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Plastics: Microbial Degradation, Environmental and Biotechnological 1

Perspectives 2

Dominik Danso1, Jennifer Chow1 and Wolfgang R. Streit1 3

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1Department of Microbiology and Biotechnology, University of Hamburg, Hamburg, 5

Germany 6

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Keywords: microbial plastic degradation, polystyrene, polyamides, polyvinylchloride, 14

polypropylene, polyurethane, polyethylene terephthalate, lipase, cutinase, PET esterase 15

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The authors declare no conflict of interest. 23

*Correspondence 24 E-Mail: wolfgang.streit@uni-hamburg.de 25 Department of Microbiology and Biotechnology, 26 University of Hamburg, Ohnhorststr.18, 27 D-22609 Hamburg, Germany 28 Tel. (+49) 40-42816-46329

AEM Accepted Manuscript Posted Online 19 July 2019Appl. Environ. Microbiol. doi:10.1128/AEM.01095-19Copyright © 2019 Danso et al.This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license.

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Plastics are widely used in our economy and each year, at least 350-400 million 30

tons are being produced. Due to poor recycling and low circular use, millions of tons 31

accumulate annually in terrestrial or marine environments. Today it has become clear 32

that plastic causes adverse effects in all ecosystems and that microplastics are of 33

particular concern to our health. Therefore, recent microbial research has addressed 34

the question, if and to which extent microorganisms can degrade plastics in the 35

environment. This review summarizes current knowledge on microbial plastic 36

degradation. Enzymes available act mainly on the high molecular weight polymers of 37

polyethylene terephthalate (PET) and ester-based polyurethane (PUR). Unfortunately, 38

the best PUR- and PET-active active enzymes and microorganisms known still have 39

moderate turnover rates. While many reports have been published describing 40

microbial communities degrading chemical additives, no enzymes acting on the high 41

molecular weight polymers polystyrene, polyamides, polyvinylchloride, 42

polypropylene, ether-based polyurethane and polyethylene are known. Altogether 43

these polymers stand for more than 80 % of the annual plastic production. Thus, 44

further research is needed to significantly increase the diversity of enzymes and 45

microorganisms acting on these polymers. This can be achieved by tapping into the 46

global metagenomes of non-cultivated microorganisms and dark matter proteins. 47

Only then, novel biocatalysts and organisms can be delivered that allow rapid 48

degradation, recycling or value-added use of the vast majority of most man-made 49

polymers. 50

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INTRODUCTION 59

Altogether, synthetic polymers are produced worldwide at a scale of at least 350-400 60

million metric tons annually (1-5). The main polymers that are produced and of importance to 61

our economy are: Polyurethane (PUR), Polyethylene (PE), Polyamide (PA), Polyethylene 62

terephthalate (PET), Polystyrene (PS), Polyvinylchloride (PVC) and Polypropylene (PP 63

(FIGURE 1). With an increasing production and use of plastics it is estimated that 5-13 64

million metric tons of plastic enter the ocean every year with negative consequences for 65

various ecosystems and for health of humans and animals (1, 2, 6). Regarding only the great 66

Pacific garbage patch, more than 1.8 trillion pieces of plastic with an estimated weight of 67

80,000 tons have so far accumulated with no end in sight (7-10). While recently few reviews 68

have been published focusing on the degradation of single types of plastic, only very few 69

articles have addressed plastic degradation on a more global scale addressing the 70

degradation of several synthetic polymers (11). Therefore, the two main questions addressed 71

in this review are: 72

(i) Which enzymes and microorganisms are currently known to be involved 73

in high molecular weight polymer plastic degradation? 74

(ii) What are the future challenges and technologies for identifying better 75

enzymes acting on a highly diverse range of synthetic polymers? 76

Intriguingly, the currently best known route of plastic destruction involves exposure to 77

UV light together with mechanical disruption caused by waves and winds or grinding on 78

marine rocks and sediments which eventually breaks larger plastics into smaller pieces of 79

micro- and nanoplastics (MP, with sizes < 5 mm and NP, with sizes less than 0.1 μm). The 80

so-called “weathering” and “photodegradation” are currently considered the main forces for 81

initial depletion of plastics and it mainly results in a modification of the chemical, physical and 82

mechanical properties of the plastics (12, 13). The resulting particles have a much larger 83

surface area, which makes them amenable to further degradation (14). Notably, MPs and 84

NPs are a concern to our health as it is expected that they enter the food chain and by this 85

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end up in our intestines (15, 16). The fate of MPs or NPs in human or animal intestines has 86

yet to be determined. 87

Therefore, removal of plastics from the environment using microbial enzymes has 88

been in the focus of recent research. The main challenge is that marine and terrestrial 89

displaced plastics are highly stable and durable. Plastics had mainly been introduced since 90

the 60ies of the last century and, given the relatively few decades since these man-made 91

polymers are available, nature only had very short time to evolve highly active enzymes. 92

Besides, many different types of plastics accumulate in the environment and many of the 93

frequently used plastics are mixtures containing additional solubilizers and other chemical 94

agents to alter the mechanical and physical properties. These compounds are further targets 95

for microbial biodegradation but may also interfere with degradative enzyme activities. 96

Thereby, it is assumed that the larger polymers are initially degraded by secreted 97

exoenzymes into smaller subunits (multimers, dimers) that can be incorporated into the cells. 98

Once in the cells, either the oligomers or the degradation products of these are funneled 99

through the classical degradation pathways to yield energy and/or suit as building blocks for 100

catabolism or metabolism. 101

Within this framework, this review summarizes the main findings on microbial 102

degradation of the above listed polymers. The chemical structures and some properties of 103

theses polymers are described at each of the following subchapters. For a first overview on 104

enzymes and microbes acting on the different plastics see FIGUREs 1 & 2. 105

In general, it is believed that the microbial degradation of man-made polymers is a 106

very slow process. This high resistance mainly stems from the high molecular weight of the 107

fiber, the strong C-C bonds and the extremely hydrophobic surface, which is very difficult to 108

attack by enzymes. Notably, polymers are high molecular weight molecules, and they have 109

amorphous and crystalline forms, which have different degradability. 110

111

Polyethylene terephthalate (PET). PET is mainly used for production of PET bottles, 112

PET foil and fibers in textile industry. PET is a polar, linear polymer of repeating units of the 113

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aromatic terephthalic acid and ethylene glycol. The PET monomer is designated bis(2-114

hydroxyethyl) terephthalate (19). PET is a thermoplast and partially shows crystallization. 115

The annual production of PET exceeded 30 million tons in 2017 (3). 116

Currently, only few bacteria and fungi have been described for the partial degradation 117

of PET to oligomers or monomers (11). All known PET hydrolases have relatively low 118

turnover rates. Intriguingly, the trait for PET degradation appears to be limited to a few 119

bacterial phyla and most bacterial isolates with the potential of PET degradation are 120

members of the Gram-positive phylum of Actinobacteria (20). Thereby, the best 121

characterized examples originate from the genera Thermobifida or Thermomonospora (21-122

28). The enzymes involved in the degradation (e.g. PET hydrolase and tannase, MHETase) 123

are typical serine hydrolases e.g. cutinases (EC 3.1.1.74), lipases (EC 3.1.1.3) and 124

carboxylesterases (EC 3.1.1.1). These enzymes possess a typical /-hydrolase fold and the 125

catalytic triad is composed of a serine, a histidine and an aspartate residue (23, 29). They 126

can also contain several disulfide bonds caused by cysteine residues, which promote thermal 127

stability and specific binding to PET as shown on the example of PETase from Ideonella 128

sakaiensis 201-F6 (30). 129

Also, for the bacterium I. sakaiensis, usage of PET as a major energy and carbon 130

source was described (30). In addition to the PET hydrolase, the I. sakaiensis genome codes 131

for a second enzyme that appears to be unique so far and which shares high similarity to the 132

group of tannases, capable of degrading mono(2-hydroxyethyl) terephthalic acid. PET 133

hydrolase as secreted enzyme produces the intermediate mono(2-hydroxyethyl) terephthalic 134

acid (MHET). MHET is internalized by the cell and hydrolyzed by the MHETase. The 135

resulting monomers are then used for the bacterial metabolism. I. sakaiensis is affiliated with 136

the phylum of Betaproteobacteria and belongs to Burkholderiales. 137

The I. sakaiensis PETase 3D-structure was elucidated recently (31). The overall 138

structure resembles mostly the structures of cutinases. Austin et al. showed that a double 139

mutation (S238F/W159H), narrowing the active site of the enzyme and making the protein 140

even more like a cutinase resembling the enzyme from Thermobifida fusca, lead to an 141

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improved variant. The majority of the functionally verified PET hydrolases contain a C-142

terminal disulfide bond, promoting thermal stability and also kinetic stability (32-34). The only 143

exception from this so far is a para-nitrobenzylesterase from Bacillus subtilis (35). An 144

additional disulfide bond can be found in I. sakaiensis PETase as well as in structural models 145

of the functionally tested PET hydrolases described by Danso et al. (36). The structural data 146

indicate that PETases bind the polymer with the hydrophobic surface and the substrate-147

binding cleft. In total, 4 MHET moieties are bound to the protein (one to subsite I and three to 148

subsite II), whereby the ester bond to be cleaved is located between both subsites next to 149

the catalytic serine. The MHETase from I. sakaiensis that further hydrolyzes MHET to 150

ethylene glycol and terephthalic acid has been recently crystallized ligand-free (2.05 Å) and 151

with a non-hydrolyzable MHET analogue bound (2.1 Å). The enzyme possesses a lid-domain 152

that almost exclusively confers substrate specificity and activity towards MHET with a kcat of 153

11.1 ± 1.4 s−1 (37). 154

While the I. sakaiensis enzymes are the best-studied models, other enzymes and 155

organisms have been identified as potent PET degraders. Currently, four enzymes from 156

Thermobifida species, one from Saccharomonospora, and one from the phylum of 157

Thermomonospora are known to act on PET. These actinobacterial enzymes are often Ca2+-158

dependent, especially in terms of their thermal stability (38) and they are partially inhibited by 159

their released hydrolysis products MHET and BHET (38). Therefore, efforts have been made 160

to overcome this limitation and one approach lies in the combination of polyester hydrolases 161

with other enzymes to improve substrate binding and catalytic properties (31, 39, 40). 162

Besides the actinobacterial PET hydrolases, fungal cutinases showed activity on PET 163

substrates as well. The most prominent examples are cutinases of the phyla Fusarium and 164

Humicola. The latter was also used together with the lipase CalB from Candida antarctica in 165

order to circumvent the before mentioned product inhibition by bis-(hydroxyethyl) 166

terephthalate (BHET) and MHET (41). While CalB completely converted to terephthalic acid, 167

the Humicola-derived enzyme was limited in the last reaction step and accumulated the 168

intermediate MHET. 169

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Complementary to the above outlined activity-based approaches, a HMM (Hidden-170

Markov-Model) motif-based large-scale global search of existing genome and metagenome 171

databases has been developed for the presence of potential PET hydrolases (36). Using this 172

approach, >800 potential PET hydrolases were identified in bacteria, archaeal genomes and 173

metagenomes and several enzymes were functionally verified (e.g. PET2, PET4, PET6 and 174

PET12). These findings imply that PET hydrolase-encoding genes are globally distributed in 175

marine and terrestrial metagenomes (36). 176

Using an in silico genome mining approach, a cutinase from Pseudomonas 177

pseudoalcaligenes (PpCutA) and a putative lipase from Pseudomonas pelagia (PpelaLip) 178

were identified as potential enzymes acting on polyesters in general. Further experimental 179

work using recombinant enzymes of PpCutA and PpelaLip verified the hydrolytic activities of 180

both enzymes on different types of polyesters including the hydrolysis of polyoxyethylene 181

terephthalate (42). In their study, the authors used structurally different ionic phthalic acid-182

based polyesters with an average molecular weight ranging from 1,770 to 10,000 g/mol and 183

semi crystalline polyesters with crystallinity below 1% to test and verify the microbial 184

degradation. Notably, the identified organism belongs to a biotechnologically important novel 185

species within the genus Pseudomonas designated P. pertucinogena (43). 186

In addition to the metagenome-derived PET esterases described above, colleagues 187

recently reported on the functional screening of metagenomes and the characterization of 188

selected enzymes. Among those, the metagenome-derived esterases MGS0156 and 189

GEN0105, which hydrolyzed polylactic acid (PLA), polycaprolactone as well as 190

bis(benzoyloxyethyl)-terephthalate. For MGS0156, 3D-structural data at 1.95 Å indicate a 191

modified α/β-hydrolase fold, with a lid domain and a highly hydrophobic active site (44). The 192

closest homologue to MGS0156 is an enzyme from Desulfovibrio fructosivorans with 70% 193

sequence similarity. 194

In summary, PETases represent the best explored and studied class of enzymes with 195

respect to the hydrolysis of synthetic polymers. 196

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Polyurethanes (PUR) can be synthesized by using different polyether or polyester 198

polyols. PUR is a polymer of organic units connected by carbamate. The additional 199

incorporation of aromatic ring structures has further impact on the physical and chemical 200

properties of the polymer. PUR is a widely used synthetic polymer for the production of 201

foams, insulation materials, textile coatings and paint to prevent corrosion (45). With over 27 202

tons produced annually (2) it ranks as number 5 of the most often produced synthetic 203

polymers. 204

Up to date, only bioactivities have been reported that act on the ester-based PUR 205

(46, 47). Biodegradation was achieved by either bacteria or fungi. With respect to bacteria 206

capable to degrade PUR, most frequently Gram-negative ß-Proteobacteria from the genus 207

Pseudomonas have been linked with PUR activities. One of the first enzymes identified to act 208

on PUR was the PueB lipase from Pseudomonas chlororaphis (48, 49). This organism codes 209

for at least one additional enzyme active on PUR which was designated PueA (50). Both 210

enzymes are lipases and PUR is degraded by the secreted hydrolases and the degradation 211

is tightly regulated. The respective genes are part of a larger gene cluster encompassing 212

seven ORFs (51). P. protegens strain Pf-5 uses a similar mechanism to degrade dispersions 213

of the polyester PUR. In this strain, it was however shown that PUR degradation is tightly 214

regulated by mechanisms of carbon catabolite control and that both lipase genes, designated 215

pueE and pueB appear to be essential for growth on PUR dispersions (52). In a similar 216

manner Pseudomonas putida was reported to degrade PUR at relatively high rates (53). The 217

bacterium needed four days to grow and consume the added colloidal PUR. Yet another 218

example comes from Comamonas acidovorans TB-35. The strain produces a PUR-active 219

enzyme which is an esterase, and which was designated PudA (54, 55). PudA shows a 220

hydrophobic PUR-surface-binding domain and a distinct catalytic domain and its surface-221

binding domain was considered as being essential for PUR degradation. PudA acts as a 62 222

kDa monomer and it releases diethylene glycol and adipic acid at an optimum temperature of 223

45 °C and a pH optimum of 6.5. 224

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Within this context, it is perhaps notable that often enzyme activities are reported 225

which are based on clearing zones in agar plates. However, these assays are not fully 226

reliable. For instance, different enzymes from Pseudomonas spp. and Bacillus spp. showed 227

significant esterase activities and partially or even completely cleared plates containing 228

colloidal PUR. However, only the Pseudomonas sp. lipase significantly degraded the added 229

PUR based on NMR and IR data (56). Furthermore, there is strong evidence that some B. 230

subtilis and Alicycliphilus sp. isolates are able to degrade PUR (57-59). 231

In a recent publication, Schmidt and colleagues reported on microbial degradation of 232

PUR (i.e. Impranil DLN). The authors of this study employed the known polyester hydrolases 233

LC cutinase, TfCut2, Tcur1278 and Tcur0390 in their assays and observed significant weight 234

loss of the tested foils when incubated for extended time periods (200 hours) at a 235

temperature of 70 °C (60). The observation that cutinases, otherwise known to degrade 236

polyethylene terephthalate, also act on PUR could be attributed to a promiscuous nature of 237

the Thermobifida-derived cutinases. Recent research on promiscuity of enzymes implies that 238

especially lipolytic enzymes such as cutinases, are very often highly promiscuous and can 239

convert up to 78 different substrates (61). 240

While the list of PUR-active bacteria is steadily increasing, also a larger number of 241

fungi have been reported to degrade polyurethane (47). Notably, the authors of this study 242

identified a 21 kDa metallo-hydrolase from Pestalotiopsis microspora as the responsible 243

enzyme. 244

Additional studies identified Fusarium solani and Candida ethanolica (62) and C. 245

rugosa (63) as PUR degraders. While for C. rugosa, a lipase has been identified as the key 246

enzyme involved in PUR metabolism, no enzymes were yet identified for C. ethanolica and 247

S. solani. Other fungi reported belong to the Cladosporium cladosporioides complex, 248

including the species C. pseudocladosporioides, C. tenuissimum, C. asperulatum, and C. 249

montecillanum, and the three others were identified as Aspergillus fumigatus and Penicillium 250

chrysogenum (64) and Aspergillus flavus (65). In the case of A. flavus, it is assumed that 251

secreted esterases are responsible for the degradation. However, no defined enzyme has 252

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yet been linked to the observed activities. In a similar study, it was recently reported that 253

Aspergillus tubingensis colonizes PUR and acts on the surface of films made of PUR. 254

However, no enzyme was linked with the PUR activities (66). 255

Noteworthy, the above mentioned PUR-active enzymes and organisms were all 256

acting on ester-linked PUR. However, to the best of our knowledge, no enzymes have yet 257

been described acting on polyurethane ethers. 258

259

Polyethylene (PE) consists of long chain polymers of ethylene and it is produced 260

as either high-density (HD-PE) or low-density polyethylene (LD-PE). PE is chemically 261

synthesized by polymerization of ethane and is highly variable, since side chains can be 262

obtained depending on the manufacturing process. Such modifications have mainly 263

influence on crystallinity and molecular weight. The polymer is most frequently used in the 264

packaging industry as one of the main packaging materials and more than 100 million tons 265

of PE are globally produced (2, 67) (FIGURE 2). 266

Possible PE degradation has been affiliated with a surprisingly large number of 267

bacterial genera. Among those were Gram-negatives affiliated with the genera of 268

Pseudomonas, Ralstonia and Stenotrophomonas and but also many Gram-positives (e.g. 269

Rhodococcus, Staphylococcus, Streptomyces, Bacillus, and others) see references in (68) 270

and (69). In addition, fungal genera affiliated with assumed PE-degradation were reported 271

including Aspergillus, Cladosporium, Penicillium, and others (see references in (68, 69 70-272

75)). In addition, few studies linked the PE-degrading microbes with complex gut 273

microbiomes of invertebrates (76, 77). 274

It is notable, that in almost all the above-mentioned studies on PE-degrading 275

microorganisms, the authors reported on degradation of the polymers using commercial 276

polymers that possibly contained chemical additives and degradation was determined by 277

measuring weight loss and by FTIR spectroscopy. Since weight loss and surface structure 278

changes are most likely attributed to the degradation of chemical additives, often making a 279

significant fraction of the polymer, the results in these studies need to be verified using more 280

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advanced technologies. None of these studies reveled biochemical mechanisms and 281

enzymes involved in PE-breakdown. Within this framework, a more recent publication 282

identified a Penicillium-derived laccase as potentially involved in PE-breakdown (78). 283

Unfortunately, no detailed biochemical characterization was performed and no sequence of 284

the protein or the corresponding gene was deposited. 285

286

Polyamide (PA) is a polymer of repeating units of aliphatic, semi-aromatic or 287

aromatic molecules linked via amide bonds. Since the monomers for making this polymer 288

can be very versatile, there are many different types of synthetic polyamides, with the most 289

popular being Nylon and Kevlar. Synthetic polyamides are mainly used in textiles, automotive 290

applications, carpets and sportswear (79). 291

Remarkably, proteins as well as natural silk are polyamides per se. Based on this, it 292

should be expected that nature has evolved enzymes acting on these non-native polymers. 293

However, to date, there is no microorganism known being able to fully degrade the intact 294

high molecular weight polymer. On the contrary, several studies are available on bacteria 295

acting on either linear or cyclic nylon oligomers with rather short chain lengths. In one of the 296

first studies, different bacteria were described to grow on various oligomers derived from 297

nylon production (80). In wastewater of nylon factories, 8-caprolactam, 6-aminohexanoic 298

acid, 6-aminohexanoic acid cyclic dimer and 6-aminohexanoic acid oligomers accumulate. 299

These compounds can serve as carbon and nitrogen source for specially adapted bacteria. 300

One of the first bacteria described growing on these mixtures of oligomers was 301

Flavobacterium sp. KI72, which was later renamed to Achromatobacter guttatus KI72 and 302

only recently named Arthrobacter sp. KI72 (80, 81). Nylon oligomer-degrading Arthrobacter 303

isolates code in their genomes for different hydrolases and several aminotransferases 304

involved in the initial degradation of the oligomers and the subsequent metabolism. In the 305

case of the strain KI72, the respective genes are located on an accessory plasmid pOAD2 306

(82-84). 307

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Mainly three enzymes are essential for the initial hydrolysis of cyclic and linear 6-308

aminohexanoate oligomers. The first one is a cyclic-dimer hydrolase (NylA), the second a 309

dimer hydrolase (NylB) and finally, the third an endo-type oligomer hydrolase (NylC). NylC is 310

a typical esterase, but its 3-D structure also reveals motifs with ß-lactamase folds (85-93). 311

Once the oligomers are hydrolyzed, the monomers are metabolized by different amino 312

transferases. The draft genome of Arthrobacter sp. KI72 carries, among others, two genes 313

designated nylD1 and nylE1 that are responsible for the secondary 6-aminohexanoate 314

metabolism. The 6-aminohexanoate aminotransferase (NylD1) catalyzes the reaction of 6-315

aminohexanoate to adipate semialdehyde. It uses α-ketoglutarate, pyruvate and glyoxylate 316

as amino acceptors and generates glutamate, alanine, and glycine, respectively. The 317

reaction relies on pyridoxal phosphate as a cofactor. The second enzyme, the adipate 318

semialdehyde dehydrogenase (NylE1) catalyzes the reaction leading from adipate 319

semialdehyde to adipate. This enzyme requires NADP+ as a cofactor and is an 320

oxidoreductase (94, 95). 321

More recently, diverse marine bacteria were reported to act on nylon. The authors of 322

this study reported a significant weight loss over a time period of three months. In their study, 323

Bacillus cereus, Bacillus sphaericus, Vibrio furnisii and Brevundimonas vesicularis were 324

identified as potential nylon degraders (96). The genes and enzymes associated with the 325

nylon degradation, however, remain to be identified and it cannot be excluded that the weight 326

loss observed was primarily linked to the degradation of chemical additives as outlined 327

above. 328

Rather than using the synthetic polymer, Oppermann and colleagues reported on 12 329

bacterial species capable to degrade the natural polymer poly-γ-glutamic acid. The high 330

molecular weight polymer is synthesized by many Gram-positive bacteria as a major 331

component of capsules and slime. In contrast to the synthetic polymer, it is, however, a 332

water-soluble molecule and thus easier accessible to microbial degradation (97). 333

The only enzyme that was so far reported to act on high molecular weight nylon fibers 334

was classified as a manganese-depending peroxidase and originated from a white rot 335

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fungus. The activity of the native and purified enzyme, however, differed from that of 336

lignolytic enzymes. Nylon-degrading activity was quantified by measuring the structural 337

disintegration of nylon-66 membranes. The enzyme had a molecular weight of 43 kDa and 338

was dependent on the presence of lactate and other alpha-hydroxy acids. Unfortunately, no 339

gene or protein sequence was determined (98). 340

While the first reports have been published already in 1965 stating that among others 341

Pseudomonas aeruginosa is able to convert oligomeric nylon, further studies have confirmed 342

this and evolved strain PAO1 to efficiently degrade 6-aminohexanoate linear dimers (80, 99). 343

The main enzymatic activities were assigned to a 6-aminohexanoate cyclic dimer hydrolase 344

and a 6-aminohexanoate dimer hydrolase. Other Pseudomonas species have, however, also 345

been reported to utilize 6-aminohexanoate-dimers as sole carbon and nitrogen source (100). 346

347

Polystyrene (PS) (poly(1-phenylethene) polymer consists of styrene monomers. PS 348

is a widely used synthetic polymer for packaging industries but also many daily use articles 349

(CD cases, plastic cutlery, petri dishes, etc.) are produced from this polymer (101). In 2016, 350

about 14 million tons were produced (3). 351

Unfortunately, today there is no enzyme known that can degrade the high molecular 352

weight polymer. With this respect, a first report was published recently by Krueger and 353

colleagues on the identification of brown-rot fungi able to attack polystrol employing 354

hydroquinone-driven Fenton reactions. In this preliminary study, Gloeophyllum striatum DSM 355

9592 and Gloeophyllum trabeum DSM 1398 caused substantial depolymerization after 20 356

days of incubation. The most active Gloeophyllum strains caused almost 50% reductions of 357

the molecular weight (102). In an earlier study, the white rot fungi Pleurotus ostreatus, 358

Phanerochaete chrysosporium, Trametes versicolor and the brown rot fungus Gloeophyllum 359

trabeum have been affiliated with the depolymerization of polystyrene when co-incubation 360

together with lignin was performed (103). While these are first and promising reports on the 361

degradation of the high molecular weight polymer, the enzymes involved in the 362

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depolymerizing reaction, however, remain to be elucidated. As already outlined above, 363

weight loss may have been caused by the degradation of chemical additives. 364

Similarly, several bacteria have been reported to form either alone or as members of 365

consortia biofilms on polystyrene films and particles and thereby degrading the polymer. In 366

these studies, mainly weight loss has been assayed. Unfortunately, in none of these studies 367

enzymes were linked to the assumed depolymerization (104, 105). 368

While not a single bacterium is known to degrade the polymer, a larger number of bacterial 369

genera are known that are capable of metabolizing the monomer styrene as a sole source of 370

carbon. The biochemistry of styrene metabolism is well understood and for reviews that are 371

more detailed see (104, 106-109) and references therein. Styrene degradation in bacteria is 372

well studied in Pseudomonas, Xanthobacter, Rhodococcus, Corynebacterium and others. It 373

appears to be a widespread metabolism. Under aerobic conditions, styrene is oxidized by 374

two different pathways: (i) attacking the vinyl side chain and (ii) a rather unspecific aromatic 375

ring. Thereby, mainly the intermediates 3-vinylcatechol, phenylacetic acid, and 2-376

phenylethanol are formed. These intermediates are channeled into the Krebs cycle after ring 377

cleavage. The degradation of the vinyl side chain involves the action of three key enzymes: a 378

styrene monooxygenase, a styrene oxide isomerase, and a phenylacetaldehyde 379

dehydrogenase (110). The styrene monooxygenase attacks the vinyl side chain to release 380

epoxystyrene, which is then subjected to isomerization to form phenylacetaldehyde. The 381

latter is oxidized to phenylacetic acid though the involvement of a dehydrogenase. In P. 382

putida, the phenylacetic acid is activated to phenylacetyl-CoA and then subjected to ß-383

oxidation to yield acetyl-CoA, which is directly fed into the Krebs-cycle. The respective genes 384

for the side-chain oxygenation are frequently located in a single conserved gene cluster often 385

designated styABC(D) (111). Thereby, the styA and styB genes code for the styrene 386

monooxygenase complex. The styrene monooxygenase is a two-component flavoprotein that 387

catalyzes the NADH- and FAD-dependent epoxidation of styrene to styrene oxide. StyA is 388

the actual monooxygenase and StyB functions as FAD reductase that transfers the electrons 389

from NADH to FAD+, to supply StyA with the required electrons (112). The styC gene codes 390

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for the styrene isomerase (113) and styD is a phenylacetaldehyde dehydrogenase gene 391

(114). The expression of the conserved cluster is regulated through either a two-component 392

regulatory systems or LysR-type regulators (115-117). 393

The direct ring cleavage of styrene is initiated by a dihydroxylation of the aromatic 394

ring. This reaction is catalyzed by a 2,3-dioxygenase and followed by a 2,3-dihydrodiol 395

dehydrogenase. The two key products that are formed are styrene cis-glycol and 3-396

vinylcatechol. The latter can then be degraded by subsequent meta- or ortho-cleavage to 397

form acrylic acid, acetaldehyde and pyruvate. The pathway is rather unspecific for the 398

general degradation of various aromatic compounds such as phenol or toluene (106-108). 399

The produced phenylacetaldehydes are of interest to different industries as they can 400

be considered as building blocks for the production of different fine chemicals or 401

pharmaceutical compounds. They can serve as the starting material to synthesize 402

fragrances, flavors, pharmaceuticals, insecticides, fungicides or herbicides (118). With this 403

respect, recent studies have also shown that Pseudomonas putida, Rhodococcus zopfii, and 404

other Gram-negatives can convert polystyrene (i.e. styrene oil) into the biodegradable 405

polymer polyhydroxyalkanoate or other valuable compounds. The approach involves as a 406

first step the pyrolysis of polystyrene to styrene oil. The styrene oil is then converted in a 407

second step to polyhydroxyalkanoate or other compounds. While the overall concept of this 408

two-step process is intriguing, it may not be feasible on large scale as the pyrolysis is a 409

process that runs at 520°C and this is energetically very demanding (119-121). 410

411

Polyvinylchloride (PVC) and Polypropylene (PP) are both important polymers 412

produced at higher levels compared to the above-named polymers. PVC is the third most 413

frequently produced polymer and only PE and PP are produced at higher levels. PVC is 414

composed of repeating chloroethyl units and PP of repeating units of propane-1,2-diyl units 415

(122, 123). In sharp contrast to their huge global production rate, hardly any reliable 416

information is available on microbial degradation of both of these important polymers. Only 417

very few reports have been published that describe the degradation of the polymers based 418

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on weight loss and using mixed species microbial communities (124, 125). However, it is 419

likely, that these reports were in part misled by the degradation of the chemical additives 420

rather than the polymer. Consequently, no defined enzymes or pathways are known that are 421

responsible for the degradation of any of these two high molecular weight polymers. 422

423

Microbiomes of invertebrates as possible sources of plastic degrading 424

bacteria 425

Recently, it was reported that invertebrates can degrade different plastics (76, 426

77,126-129). While these studies demonstrated that the insects perform a mechanical 427

grinding and shredding of the plastics, it has been critically discussed, if, and to which extent, 428

the microbiomes associated with the different insects are capable to truly degrade the 429

synthetic polymers. In one of those studies, Yang and colleagues provided convincing 430

evidence that Tenebrio molitor Linnaeus (mealworms) digested Styrofoam. The larvae lived 431

over a month, when fed on the Styrofoam. Within a 16-day period, nearly 50% of the 432

ingested Styrofoam carbon was converted into CO2 and the residual Styrofoam was found in 433

the feces. Labeling studies using α 13C- or β 13C-labeled polystyrol implied that the carbon 434

compound was preferentially used to build lipids (77). One of the earliest reports on insects 435

digesting plastics came from caterpillars. In 2017, a Spanish team reported on the fast bio-436

degradation of PE by larvae of the wax moth (Galleria mellonella). The authors of this study 437

presented evidence that larvae of the wax moth produced holes in PE films with considerable 438

speed (126). The finding of this study was critically discussed later on, as the occurrence of 439

ethylene glycol as well as the correct usage of the FTIR method could not be immediately 440

verified (127). Further work by a Chinese and US-based research team identified Bacillus sp. 441

YP1 as polyethylene-degrading bacterium responsible for the PE-degradation in Indian 442

mealworms (76, 128). A related study from the same group identified bacteria affiliated with 443

the genera Citrobacter and Kosakonia as main degraders for PE and PS in the guts of 444

Tenebrio molitor (129). 445

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Thus, grinding of larger plastic pieces into smaller parts might offer a solution that it 446

increases the surface and thereby allows microorganisms to better attach to the surfaces. 447

448

Future challenges in microbial plastic degradation research. 449

The diversity of known enzymes and microbes acting on synthetic polymers is still 450

rather limited. Therefore, future work has to address the identification of organisms acting on 451

the most dominant polymers. Thereby, the main bottleneck lies in the initial breakdown of the 452

high molecular weight and highly robust polymer and its crystalline structure. Further, the 453

implementation of enzymes in processes that would allow the degradation of plastic polluting 454

environmental niches is a challenge for future generations of microbiologist. Since current 455

cultivation technologies have not yet resulted in the identification of highly active enzymes for 456

most plastics, the diversity of the non-cultivated (i.e. global metagenomes) and the so-called 457

dark matter proteins offer a promising source for the identification of such biocatalysts. Thus, 458

the further development of smart search algorithms for mining metagenome data sets is 459

certainly a rewarding task. In parallel, the set-up of reliable function-based assays for the 460

detection of high molecular weight polymer-active enzymes is important as well. 461

Since often commercially available polymers and films thereof are used as substrate, 462

they contain additives, plasticizers and other biodegradable impurities (for example 463

phthalates), which are much easier broken down than the actual backbone. This however 464

interferes with the results and frequently leads to the identification of false positives. Thus, 465

the overall methodology linked to the analysis of microbial plastic degradation needs to be 466

standardized and optimized. 467

Similarly, the development of cellulosome-like structures (i.e. “plastosomes”) in 468

microbes to attack the intact and crystalline fiber would certainly be a worthwhile project. In 469

this line, the simple development of highly active enzymes for textile industries could already 470

significantly reduce the annual plastic pollution and would perhaps be one of the more 471

realistic short-term goals. 472

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Further, using synthetic biology to generate microorganisms that would produce high-473

value compounds from plastic waste would be a future challenge and it would contribute to 474

an improved circular use of plastics. Thereby monomers and oligomers formed after the 475

degradation could be used to build value-added products or even new (biodegradable) 476

polymers. 477

Lastly, obtaining plastic-active enzymes and implementing them in the production of 478

true biopolymers is a highly rewarding research task and would significantly reduce our 479

global plastic problem. 480

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853 854

855

856

ACKNOWLEDGMENTS 857

This work was in part supported by the BMBF within the program MarBiotech (FKZ 858

031A565), the EU Horizon 2020 project INMARE and MetaGenLig (FKZ 031B0571B) 859

at the University of Hamburg. 860

861

FIGURE LEGENDS 862

863

FIGURE 1: Main synthetic polymers globally produced in 2016. Numbers in the chart 864

indicate the global annual production (millions of tons) of the specified synthetic 865

polymer. Global annual plastic production was extracted from references (1-7). 866

Monomers are depicted above the chart. Indicated are the names of bacterial genera 867

producing verified enzymes with available protein sequences that are known to be 868

involved in the breakdown of the high molecular weight polymers (not the additives, 869

plasticizers, etc.). For detailed references on the individual enzymes, we refer to the 870

main text. For PA, PE, PS, PVC and PP no defined enzymes on the level of amino 871

acid or DNA sequences haven been identified that act on the polymer. For enzymes 872

acting on dimers or oligomers and feeding them into the different metabolic pathways 873

see main text. For additional structural information on the polymers we refer to 874

https://www.ebi.ac.uk/chebi/init.do. 875

876

FIGURE 2: A) Electron microscopic images of Comamonas sp. strain DDHH 01 877

attached and hydrolyzing PET fibers. Comamonas sp. DDHH 01 was isolated from a 878

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sewage enrichment culture. Red arrows indicate PET fibers. Black and white arrows 879

indicate bacterial cells. Top image: Transmission electron microscopy image of a 880

PET fiber with attached Comamonas sp. cells. Middle image: Scanning electron 881

microscope image of PET yarn with microcolonies. Bottom: Close‐up of a single cell 882

on the surface of a single PET fiber. B) Topology of a neighbor joining tree 883

containing representative sequences of most of the currently known synthetic 884

polymer or oligomer/monomer degrading enzymes. The tree is based on amino acid 885

sequence homologies. Overall, 886

27 known functional and verified enzymes were included in this alignment. This 887

represents the majority of the currently known and biochemically characterized 888

enzymes. PET hydrolases represent the largest fraction of known and studied 889

enzymes. The alignment was calculated using T-Coffee in accurate mode (17). The 890

tree was calculated with Mega6 (18) and is not rooted. A similarity and identity matrix 891

for all included sequences is provided in the supplementary material as TABLE S1 892

together with their accession numbers. 893

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