Biodegradation of Aliphatic Homopolyesters and Aliphatic−Aromatic

Biomacromolecules 2004, 5, 1687-1697

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Biodegradation of Aliphatic Homopolyesters and Aliphatic-Aromatic Copolyesters by Anaerobic Microorganisms Dunja-Manal Abou-Zeid,† Rolf-Joachim Mu¨ller,* and Wolf-Dieter Deckwer GBF, Gesellschaft fu¨r Biotechnologische Forschung mbH, Mascheroder Weg, D 38124 Braunschweig, Germany Received February 2, 2004; Revised Manuscript Received May 19, 2004

The anaerobic degradability of natural and synthetic polyesters is investigated applying microbial consortia (3 sludges, 1 sediment) as well as individual strains isolated for this purpose. In contrast to aerobic conditions, the natural homopolyester polyhydroxybutyrate (PHB) degrades faster than the copolyester poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV). For the synthetic polyester poly(-caroplacton) (PCL), microbial degradation in the absence of oxygen could be clearly demonstrated; however, the degradation rate is significantly lower than for PHB and PHBV. Other synthetic polyesters such as poly(trimethylene adipate) (SP3/6), poly(tetramethylene adipate) (SP4/6), and aliphatic-aromatic copolyesters from 1,4butanediol, terephthalic acid, and adipic acid (BTA-copolymers) exhibit only very low anaerobic microbial susceptibility. A copolyester with high amount of terephthalic acid (BTA 40:60) resisted the anaerobic breakdown even under thermophilic conditions and/or when blended with starch. For the synthetic polymers, a number of individual anaerobic strain could be isolated which are able to depolymerize the polymers and selected strains where identified as new species of the genus Clostridium or Propionispora. Their distinguished degradation patterns point to the involvement of different degrading enzymes which are specialized to depolymerize either the natural polyhydroxyalkanoates (e.g., PHB), the synthetic polyester PCL, or other synthetic aliphatic polyesters such as SP3/6. It can be supposed that these enzymes exhibit comparable characteristics as those described to be responsible for aerobic polyester degradation (lipases, cutinases, and PHB-depolymerases). Introduction The past two decades have witnessed a growing public and scientific concern regarding the use of biodegradable plastic material as a solution for the existing problem of plastic waste. A number of biodegradable plastics have been successfully developed over the past few years to meet the specific demands, e.g., in agriculture and packaging industries.1,2 The best understood and most extensively studied plastics with regard to biodegradation are poly(hydroxyalkanoates) (PHA), which are polymers naturally produced by bacteria.3,4 However, for practical applications biodegradable aliphatic synthetic polyesters such as poly(-caprolactone) (PCL), poly(ethylene succinate-co-butylene succinate) (trade name “Bionolle”), or polylactide (PLA) have predominantly been used up to now.5 Due to the limited material properties of aliphatic polyesters, new biodegradable aliphatic-aromatic copolyesters have been developed and recently introduced into the market, e.g., under the trade name Ecoflex (BASF AG, Germany) or Eastar Bio (Eastman Chemicals, U.S.A.).6,7 Most research on biodegradation processes is focused on aerobic environments such as surface water, soil, or compost. In contrast, only little is known about anaerobic biodegradation of plastics, although anaerobic digestion of biowaste * To whom correspondence should be addressed. Tel: +49 531 6181619. Fax: +49 531 6181175. E-mail: [email protected]. † Present address: University of Alexandria, Alexandria, Egypt.

becomes more and more established because of the additional energetic benefit of biogas recovery. Most studies published on anaerobic biodegradation of plastics focus on mixed and unspecified microbial communities such as diverse anaerobic sludges and/or sediments evaluating the anaerobic biodegradation of polyhydroxyalkanoates, PCL and PLA,8-18 or starch- or cellulosesters.19-21 Nishida and Tokiwa22 characterized the distribution of PCL degrading aerobic and anaerobic microorganisms in different environments, but did not isolate or identify any strain. Investigations using individual cultures were restricted to poly(hydroxybutyrate) (PHB) degradation by an organism described as Ilyobacter delafildii which was isolated and identified by Janssen and co-workers.23,24 Recently, we published systematic investigations on the anaerobic degradation of polyhydroxybutyrate (PHB) and poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) and the synthetic aliphatic polyester poly(-caprolactone) (PCL), using mixed microbial consortia and especially isolated anaerobic individual strains (four strains of the genus Clostridium).25,26 Although contrary findings had been published about the anaerobic biodegradability of the synthetic aliphatic polyester PCL, we could unambiguously demonstrate the biodegradation of PCL by anaerobic microorganisms. However, the observation of a faster degradation of the PHB homopolyester compared with the copolyester PHBV under anaerobic conditions indicates that the

10.1021/bm0499334 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/10/2004

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Figure 1. Chemical formula of the polyesters used in this study. (The index n represents the degree of polymerization, and the indices l and m represent the composition of the random copolyesters.)

substrate specificity of the microorganisms and possibly also degradation mechanisms are different from that of the aerobic situation, where usually PHBV was found to degrade faster than PHB. Since it is obviously not possible to conclude from the aerobic degradation behavior of polyesters on their microbial susceptibility in the absence of oxygen, it was of particular interest to look if also other synthetic aliphatic polyesters and especially aliphatic-aromatic copolyesters, which are of high commercial relevance, can be degraded under anaerobic conditions. Therefore, we investigated the anaerobic degradability of the polyesters poly(butylene succinate), poly(butylene adipate), and poly(butylene terephthalate-cobutylene adipate) and compared it with the results obtained for the natural poly(hydroxyalkanoates) and PCL tested in a previous study.25 For the investigations presented here, we applied microbial consortia as well as specifically isolated individual anaerobic strains. Material and Methods Polymers. The chemical structures of the different polyesters chosen for the degradation studies are shown in Figure 1. Table 1 summarizes the chemical components, the composition, and some important physical parameters. Sample Preparation and Sterilisation. Polyester processing into thin films by compression molding and sterilization by UV-radiation was performed as described by Abou-Zeid et al.25 PCL Tone 787 and PCL-S as well as BTA45:55 and BTA-S were available as films. Using punches with defined diameters, test specimens of defined surface areas were cut. For degradation tests in liquid cultures, the processed films were sterilized by H2O2 treatment. Films were inserted singly in small Petri dishes (q ) 35 mm, Greiner, Frieckenhausen) and exposed to 10% (vol/vol) H2O2 for 1 h per each side, dried under a laminar air flow at room temperature overnight, and washed thereafter in three subsequent volumes of 500

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mL sterile distilled water using sterile forceps. No significant changes in the material properties due to the treatment could be observed. Source of Inocula. Three different sources of technically managed and controlled disposal systems were used as a source of anaerobic bacteria for all degradation tests and the isolation of microbial strains: (I) Anaerobic sludge (wastewater sludge: WWS) from an anaerobic digester of a municipal wastewater treatment plant (Gifhorn, Germany) (II) Anaerobic methane producing sludge collected from an anaerobic laboratory reactor of the Institute for Technology of Carbohydrates (Technical University, Braunschweig, Germany) fed with wastewater from the sugar industry (laboratory sludge: LS) (III) Thermally treated biowaste (TBW) from the anaerobic biowaste treatment plant in Braunschweig-Watenbu¨ttel, Germany (IV) Additionally, a sample from a natural environment was taken, namely an anaerobic river sediment (AS) from Spittelwasser, a sidearm of the Elbe river, Germany. Sediments of the Spittelwasser are highly loaded with organic contaminants. It is known that for such sediments only the upper few millimeter exhibit aerobic conditions, while the microbial activity in this layer consumes the entire oxygen diffusing from the surface into the sediment. The sludges were used directly after sampling for preliminary degradation tests and the preparation of polyester degrading enrichment cultures. Sludge samples were additionally stored under nitrogen at 4 °C for further experiments. Media for Cultivation and Degradation Experiments. The composition of the mineral salt medium used in this work was according to Abou-Zeid.25,26 MSV Medium (Components per 1 L Medium). K2HPO4, 0.35 g; KH2PO4, 0.27 g; NH4Cl, 0.5 g; CaCl2‚2H2O, 0.075 g; FeCl2‚4H2O, 0.02 g; MgCl2‚6H2O, 0.1 g; trace element solution, 1 mL; vitamin solution, 1 mL; selenite/tungstate solution; resazurin (0.1%), cystein-HCl (0.025%), and Na2S (0.025%) were added to ensure anaerobic conditions. Trace Element Solution. MnCl2‚4H2O, 0.5 g; H3BO3, 0.05 g; ZnCl2, 0.05 g; CuCl2‚2H2O, 0.03 g; CoCl2‚6H2O, 0.5 g; NiCl2‚6H2O, 0.05 g; Na2MoO4‚2H2O, 0.01 g. Demineralized water was added to complete 1 liter. Vitamin Solution. Biotine, 2 mg; folic acid, 2 mg; pyridoxalhydrochloride, 10 mg; thiamindichloride, 5 mg; riboflavine, 5 mg; nicotinic acid, 5 mg; DL-calciumpanthotenate, 5 mg; vitamin B 12, 0.1 mg; p-aminobenzoate, 5 mg; lipoic acid, 5 mg;. Components were dissolved in 1 l of demineralized water. The solution was membrane filtered (pore size 0.2 µm) and stored at 4 °C. Selenite/Tungstate Solution. NaOH, 0.5 g; Na2SeO3‚5H2O, 3 mg; Na2WO4‚2H2O, 4 mg. The salts were added to 1 l demineralized water and stored at 4 °C. For preparing agar plates, 20 g of agar/L was added to the MSV medium; The preparation of the polyester supplemented agar plates for clear zone experiments is described elsewhere.26

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Biodegradation of Polyesters Table 1. Polyesters Used for Anaerobic Degradation Experiments polymer

component(s)

Tma

Mwb

source ICI, Billingham United Kingdom (as Biopol BX G08) ICI, Billingham United Kingdom (as Biopol BX PO270) Polyscience Inc. Warrington, USA Novamont S.p.A. Novara, Italy Novamont S.p.A. Novara, Italy GBF Braunschweig, Germany GBF Braunschweig Germany GBF Braunschweig Germany

PHB

3-hydroxybutyrate

174

540 000

PHBV

3-hydroxybutyrate/ 3-hydroxyvalerate (11,6 mol %)

150

397 000

PCL

-caprolactone

60

50 000

PCL Tone 787

-caprolactone

63

200 000

PCL-S

-caprolactone and starch (40% w/w)

63

187 000c

SP 3/6

1,3-propanediol/adipic acid

44

38 000

SP 4/6

1,4-butanediol/adipic acid

62

40 000

BTA 10:90

1,4-butanediol (50 mol %) adipic acid (45 mol %)/ terephthalic acid (5 mol %) 1,4-butanediol (50 mol %) adipic acid (40 mol %)/ terephthalic acid (10 mol %) 1,4-butanediol (50 mol %) adipic acid (30 mol %)/ terephthalic acid (20 mol %) 1,4-butanediol (50 mol %) adipic acid (27.5 mol %)/ terephthalic acid (22.5 mol %) 1,4-butanediol (50 mol %) adipic acid(27.5 mol %)/ terephthalic acid (22.5 mol %) and starch (32% w/w)

56

25 000

52

36 000

GBF Braunschweig Germany

99

47 600

Hu¨ls AG Marl, Germany

120

66 500

BASF AG Ludwigshafen, Germany

92

145 000c

BTA 20:80

BTA 40:60

BTA 45:55

BTA-S

Novamont S.p.A. Novara, Italy

a T : melting temperature; temperature at the maximum of melting peak (DSC). b M : weight average molar mass (determined by GPC with polystyrene m w calibration). c Mw of the polyester component.

Mass Loss Determination. For mass loss measurements of films after incubation the polyester samples were washed twice with distilled water and dried to constant mass under vacuum. The mean mass difference of the films (at least triplicates) was expressed as mass loss (∆m in mg) or optionally expressed as ∆m A-1 in mg cm-2 (A ) total surface area of both sides of the polyester strip in cm2) since polymer depolymerization is a surface process. Determination of the Biogas Formation. Monitoring of biogas production for the quantitative comparison of anaerobic degradation with mixed cultures was done as described by Abou-Zeid et al.25 according to ASTM D 5210-91. Biodegradation is expressed as percentage of the maximum theoretical biogas formation as calculated from the so-called Buswell equation:27 CnHaOb + (n - a/4 - b/2)H2O f (n/2 - a/8 + b/4)CO2 + (n/2 + a/8 - b/4)CH4 Determination of the Starch Content of PolyesterStarch Blends. The relative starch content of the polyesterstarch blends was determined to follow up changes in the composition during degradation, either by a gravimetric method or by applying GPC. For the gravimetric determination of the starch content, a preweighed circular polyester film (q 38 mm) was dissolved in 1 mL dichloromethane in

a preweighed 1 mL test vial (Eppendorf). After centrifugation (15 min, 15 000 rpm), the supernatant was carefully removed. The remaining pellet was dispersed again in 1 mL of dichloromethane and centrifuged. This washing procedure was performed three times for each sample. Finally, the cups were dried under vacuum at 37 °C for 36 h and re-weighed. The content of insoluble starch was calculated from the weight differences. To estimate possible mass losses due to the dissolution of the cup-material caused by the dichloromethane treatment, the cups were treated the same way as mentioned above without polyester samples. An increase in mass only up to 0.2% was noticed. The determination of the starch content by GPC was performed by comparing the polyester content of pure polyester films to that of the starch containing blends. The blends were dissolved in chloroform to a known concentration. The residual solid starch was thereafter separated by filtration (0.8 and 0.45 µm filter paper), and the solution containing the polyesters was analyzed by GPC. The polymer concentrations were calculated from the GPC peak areas. GPC calibration curves of pure PCL and BTA were established in a concentration range of 0.25-1.5 mg‚ml-1 in chloroform (R2 for PCL: 0.9997; R2 for BTA: 0.9996). Enrichment Cultures. Enrichment cultures were prepared by adding to the three mesophilic sludges (sludge I, WWS; sludge II, LS; sludge IV, AS) strips of one of the six

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Figure 2. Biological hydrolysis of the polyesters in different anaerobic environments after 14 weeks at 35 °C. (Polyester films, q ) 25 mm; initial mass of films, 39-49 mg; average of three films per test.) The numbers above the bars indicate the absolute mass of the residual polymer.

polyesters (see Figure 1) and optionally of all six polyesters together. Screening for polyester degrading microorganisms was then performed after 14 weeks and optionally after 18 month incubation using the so-called “clear-zone method” with polyester containing agar plates and roll tubes as described elsewhere.25,26 The isolates were preserved in airtight vials containing 50% (v/v) glycerol (87%) flushed and headspace filled with oxygen free N2 gas. The anaerobic vials were additionally put in airtight bags containing an anaerobic catalyst (Anaerocult A, Merck, Darmstadt, Germany) and stored at -20 °C. Identification and Biochemical Characterization of the Isolated Strains. The identification of the isolated polyester degrading strains through partial 16S rDNA sequence analysis was carried out by the German Collection of Microorganisms and Cell Cultures (DSMZ, Braunschweig, Germany). Selected isolates were characterized according to the standard methods described by Holdemann et al.28 as well as Krieg.29 These tests were also partially performed by the DSMZ, Germany. More information are given elsewhere.49 Results Evaluation of Anaerobic Biodegradablity by Mass Loss Measurements. The incubation of the polyester films in the anaerobic sludges served as a first indication to what extent the different materials are susceptible to an anaerobic microbial attack. In our previous publication,25 we found a significant mass loss of PHB and PHBV films in anaerobic methane sludge (LS) after 10 weeks at 37 °C. Under the same conditions, the synthetic aliphatic polyester PCL also exhibited a clear microbial attack but mass losses were only about 30% of those of the poly(hydroxyalkanoates). In Figure 2, mass losses of these polymers and additionally of the synthetic aliphatic polyester SP 4/6 and the aliphaticaromatic copolyester BTA 40:60 in cultures with inocula from three different anaerobic environments after 14 weeks incubation at 35 °C are shown.

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As previously observed, PHB and PHBV disintegrated rapidly while PCL exhibited a significant slower degradation. In contrast, for SP4/6 and BTA 40:60 even after an incubation time of more than 3 month, only minor mass losses (less than 2 mg) of the samples could be observed, mainly in the laboratory sludge (LS). Obviously anaerobic degradability of synthetic polyesters is strongly depending on the structure or properties of the material. From the mass loss data shown here, it is not possible to decide reliably if a microbial attack in principle took place or not for the SP 4/6 and BTA 40:60. The tendency of the degradation behavior of the polyesters in the different sludges used here is generally comparable. However, no general rule of the degradation potential of the different sludges can be derived from the data presented here. Evaluation of Anaerobic Biodegradability by Biogas Determination. Although mass loss measurements only give an indication if microorganisms principally can attack a material, the determination of the biogas (predominantly CO2 and CH4) which is formed during degradation reflects directly the metabolic transformation of the material in the microbial cells. In Figure 3, the biogas formation from the polyester samples in the laboratory sludge (LS) and the wastewater sludge (WWS) at 37 °C is shown as a function of the exposure time. The successively decreasing anaerobic degradability PHB > PHBV > PCL, which was already previously observed in the laboratory sludge 25 (Figure 3a), is confirmed by the data obtained in the wastewater sludge (Figure 3b). For the synthetic polyesters SP4/6 and BTA40:60 only very low biogas formation could be observed in LS ( PHBV > PCL . SP 3/6 - SP 4/6 > BTA 10:90 BTA 20:80 (. BTA 40:60) The polyester degrading isolates from Table 7 can be divided into three main groups: (1) The PHB and PHBV degrading isolates are specialized to depolymerize only the natural polyhydroxyalkanoates and cannot attack synthetic polyesters and vice versa. (2) PCL degrading strains showed no depolymerization activity toward other polymers. Hence, no organisms originally screened on PCL degrade SP 3/6, SP 4/6 or BTAcopolymers. (3) In contrast, strains isolated from SP 3/6-enrichment cultures (total of 9) show a wide substrate spectrum within the synthetic polyesters but do not hydrolyze the polyhydroxyalkanoates. Discussion Biological degradation of polyesters under anaerobic conditions differ from that in the presence of oxygen with

Biodegradation of Polyesters

respect to the absolute degradation rates and the influence of the polymer structure on the biodegradation. A large number of publications are available focusing on aerobic biodegradation of natural and also synthetic polymers, but only few papers are dealing with the behavior of biodegradable plastics under anaerobic conditions. Furthermore most of these papers focused on biodegradation in complex microbial environments such as anaerobic sewage sludge or sediments, from which only few reliable facts about mechanistic aspects can be drawn, since anaerobic microbial communities are complex systems. In a previous paper,25 we demonstrated that it is also possible to use individual strains besides anaerobic microbial consortia (four new species of the genus Clostridia were identified25,26) to investigate enzymatically catalyzed depolymerization of poly(hydroxyalkanoates) and PCL, although anaerobic digestion is recognized as a complex process involving the coordinated activity of a number of different bacterial trophic groups.36 It could clearly be shown that PHB, PHBV, and also PCL, which are all well degradable under aerobic conditions, are also microbially attacked in the absence of oxygen. However, differences in aerobic conditions were detected, since the copolyester PHBV exhibited clearly no faster degradation as the hompolyester PHB. In addition to the polyesters mentioned above, only very few other polymers have been evaluated with respect to their anaerobic biodegradability up to now. From the investigations presented here, it can be suspected that synthetic polyesters others than PCL (such as SP 3/6, SP 4/6 and BTA-copolyesters) degraded very slowly under anaerobic conditions, although their ready aerobic degradation has been demonstrated.31,37-39 The long time period of enrichment (18 month), necessary to isolate strains capable to depolymerize these synthetic polyesters, point to the fact that only few special anaerobic organisms are able to attack the chain structures of these polymers. This supposition is supported by the number of degrading strains isolated from the enrichment cultures, decreasing from 30 strains capable to attack the natural poly(hydroxyalkanoates) and 16 strains degrading PCL to only 9 isolates forming clear zones with SP 3/6. The PHB degrading anaerobic isolates seem to be specialized on PHB degradation and metabolization, only. The enzyme, presumably a specific PHB depolymerase, is apparently induced by PHB. Generally, the addition of a second carbon substrate (additional energy source) such as acetate, crotonate or citrate (Table 4) obviously enhanced biomass formation and hence increased indirectly polyester degradation (clear zone formation). PHB depolymerization is mainly stimulated by acetate (80% of the total number of isolates) and crotonate (57% of the 30 isolates), a fact which can be explained by acetate being an intermediary metabolite of many anaerobes 40 and crotonate is a key metabolite in 3-hydroxybutyrate metabolism.41 Glucose on the other hand supported growth of the isolates but not degradation (clear zone formation) with one exception. Obviously, glucose suppresses the PHB depolymerizing enzyme secretion, probably by catabolite repression.40,42 It is known for clostridia that easily degradable substrates might mask

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abilities for biosynthesis and biodegradation and hence extracellular enzymes of polymeric substrates.40,42 It is assumed that the presence of glucose as a growth substrate results in what is known as inducer (3-hydroxybutyrate) expulsion as has been observed for other clostridia.42,43 Accordingly, the PHB depolymerizing enzyme is not induced and PHB is not degraded. In contrast, catabolite repression of the polymer degradation was not observed for the organisms depolymerizing synthetic polyester and rich medium addition enhanced polyester depolymerization. Thus, here no problems with preservation of organisms and/or instability of degradation character occurred. Concerning the anaerobic PCL-degrading strains isolated (Tables 5 and 6), to our knowledge, no reports on characterized anaerobic single strains exist in the literature up to now. These strains are also specialists since they only showed depolymerization activity toward PCL. No organism originally screened on PCL, degrades the other synthetic polyesters SP 3/6, SP 4/6, BTA 40:60 or the natural PHAs. Two strains, identified as Clostridium sp. noV., are lipase negative, although the described aerobic PCL depolymerizing enzymes are reported to be also lipases besides cutinases.44,45,46 Catabolite repression was not observed for PCL degradation, and the strains did not metabolize the depolymerization products, i.e., the monomers of PCL. The hydrolyzing enzyme depolymerizes PCL probably due to structural similarities between PCL and another natural polymer, such as cutin, possessing structurally similar elements.22 Also Murphy et al.47 presented genetic, regulatory, and enzymatic evidence for the involvement of a cutinase in aerobic PCL degradation. In addition, they showed that PCL dimers and trimers are structurally similar to natural inducers of cutinase. It is therefore possible that anaerobic PCL degradation follows the same principle as aerobic PCL degradation. The nine strains (Table 6) able to degrade the other synthetic polyesters (none of them did attack the natural polyhydroxyalkanoates) showed a wide substrate spectrum within the synthetic polyesters. One isolate was taxonomically characterized as Propionispora sp. noV.48 (since the genus Propionispora Vibroides noV. gen., noV sp. has just recently been established by Biebl et al.,49 only a little information is available about these organisms). To our knowledge, this is the first report describing the anaerobic depolymerization of these synthetic polyesters by an individual culture. From the observation that the strain only exhibited a very limited growth of the polyesters, it can be supposed that the isolated strain did not metabolize the depolymerization products of the polyesters. The involved depolymerizing enzyme seems to be quite unspecific and represents probably lipase-like enzyme induced by the presence of gratuitous inducers, i.e., the synthetic aliphatic polyesters. Kleeberg32 documented a similar situation where aerobic BTA depolymerizing strains of Thermobifida fusca (former name: Thermomonospora fusca) secreted an extracellular hydrolase which unspecifically depolymerized the copolyester and several other synthetic aliphatic polyesters, too. The resulting depolymerization products were also not metabolized by the strain.39 In the present case, probably a

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nonspecific lipase producing organism had to adapt its enzyme regulation mechanisms to the synthetic and unusual polyester substrate. Similarly, several strains of clostridia such as C. thermocellum synthesize xylanases, for instance, but grow only poorly on xylan owing to an inability to metabolize the degradation products.50-53 The degradation rate of the aliphatic synthetic polyesters by Propionispora sp. noV. decreases with increasing melting point of the materials (SP 3/6 (44 °C) > PCL (60 °C) > SP 4/6 (62 °C)). This is congruent to observations reported for aerobic degradation of polyesters.31,33,55 Lipases attack the polyester chains quite nonspecifically. Degradation is not controlled predominantly by the chemical structure of the ester bonds but by the ability of the polymer chains to fit into the active site of the lipases, which is located within the enzyme interior. Thus, the degradation is strongly depending on the mobility of the chains, which can be characterized by the difference between the melting temperature of the given polyester and the incubation temperature.33 In contrast to aerobic conditions, the aliphatic-aromatic copolyesters were anaerobically only attacked at low contents of aromatic component (up to 20 mol % terephthalic acid of the acid component). Here, two reasons for this different behavior can be discussed. Generally, the degradation rates in the anaerobic environment are slower than in the presence of oxygen, and thus, it may be that the absolute degradation rates of the BTA-copolyesters with higher amounts of terephthalic acid (exhibiting also high melting points) are to low to be detected with the experimental tools used in this work. However, it cannot be excluded that the anaerobic enzymes (lipases) are not able to cleave any ester bond in vicinity of an aromatic group and, thus, can only attack the copolyesters when extended aliphatic sequences exist. Obviously, under anaerobic as well as under aerobic conditions, at least three different enzyme systems are supposed to be involved in the anaerobic degradation of the different polyesters: PHB depolymerases, lipases, and enzymes described as cutinases. None of the PHA depolymerases shows significant lipase activity or attacks synthetic polyesters.54 However, several lipases hydrolyze polyesters of ω-hydroxyalkanoic acids such as PCL. Cutinases, on the other hand, are serine hydrolases for primary alcohol esters55,56 which depolymerize cutin and synthetic polyesters such as PCL. Acknowledgment. We gratefully acknowledge the support of the scholarship of Mrs. D.-M. Abou-Zeid by the DAAD (Deutscher Akademischer Austauschdienst e.V.). We thank Dr. I. Wagner-Do¨bler for supplying the anaerobic river sediment, BASF, and Novamont for providing polyester samples. References and Notes (1) Gross, R. A.; Kalra, B. Science 2002, 297, 803. (2) Amass, W.; Amass, A.; Tighe, B. Polym. Int. 1998, 47, 89. (3) Braunegg, G.; Lefebvre, G.; Genser, K. F. J. Biotechnol. 1998, 65, 127. (4) Doi, Y. Microbial polyesters; VCH Publishers Inc.: New York, 1990. (5) Okada, M. Prog. Polym. Sci. 2002, 27 (1), 87.

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