival strategies under distinct growth conditions. During both feast and famine conditions the cells were found to proceed through sharply separated phases of life. Two sources of carbon and energy, one poor (0.02% phenol) and one rich (0.2% pyruvate and 0.1% yeast extract) were chosen to study the cellular responses. Despite the major differences in carbon source, when growth stages of the bacteria on the two substrates were characterised in batch growth, only minor differences were found in the time course of the membrane potential related fluorescence intensity (MPRFI). This also applied to the rRNA content and the size-correlated forward scatter (FSC) signal of the cells, both of which increased to high levels during the (early) exponential growth phase. On the rich medium, DNA synthesis initially occurred in an uncoupled manner, then a high rate of PHB formation followed when nutrients began to be limiting. Under famine conditions, the cellular responses were much more complex. PHB was synthesised, then DNA synthesis occurred in a 'eukaryotic' mode, to be succeeded by renewed PHB synthesis. To obtain defined cell physiological states, the chemostat technique was used in addition to batch experiments. The results obtained clearly indicated that key events in cell physiology, including initiation of DNA replication and overflow metabolism, occurred in a hierarchically ordered manner and were tightly correlated with changes in the environmental conditions of the bacterial cells.     

Stoichiometry and kinetics of poly-beta-hydroxybutyrate metabolism in aerobic, slow growing, activated sludge cultures

This paper discusses the poly-beta-hydroxybutyrate (PHB) metabolism in aerobic, slow growing, activated sludge cultures, based on experimental data and on a metabolic model. The dynamic conditions which occur in activated sludge processes were simulated in a 2-L sequencing batch reactor (SBR) by subjecting a mixed microbial population to successive periods of external substrate availability (feast period) and no external substrate availability (famine period). Under these conditions intracellular storage and consumption of PHB was observed. It appeared that in the feast period, 66% to almost 100% of the substrate consumed is used for storage of PHB, the remainder is used for growth and maintenance processes. Furthermore, it appeared that at high sludge retention time (SRT) the growth rate in the feast and famine periods was the same. With decreasing SRT the growth rate in the feast period increased relative to the growth rate in the famine period. Acetate consumption and PHB production in the feast period both proceeded with a zero-order rate in acetate and PHB concentration respectively. PHB consumption in the famine period could best be described kinetically with a nth-order degradation equation in PHB concentration. The obtained results are discussed in the context of the general activated sludge models.     

Accumulation of the PhaP Phasin of Ralstonia eutropha Is Dependent on Production of Polyhydroxybutyrate in Cells

Polyhydroxyalkanoates (PHAs) are polyoxoesters that are produced by diverse bacteria and that accumulate as intracellular granules. Phasins are granule-associated proteins that accumulate to high levels in strains that are producing PHAs. The accumulation of phasins has been proposed to be dependent on PHA production, a model which is now rigorously tested for the phasin PhaP of Ralstonia eutropha. R. eutropha phaC PHA synthase and phaP phasin gene replacement strains were constructed. The strains were engineered to express heterologous and/or mutant PHA synthase alleles and a phaP-gfp translational fusion in place of the wild-type alleles of phaC and phaP. The strains were analyzed with respect to production of polyhydroxybutyrate (PHB), accumulation of PhaP, and expression of the phaP-gfp fusion. The results suggest that accumulation of PhaP is strictly dependent on the genetic capacity of strains to produce PHB, that PhaP accumulation is regulated at the level of both PhaP synthesis and PhaP degradation, and that, within mixed populations of cells, PhaP accumulation within cells of a given strain is not influenced by PHB production in cells of other strains. Interestingly, either the synthesis of PHB or the presence of relatively large amounts of PHB in cells (>50% of cell dry weight) is sufficient to enable PhaP synthesis. The results suggest that R. eutropha has evolved a regulatory mechanism that can detect the synthesis and presence of PHB in cells and that PhaP expression can be used as a marker for the production of PHB in individual cells.     

Denitrification with endogenous carbon source at low C/N and its effect on P(3HB) accumulation

The objective of the work reported here was to determine whether the ratio of COD/Nox has an impact on poly-beta-hydroxybutyrate (PHB) metabolism in activated sludge. Furthermore, it was tested if the ratio influenced the percentage use of organic compounds present in wastewater, for endogenous respiration, oxidation, accumulation and denitrification. Gas flow rate in SBR reactor was controlled by thermal mass flow controller (TMFC). Constant amount of air entering sequencing batch reactor was automatically adjusted to stable set-point 2mg O2 L(-1). It means that DO concentration in the reactor could change with oxygen uptake. During the filling period and part of the reaction time DO was nearly zero. Feast period of the external substrate availability and famine period of little amount or no external carbon availability were determined. At 23 h of the reaction time, and COD/Nox ratio 8, denitrification took place only during feast period. What was interesting, poly-beta-hydroxybutyrate degradation was observed in the feast period as well. However, at 11h of the reaction time and COD/Nox ratio 37, denitrification occurred in feast and famine period. In the feast period PHB was accumulated and in the famine period was used as the endogenous carbon source. COD consumption to reduce 1mg N-nitrate was ranging from 1.15 to 6.26 depending on carbon source and increased when exogenous and endogenous carbon were used by activated sludge. The increase in PHB content from 0.25 to 0.43 Cmol/Cmol resulted in a double increase in the amount of nitrogen removed due to denitrification was observed.     

Simultaneous nitrification and denitrification using stored substrate (PHB) as the electron donor in an SBR

The potential for PHB (poly-beta-hydroxybutyrate) to serve as the electron donor for effective simultaneous nitrification and denitrification (SND) was investigated in a 2-L sequencing batch reactor (SBR) using a mixed culture and acetate as the organic substrate. During the feast period (i.e., acetate present), heterotrophic respiration activity was high and nitrification was prevented due to the inability of nitrifying bacteria to compete with heterotrophs for oxygen. Once acetate was depleted the oxidation rate of PHB was up to 6 times slower than that of soluble acetate and nitrification could proceed due to the decreased competition for oxygen. The slow nature of PHB degradation meant that it was an effective substrate for SND, as it was oxidised at a similar rate to ammonium and was therefore available for SND throughout the entire aerobic period. The percentage of nitrogen removed via SND increased at lower DO concentrations during the famine period, with up to 78% SND achieved at a DO concentration of 0.5 mg L(-1). However, the increased percentage of SND at a low DO concentration was compromised by a 2-times slower rate of nitrogen removal. A moderate DO concentration of 1 mg L(-1) was optimal for both SND efficiency (61%) and rate (4.4 mmol N x Cmol x(-1) x h(-1)). Electron flux analysis showed that the period of highest SND activity occurred during the first hour of the aerobic famine period, when the specific oxygen uptake rate (SOUR) was highest. It is postulated that a high SOUR due to NH(4) (+) and PHB oxidation decreases oxygen penetration into the floc, creating larger zones for anoxic denitrification. The accumulation of nitrate towards the end of the SND period showed that SND was finally limited by the rate of denitrification. As PHB degradation was found to follow first-order kinetics (df(PHB)/dt = -0.19 x f(PHB)), higher PHB concentrations would be expected to drive SND faster by increasing the availability rate of reducing power and reducing penetration of oxygen into the floc, due to the corresponding increased SOUR. Process control techniques to accumulate higher internal PHB concentrations to improve PHB-driven SND are discussed.     

A framework for multidimensional modelling of activity and structure of multispecies biofilms

Concepts from previous biofilm models were integrated to create a framework for the implementation of multidimensional (2D and 3D) multispecies biofilm models. The framework is here described at three levels: (i) mathematical representation of the processes involved in biofilm formation, (ii) numerical implementation into a computer program (freely available from our website http://www.biofilms.bt.tudelft.nl/frameworkMaterial) and (iii) using the program for the creation of biofilm models with multiple bacterial and solute species. An improved version of the individual-based modelling (IbM) that allows structured biomass was used. In this approach biomass composition may be discriminated into any number of particulate species, including extracellular polymeric substances (EPS) for which specific functionality was included. Detachment is also included, described as occurring at the biofilm surface with variable local rates derived from functions of state variables. The application of this modelling framework to a multispecies system with structured biomass is illustrated in a case study where the competition between an organism capable of accumulating polyhydroxybutyrate (PHB, an internal storage compound) and an EPS-producing organism in a two-species biofilm is analysed. Results illustrate that biofilms enriched in PHB-producing organisms may be obtained by supplying substrate intermittently in feast/famine cycles.     

High levels of nitrifying bacteria in intermittently aerated reactors treating high ammonia wastewater

Changes in the fractions of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in two laboratory-scale reactors were investigated using 16S rRNA probe hybridizations. The reactors were operated in intermittent aeration mode and different aeration cycles to treat anaerobically digested swine wastewater with ammonia concentrations up to 175 mg NH(3)-N/L. High ammonia removals (>98.8%) were achieved even with increased nitrogen loads and lower aeration: non-aeration time ratios of 1h:3h. Nitrosomonas/Nitrosococcus mobilis were the dominant ammonia-oxidizing bacteria in the reactors. Nitrospira-like organisms were the dominant nitrite-oxidizing bacteria during most of the investigation, but were occasionally outcompeted by Nitrobacter. High levels of nitrifiers were measured in the biomass of both reactors, and ammonia-oxidizing bacteria and nitrite-oxidizing bacterial levels adjusted to changing aeration: non-aeration time ratios. Theoretical ammonia-oxidizer fractions, determined by a mathematical model, were comparable to the measured values, although the measured biomass fractions were different at each stage while the theoretical values remained approximately constant. Stable ammonia removals and no nitrite accumulation were observed even when rRNA levels of ammonia oxidizers and nitrite-oxidizers reached a minimum of 7.2% and 8.6% of total rRNA, respectively. Stable nitrogen removal performance at an aeration: non-aeration ratio of 1h:3h suggests the possibility of significant savings in operational costs.     

Substrate consumption and biomass growth of Ralstonia eutropha at various S0/X0 levels in batch cultures

The biomass growth, substrate consumption and polyhydrobutyrate (PHB) production of Ralstonia eutropha with butyric acid and fructose as the carbon and energy sources at various ratios of initial substrate concentration (S0) to initial biomass concentration (X0) were investigated in this study. Results indicated that the PHB content increased with the increasing S0/X0 ratio. Different substrates exhibited a similar trend for cell growth and substrates consumption with the changing S0/X0 ratio. The specific consumption rates of both butyric acid and fructose increased with the increasing S0/X0 ratio. An S0/X0-dependent kinetic model was modified to describe the kinetics of biomass growth and substrate consumption for R. eutropha. This model was verified with the experimental results from this work and in literature.     

Effects of homologous phosphoenolpyruvate-carbohydrate phosphotransferase system proteins on carbohydrate uptake and poly(3-Hydroxybutyrate) accumulation in Ralstonia eutropha H16

Seven gene loci encoding putative proteins of the phosphoenolpyruvate-carbohydrate phosphotransferase system (PEP-PTS) were identified in the genome of Ralstonia eutropha H16 by in silico analysis. Except the N-acetylglucosamine-specific PEP-PTS, an additional complete PEP-PTS is lacking in strain H16. Based on these findings, we generated single and multiple deletion mutants defective mainly in the PEP-PTS genes to investigate their influence on carbon source utilization, growth behavior, and poly(3-hydroxybutyrate) (PHB) accumulation. As supposed, the H16 ΔfrcACB and H16 ΔnagFEC mutants exhibited no growth when cultivated on fructose and N-acetylglucosamine, respectively. Furthermore, a transposon mutant with a ptsM-ptsH insertion site did not grow on both carbon sources. The observed phenotype was not complemented, suggesting that it results from an interaction of genes or a polar effect caused by the Tn5::mob insertion. ptsM, ptsH, and ptsI single, double, and triple mutants stored much less PHB than the wild type (about 10 to 39% [wt/wt] of cell dry weight) and caused reduced PHB production in mutants lacking the H16_A2203, H16_A0384, frcACB, or nagFEC genes. In contrast, mutant H16 ΔH16_A0384 accumulated 11.5% (wt/wt) more PHB than the wild type when grown on gluconate and suppressed partially the negative effect of the ptsMHI deletion on PHB synthesis. Based on our experimental data, we discussed whether the PEP-PTS homologous proteins in R. eutropha H16 are exclusively involved in the complex sugar transport system or whether they are also involved in cellular regulatory functions of carbon and PHB metabolism.     

Plug flow simulating and completely mixed reactors with a premixing tank,in the control of filamentous bulking

Summary A sequence of a substrate rich and substrate starvation phase (feast and famine strategy) is created in both compartmentalized reactors and reactors with premixing tanks. This condition is known to be essential in the control of bulking. With a synthetic waste water containing glucose a reactor with 12 compartments was effective in the control of filamentous bulking. With a dairy industrial waste water, containing a slowly biodegradable COD-fraction, this reactor could not suppress the growth of filamentous bacteria. With dairy and brewery waste water, reactors with premixing tanks were used to create a more pronounced exogenous (substrate rich) phase. Using three or more premixing tanks filamentous bulking could be controlled. During the exogenous phase the floc-forming microorganisms, having a higher substrate uptake rate, can take up the largest amount of substrate and can survive better during the endogenous phase.Substrate concentration and respiration rate profiles were studied and the concentration of the reserve materials was monitored.For each type of waste water the sludge loading and the fraction of readily available COD will determine the system's design.     

New insight into the role of the PhaP phasin of Ralstonia eutropha in promoting synthesis of polyhydroxybutyrate

Phasins are proteins that are proposed to play important roles in polyhydroxyalkanoate synthesis and granule formation. Here the phasin PhaP of Ralstonia eutropha has been analyzed with regard to its role in the synthesis of polyhydroxybutyrate (PHB). Purified recombinant PhaP, antibodies against PhaP, and an R. eutropha phaP deletion strain have been generated for this analysis. Studies with the phaP deletion strain show that PhaP must accumulate to high levels in order to play its normal role in PHB synthesis and that the accumulation of PhaP to low levels is functionally equivalent to the absence of PhaP. PhaP positively affects PHB synthesis under growth conditions which promote production of PHB to low, intermediate, or high levels. The levels of PhaP generally parallel levels of PHB in cells. The results are consistent with models whereby PhaP promotes PHB synthesis by regulating the surface/volume ratio of PHB granules or by interacting with polyhydroxyalkanoate synthase and indicate that PhaP plays an important role in PHB synthesis from the early stages in PHB production and across a range of growth conditions.     

Properties of a novel intracellular poly(3-hydroxybutyrate) depolymerase with high specific activity (PhaZd) in Wautersia eutropha H16

A novel intracellular poly(3-hydroxybutyrate) (PHB) depolymerase (PhaZd) of Wautersia eutropha (formerly Ralstonia eutropha) H16 which shows similarity with the catalytic domain of the extracellular PHB depolymerase in Ralstonia pickettii T1 was identified. The positions of the catalytic triad (Ser190-Asp266-His330) and oxyanion hole (His108) in the amino acid sequence of PhaZd deduced from the nucleotide sequence roughly accorded with those of the extracellular PHB depolymerase of R. pickettii T1, but a signal peptide, a linker domain, and a substrate binding domain were missing. The PhaZd gene was cloned and the gene product was purified from Escherichia coli. The specific activity of PhaZd toward artificial amorphous PHB granules was significantly greater than that of other known intracellular PHB depolymerase or 3-hydroxybutyrate (3HB) oligomer hydrolases of W. eutropha H16. The enzyme degraded artificial amorphous PHB granules and mainly released various 3-hydroxybutyrate oligomers. PhaZd distributed nearly equally between PHB inclusion bodies and the cytosolic fraction. The amount of PHB was greater in phaZd deletion mutant cells than the wild-type cells under various culture conditions. These results indicate that PhaZd is a novel intracellular PHB depolymerase which participates in the mobilization of PHB in W. eutropha H16 along with other PHB depolymerases.     

Impact of the core components of the phosphoenolpyruvate-carbohydrate phosphotransferase system, HPr and EI, on differential protein expression in Ralstonia eutropha H16

In Ralstonia eutropha H16, seven genes encoding proteins being involved in the phosphoenolpyruvate-carbohydrate phosphotransferase system (PEP-PTS) were identified. In order to provide more insights into the poly(3-hydroxybutyrate) (PHB)-leaky phenotype of the HPr/EI deletion mutants H16ΔptsH, H16ΔptsI, and H16ΔptsHI when grown on the non-PTS substrate gluconate, parallel fermentations for comparison of their growth behavior were performed. Samples from the exponential, the early stationary, and late stationary growth phases were investigated by microscopy, gas chromatography and (phospho-) proteome analysis. A total of 71 differentially expressed proteins were identified using 2D-PAGE, Pro-Q Diamond and Coomassie staining, and MALDI-TOF analysis. Detected proteins were classified into five major functional groups: carbon metabolism, energy metabolism, amino acid metabolism, translation, and membrane transport/outer membrane proteins. Proteome analyses revealed enhanced expression of proteins involved in the Entner-Doudoroff pathway and in subsequent reactions in cells of strain H16 compared to the mutant H16ΔptsHI. Furthermore, proteins involved in PHB accumulation showed increased abundance in the wild-type. This expression pattern allowed us to identify proteins affecting carbon metabolism/PHB biosynthesis in strain H16 and translation/amino acid metabolism in strain H16ΔptsHI, and to gain insight into the molecular response of R. eutropha to the deletion of HPr/EI.     

Ralstonia eutropha H16 encodes two and possibly three intracellular Poly[D-(-)-3-hydroxybutyrate] depolymerase genes

Intracellular poly[D-(-)-3-hydroxybutyrate] (PHB) depolymerases degrade PHB granules to oligomers and monomers of 3-hydroxybutyric acid. Recently an intracellular PHB depolymerase gene (phaZ1) from Ralstonia eutropha was identified. We now report identification of candidate PHB depolymerase genes from R. eutropha, namely, phaZ2 and phaZ3, and their characterization in vivo. phaZ1 was used to identify two candidate depolymerase genes in the genome of Ralstonia metallidurans. phaZ1 and these genes were then used to design degenerate primers. These primers and PCR methods on the R. eutropha genome were used to identify two new candidate depolymerase genes in R. eutropha: phaZ2 and phaZ3. Inverse PCR methods were used to obtain the complete sequence of phaZ3, and library screening was used to obtain the complete sequence of phaZ2. PhaZ1, PhaZ2, and PhaZ3 share approximately 30% sequence identity. The function of PhaZ2 and PhaZ3 was examined by generating R. eutropha H16 deletion strains (Delta phaZ1, Delta phaZ2, Delta phaZ3, Delta phaZ1 Delta phaZ2, Delta phaZ1 Delta phaZ3, Delta phaZ2 Delta phaZ3, and Delta phaZ1 Delta phaZ2 Delta phaZ3). These strains were analyzed for PHB production and utilization under two sets of conditions. When cells were grown in rich medium, PhaZ1 was sufficient to account for intracellular PHB degradation. When cells that had accumulated approximately 80% (cell dry weight) PHB were subjected to PHB utilization conditions, PhaZ1 and PhaZ2 were sufficient to account for PHB degradation. PhaZ2 is thus suggested to be an intracellular depolymerase. The role of PhaZ3 remains to be established.     

Substrate uptake,loss, and reserve in ammonia-oxidizing bacteria (AOB) under different substrate availabilities

The controversial arguments on the true substrate in nitritation kinetics might be due to the cells' dual substrate-transport system. Our experiments revealed that, under ammonia-rich environments, it diffused into the membrane (ammonia was the direct substrate); but, under oligotrophic, ammonium ion was actively transported (ammonium was the direct substrate). Facilitating this change-over, the bacterial composition in the sludge was altered, although the predominant was Nitrosomonas eutropha in most of the six chemostats. Also, the substrate affinity constant (K_s) fell resulting in partial compensation for the reduced availability of substrate. When the environmental ammonia concentration was lower than the cytoplasmic one, a backward diffusion appeared to take place, which probably had the cells accelerate its energy-consuming ammonium transport. The % ammonium oxidizing bacteria (AOB) to the total number of bacteria in the sludge remarkably decreased when cells were grown under oligotrophic environments. This could be evidence of the cellular energy dissipation caused by ammonia loss and recovery. Intracellular total ammonium nitrogen (TAN) accumulations were observed, which gradually increased from a basal value of ∼1 M (for AOB grown under copious environments) to much higher values (grown under oligotrophic environment). It did not affect the reaction kinetics but potentially served as a reserve against famine.     

The effect of dissolved oxygen on PHB accumulation in activated sludge cultures

Nitrogen removal from wastewater is often limited by the availability of reducing power to perform denitrification, especially when treating wastewaters with a low carbon:nitrogen ratio. In the increasingly popular sequencing batch reactor (SBR), bacteria have the opportunity to preserve reducing power from incoming chemical oxygen demand (COD) as poly-beta-hydroxybutyrate (PHB). The current study uses laboratory experiments and mathematical modeling in an attempt to generate a better understanding of the effect of oxygen on microbial conversion of COD into PHB. Results from a laboratory SBR with acetate as the organic carbon source showed that the aerobic acetate uptake process was oxygen-dependent, producing higher uptake rates at higher dissolved oxygen (DO) supply rates. However, at the lower DO supply rates (k(L)a 6 to 16 h(-1), 0 mg L(-1) DO), a higher proportion of the substrate was preserved as PHB than at higher DO supply rates (k(L)a 30, 51 h(-1), DO >0.9 mg L(-1)). Up to 77% of the reducing equivalents available from acetate were converted to PHB under oxygen limitation (Y(PHB/Ac) 0.68 Cmol/Cmol), as opposed to only 54% under oxygen-excess conditions (Y(PHB/Ac) 0.48 Cmol/Cmol), where a higher fraction of acetate was used for biomass growth. It was calculated that, by oxygen management during the feast phase, the amount of PHB preserved (1.4 Cmmol L(-1) PHB) accounted for an additional denitrification potential of up to 18 mg L(-1) nitrate-nitrogen. The trends of the effect of oxygen (and hence ATP availability) on PHB accumulation could be reproduced by the simulation model, which was based on biochemical stoichiometry and maximum rates obtained from experiments. Simulated data showed that, at low DO concentrations, the limited availability of adenosine triphosphate (ATP) prevented significant biomass growth and most ATP was used for acetate transport into the cell. In contrast, high DO supply rates provided surplus ATP and hence higher growth rates, resulting in decreased PHB yields. The results suggest that oxygen management is crucial to conserving reducing power during the feast phase of SBR operation, as excessive aeration rates decrease the PHB yield and allow higher biomass growth.     

Purification and properties of an intracellular 3-hydroxybutyrate-oligomer hydrolase (PhaZ2) in Ralstonia eutropha H16 and its identification as a novel intracellular poly(3-hydroxybutyrate) depolymerase

An intracellular 3-hydroxybutyrate (3HB)-oligomer hydrolase (PhaZ2(Reu)) of Ralstonia eutropha was purified from Escherichia coli harboring a plasmid containing phaZ2(Reu). The purified enzyme hydrolyzed linear and cyclic 3HB-oligomers. Although it did not degrade crystalline poly(3-hydroxybutyrate) (PHB), the purified enzyme degraded artificial amorphous PHB at a rate similar to that of the previously identified intracellular PHB (iPHB) depolymerase (PhaZ1(Reu)). The enzyme appeared to be an endo-type hydrolase, since it actively hydrolyzed cyclic 3HB-oligomers. However, it degraded various linear 3HB-oligomers and amorphous PHB in the fashion of an exo-type hydrolase, releasing one monomer unit at a time. PhaZ2 was found to bind to PHB inclusion bodies and as a soluble enzyme to cell-free supernatant fractions in R. eutropha; in contrast, PhaZ1 bound exclusively to the inclusion bodies. When R. eutropha H16 was cultivated in a nutrient-rich medium, the transient deposition of PHB was observed: the content of PHB was maximized in the log growth phase (12 h, ca. 14% PHB of dry cell weight) and decreased to a very low level in the stationary phase (ca. 1% of dry cell weight). In each phaZ1-null mutant and phaZ2-null mutant, the PHB content in the cell increased to ca. 5% in the stationary phase. A double mutant lacking both phaZ1 and phaZ2 showed increased PHB content in the log phase (ca. 20%) and also an elevated PHB level (ca. 8%) in the stationary phase. These results indicate that PhaZ2 is a novel iPHB depolymerase, which participates in the mobilization of PHB in R. eutropha along with PhaZ1.