15 Sustainable Marine Structures | Volume 03 | Issue 02 | July 2021 Distributed under creative commons license 4.0 http://dx.doi.org/10.36956/sms.v3i2.432 Sustainable Marine Structures http://ojs.nassg.org/index.php/sms/index REVIEW Knowledge and Opportunities from the Plastisphere: A Prelude for the Search of Plastic Degrading Bacteria on Coastal Environments Luis Felipe Avilés-Ramírez1 Joanna M. Ortiz-Alcantara2 Ma. Leticia Arena-Ortiz2* 1. Faculty of Sciences, National Autonomous University of Mexico (UNAM), Yucatán, Mexico 2. Laboratory of Ecogenomic Studies, Faculty of Sciences, National Autonomous University of Mexico (UNAM), Yucatán, Mexico ARTICLE INFO ABSTRACT Article history Received: 2 August 2021 Accepted: 30 August 2021 Published Online: 15 September 2021 Plastic pollution has become an urgent issue, since its invasion to every ecosystem has led to multiple impacts on the environment and human pop- ulations. Certain microbial strains and genera had shown the ability to bio- degrade plastic sources under laboratory conditions. In this minireview, we collect and analyze scientific papers and reports of this microbial activity as we contextualize this information on the global plastic pollution problem, to provide an updated state of the art of plastic biodegradation with microbial agents. Along with a broad understanding of the general process of plastic biodegradation hosted by microorganisms. The contributions of this mini- review come from the identification of research gaps, as well as proposals for new approaches. One of the main proposals focuses on coastal environ- ments and in particular coastal wetlands as a great microbiome source with potential for plastic biodegradation, whether reported or undiscovered. Our final proposal consists of the application of this knowledge into technologic tools and strategies that have a remarkable impact on the battle against the plastic pollution problem. Keywords: Plastic biodegradation Bacteria Plastisphere Bioremediation Coastal environments 　 *Corresponding Author: Ma. Leticia Arena-Ortiz, Laboratory of Ecogenomic Studies, Faculty of Sciences, National Autonomous University of Mexico (UNAM), Yucatán, Mexico; Email: leticia.arena@ciencias.unam.mx 1. Introduction The plastic pollution problem Plastics are synthetic or semi-synthetic polymers main- ly produced from petrochemicals, characterized by their high resistance/density relationship, their great thermal and electric isolation properties, as well as resistance to acids, alkalis and solvents [1]. These materials have ap- plications in multiple industries and business sectors like trading, packing, building materials, medical and pharma- ceutical uses, automotive industry, home appliances, agri- culture and many mass production products for the every- day human consumption [2,3]. These polymers pollute and invade a lot of environ- ments due to the single-use and improper waste manage- ment. The main plastic types considered environmental pollutants are High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), Polyvinyl Chloride (PVC), Polystyrene (PS), Polypropylene (PP), Polyester (PES), Polyamide (PA) and Polyethylene terephthalate (PET) [4]. Plastic pollution has become one of the biggest envi- ronmental threats by invading every ecosystem. It has been reported the presence of plastic debris (microplastics) 16 Sustainable Marine Structures | Volume 03 | Issue 02 | July 2021 Distributed under creative commons license 4.0 from deep-sea sediments [5], to the highest areas of the mount Everest [6]. The presence of microplastics in the hu- man placenta has been documented as well [7]. Around 13 million tons of plastic have been thrown to oceans annually, besides the global annual production of 300 million tons of plastic debris [8]. In México, nearly 83,343 tons of trash are collected daily [9] and in the mu- nicipal solid waste composition, plastic spans 10.9% [10]. Main consequences of this pollutant relay on contami- nation of water sources like rivers, lakes or oceans leading to the formation of continental sized trash patches in the middle of the Pacific Ocean [11]. Also, additives and other compounds present in plastics impact on coastal, marine and inland soils, damaging their physicochemical proper- ties and fertility [12]. Biotic impacts come from mutilation, intoxication and asphyxia of marine, coastal and terrestrial organisms [13]. Human population affectations include the intake of mi- croplastic polluted food [14] to neurotoxic damage from PAH´s, Phthalates and PCB´s [15]. Furthermore, plastic residues represent public health and economic hazards to human communities. 2. Coastal Wetland Ecosystem Goods and Services Worldwide ecosystems shelter goods and services that provide human populations resources for their basic needs; these so-called ecosystem services and natural cap- ital [16] are in decay because of environmental issues such as plastic contamination. This situation is a threat to glob- al human economics and welfare, ever since biodiversity is linked in so many complex ways to ecosystem function- alities and their output ecosystem services [17]. Particularly, coastal wetlands provide human popu- lation wide and diverse goods and services due to the enormous biodiversity within these ecosystems. Some of these goods and services are categorized as ecosystem services framework [18]. Supporting ecosystem services include the primary production of microbiotic (bacteria, fungi, protozoea) and macrobiotic (migratory birds and commercial interest fish and crustaceans) life forms, soil formation and enrichment. Provisioning goods and services are represented by sources like fresh water, seafood, honey and woods. Regulating functions such as climate regulation by carbon dioxide sequestration and flood and storms mitigation. The Cultural category is covered as aesthetics, recreational, educative and spiritual values for native human population as well as foreign people through ecotourism [19]. 3. Microbial Biodegradation of Plastics Environmental plastic pollution solutions have become an urgent subject and an interesting approach could be just below ourselves. Diverse microbes have shown the ability to degrade plastics; particularly bacterial strains isolated from different environments that have been studied and reported in diverse scientific publications. Some of these papers are reviewed in the present study. Bacteria have been widely researched in plastic bio- degradation matter, since they are easy for cultivation and isolation, and the facility for bioprospecting met- abolic pathways, ecological functions and subproduct related information through metagenomic analysis and sequencing of the 16S ribosomal gene [20,21]. In this re- view, scientific evidence of plastic biodegradation host- ed by bacteria dates from at least 1991 [22,23]. In general, research has focused on confirming and assessing plas- tic degradation by bacteria either through in vitro or in situ assays. Depolymerization activity is usually measured with diverse methodology such as visual assessment through detecting roughening, cracking and biofilm formation [24], clear-zone tests [25], weight loss estimates in microbial exposure [26], respiratory activity evaluation such as CO2 production or O2 consumption [27]. As well as detection of the activity of specific enzymes or byproducts of depo- lymerization activity [28,29]. 4. Bacterial Biodegradation State of the Art In the present review, we summarize some bacterial strains and species that have the ability to biodegrade plastic, with complementary information about the hab- itat of the bacteria, type of plastic degraded, byproducts, enzyme and the bibliographic reference to the paper. This information is categorized and divided based on the type of environment where the bacteria were isolated. Table 1 shows previous scientific research results from bacteria isolated from anthropic environments, such as municipal waste disposal sites, sewage water, industrial activated sludges or purchased pre-cultured strains from laboratories and microbial strains collections. Aquatic environment native bacteria are represented in Table 2. Aquatic environments that hold bacterial sources are wide and diverse and some of the most studied envi- ronments are sea water and soil, abyssal water and soil, freshwater like rivers and lakes. Some of the least studied are coastal soil and water, highlighting coastal wetland ecosystems. http://dx.doi.org/10.36956/sms.v3i2.432 17 Sustainable Marine Structures | Volume 03 | Issue 02 | July 2021 Distributed under creative commons license 4.0 Table 1. Municipal waste disposal sites/Other Human environments ID Habitat Plastic type degraded Degradation by- products Enzyme Reference Pseudomonas, Penicillium, Rhodotorula, Hyalodendron Landfill Low Density Polyethylene (LDPE), Polyurethane (PU), Polyvinyl chloride (PVC) Polyethylene glycol — Alcano monoxigenase, same as found on hydrocarbon biodegradation (Seneviratne, 2006) [26] Ideonella sakaiensis Sediment from PET- recycling site. Polyethylene terephthalate (PET) Terephthalic acid (TPA) & ethylene glycol (EG) Mono(2-hydroxyethyl) (MHET) terephthalic acid [30] Pseudomonas Final waste deposition site Low Density Polyethylene (LDPE) — — [31] Pseudomonas MYK1 and Bacillus MYK2 Digester sewage sludge Polylactic Acid (PLA) CO2 and CH4 — [27] Comamonas acidovorans City soil samples Polyester-type polyurethanes Adipic acid and diethylene glycol — [32] Pseudomonas aeruginosa Previously isolated microorganisms (Microteca/microlibrary) PET, PU, PP, ABS, HDPE, PVC, ABS, PS — — [24] Acidovorax delafieldii City soil samples Poly(tetramethylene succinate)-co- (tetramethylene adipate) (PBSA) — Lipase [33] Bacillus subtilis, Bacillus cereus, Bacillus lentus, Pseudomonas aeruginosa, Staphylococcus aureus, Klebsiella pneumoniae, Streptococcus pyogenes, Escherichia coli, Proteus vulgaris, Micrococcus. FADAMA soil Polythene plastic bags and environmental plastic materials — Polyurethanases (Koutny et al., 2006). [34] Brevibacillus borstelensis Soil from Polyethylene waste deposition site Branched low-density polyethylene — — [35] Rhodococcus ruber Sediments with polyethylene debris from agriculture use Polyethylene — Esterases [36] Streptomyces viridosporus, Streptomyces badius and Streptomyces setonii Enzymes from cultured S. viridosporus, S badius and S setonii Starch-polyethylene degradable plastic films Primary and secondary alcohols — [23] Pseudomonas putida (AJ) and Ochrobactrum (TD) Dangerous waste disposal site Vinyl Chloride — — [37] Pseudomonas fluorescens Naval Research Laboratory, Wahington D.C. Polyester polyurethane — Enzyme with protease activity [38] Thermomonospora fusca Compost from green waste Aliphatic-Aromatic Copolyesters (synthesized from 1,4-butanediol, adipic acid, and terephthalic acid (BTA)) — — [39] Schlegelella thermodepolymerans and Pseudomonas indica (K2) Activated sludge Poly(3- hydroxybutyrate-co-3- mercaptopropionate). [poly(3HB-co- 3MP)] 3HB Oligomer linked as thioester Poly(3- hydroxybutyrate)(3HB) depolymerase [40] http://dx.doi.org/10.36956/sms.v3i2.432 18 Sustainable Marine Structures | Volume 03 | Issue 02 | July 2021 Distributed under creative commons license 4.0 ID Habitat Plastic type degraded Degradation by- products Enzyme Reference Clostridium botulinum and Clostridium acetobutylicum Sewage slugde and methane sludge Poly(b-hydroxybutyrate) (PHB), poly(b- hydroxybutyrate- co-11.6%-b- hydroxyvalerate) (PHBV) and the synthetic polyester poly(o- caprolactone) (PCL) — PCL depolymerizing [25] Bacillus amylolyticus, Bacillus firmus, Bacillus subtilis, Pseudomonas putida, Pseudomonas fluorescens Municipal solid waste from compost plant Polyethylene bags — — [41] Bacillus cereus, B. megaterium, B. subtilis, Brevibacillus borstelensis Culture fields Polyethylene Hydrocarbons (saturated and unsaturated) and alcohols of higher molecular weight — [42] Rhodococcus rhodoshrous Purchased isolates from American Type Culture Collection High density polyethylene (HDPE), Low density polyethylene (LDPE) and Linear low-density polyethylene (LLDPE) films with balanced content of antioxidants and pro-oxidants (manganese+iron or manganese+iron+cobalt) — — [43] Actinomadura sp. S14, Actinomadura sp. TF1, Streptomyces sp. APL3 y Laceyella sp. TP4 Compost soil Polyester biodegradable plastics; polylactic acid (PLA), polycaprolactone (PCL), poly-(butylene succinate) (PBS) & polybutylene succinate- co-adipate (PBSA) — S14, TF1 & TP4 produced PLA and PBSA depolymerase (50°C), APL3 (40°C). Actinomadura sp. S14 (PCL depolymerase) Actinomadura sp.TF1 (PLA depolymerase), Streptomyces sp. APL3 (PBS depolymerase), Laceyella sp.TP4 (PBSA depolymerase) [44] Paenibacillus amylolyticus TB-13 (Bacillus amylolyticus) Sediment samples from multiple sites Poly(lactic acid), poly(butylene succinate), poly(butylene succinateco-adipate), poly(caprolactone) and poly(ethylene succinate) — Proteases and esterases [45] Streptomyces sp. AF-111 Sewage sludge from Treatment Plant Rawalpindi Pakistan Poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV) — PHBV Depolymerase [28] Bacillus sp. AF8, Pseudomonas sp AF9, Micrococcus sp. 10, Arthrobacter sp. AF11, and Corynebacterium sp. AF12 Soil from plastic deposition sites Poly [4,4’-methylenebis (phenyl isocyanate)-alt- 1,4-butanediol/poly (butylene adipate)] (Polyurethane, PU) p-nitrophenol (Spectroscopy), CO2 (Sturm test) Esterase Polyurethanases (plate assay with Coomassie blue R 250). [29] PN24 Bacillus cereus, PN12 Bacillus pumilus, LNR3 Arthrobacter. Waste deposition sites and artificially developed soil beds containing maleic anhydride glucose, and small pieces of polyethylene. High and low-density polyethylenes (HDPE/ LDPE) — — [46] http://dx.doi.org/10.36956/sms.v3i2.432 19 Sustainable Marine Structures | Volume 03 | Issue 02 | July 2021 Distributed under creative commons license 4.0 Table 2. Marine, Freshwater and coastal environments ID Habitat Plastic type degraded Degradation by-products Enzyme Reference Pseudomonas, Staphyloccoccus, Moraxella, Micrococcus, Streptococcus Mangrove soil Polyethylene bags and plastic cups — — [47] Pseudomonas (Pseudomonas stutzeri) River water High molecular weight Polyethylene Glycols (PEG´s) Glyoxylic acid PEG dehydrogenase (Single polypeptide) [22] Rhodobacteraceae, Rhodospirillaceae, Oceanospirillaceae, Glaciecola Seawater Poly(3-hydroxybutyrate- co-3hydroxyhexanoate) (PHBH) — — [48] Shenawella (CT01), Moritella (CT12, JT01, JT04), Psychrobacter (JT05) and Pseudomonas (JT08) Marine dephts soil Poly ε-caprolactone (PCL), aliphatic polyesters — — [49] Terrabacter tumescens, Terracoccus luteus, Brevibacillus reuszeri, Agrobacterium tumefaciens, Burkholderia vietnamiensis, Duganella zoogloeoides, Pseudomonas lemoignei, Ralstonia eutropha, Ralstonia pickettii, Matsuebacter chitosanotabidus, Roseateles depolymerans, Rhodoferax fermentans, Variovorax paradoxus, Serratia marcescens, Acinetobacter calcoaceticus, Acinetobacter junii, Pseudomonas pavonaceae, Pseudomonas rhodesiae, Pseudomonas amygdali, Pseudomonas veronii Soil samples (ando-soil, woody area at Shima- Tsakuba, brown lowlands soils, sandy riverside soil and riverside mud) Poly(β- hydroxyalkanoate), poly(ε- caprolactone), poly(hexamethylene carbonate), or poly(tetramethylene succinate) — — [50] Bacillus mojavensis TH309 Bio-deteriorated plastic waste from Tidal zone on Carsamba coast of Samsun Poly(e-caprolactone) (PCL) — Esterase (BmEST) [51] Alcalinovorax, Hyphomonas and Cycloclasticus. Seawater and soil samples Poly(ethylene terephthalate) (PET) & Biodegradable Plastic bags (BD) Insoluble by-products of the hydrolytic degradation of PET. Esterase [52] Pseudomonas pachastrellae JCM12285 Marine plastic debris in coastal seawater Poly(ε-caprolactone) (PCL) — PCL hydrolase [53] Gammaproteobacteria, Alphaproteobacteria, and Flavobacteria (Class level) Microplastics exposed to seawater from coastal zones Polypropylene (PP) and polyvinyl chloride (PVC) microplastics — — [54] Arcobacter and Colwellia Three types of coastal marine sediment from Spurn Point, Humber Estuary, U.K. Low Density Polyethylene (LDPE) microplastics — — [55] Bacillus cereus and Bacillus gottheilii Sediments from Matang mangrove in Perak Polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), and polystyrene (PS) — — [56] Lysinibacillus fusiformis strain VASB14/WL and Bacillus cereus strain VASB1/TS Rhizosphere samples from mangrove (Aviccenia marina) soil Polythene — — [57] http://dx.doi.org/10.36956/sms.v3i2.432 20 Sustainable Marine Structures | Volume 03 | Issue 02 | July 2021 Distributed under creative commons license 4.0 Table 3. Macrofauna gut microbiome (Holobionts) ID Habitat Plastic type degraded Degradation by-products Enzyme Reference Escherichia, Shigella, Asaia and Acinetobacter,Rhodocytophaga, Bergeyella, Diaphorobacter, Hydrogenophaga, Zhizhongheella. Gut microbiome from Galleria mellonella Low-density polyethylene (LDPE) Glycol — [59] Enterobacteriaceae, Spiroplasmataceae, and Enterococcaceae Gut microbes from T. obscurus and T. molitor (purchased from insect breeding plants) Polystyrene (PS) — — [60] Dyella, Lysobacter, and Leptothrix Microbes from larvae T. molitor gut. larvae purchased from market Polystyrene (PS) and Low-density polyethylene (LDPE) Foams — — [61] Family Enterobacteriaceae, Sphingobacteriaceae, and Aeromonadaceae Microbiome gut from Soil Snail A. fulica purchased from Jiaxing Hong- Fu Breeding farm (Zhejiang, China), Expanded polystyrene (PS) foam — — [62] Serratia sp. strain WSW Microflora from P. davidis larvae gut Polystyrene (PS) Styrofoam — — [63] Actinobacteria (Microbacterium awajiense, Rhodococcus jostii, Mycobacterium vanbaalenii and Streptomyces fulvissimus) and Firmicutes (Bacillus simplex and Bacillus sp.) Earthworm´s gut LDPE Volatile compounds (octadecane, eicosane, docosane and tricosane) and nanoplastics [64] Bacillus and Serratia Microbial gut from G. mellonella L. larvae purchased from Huiyude Co. Polyethylene (PE) and polystyrene (PS) Long chain fatty acids as the metabolic intermediates of plastics in the residual polymers — [65] Microbial research has focused on new sources. One of the most popular in recent years is the gut microbiome from organisms such as Coleoptera, Lepidoptera, Lum- bricidae and certain mollusks. Some organisms contain bacterial strains capable of degrading complex chemical structured materials such as wax or wood timbers. Assays on these organisms and their gut microbiota have shown a certain capacity of degrading plastic samples as well (Table 3). These host organisms could also be referred to as holobionts [58]. 5. General Process of Biodegradation by Microorganisms For a better comprehension of the topic, a deeper un- derstanding of the biodegradation process is required. Thus this metabolic mechanism is the core of the activity and eventual application of plastic degradation through bacterial strains. The biodegradation process may differ depending on the genera and species, but the main process is illustrated in Figure 1 and described as follows: The general microbial biodegradation process can be described in three stages: Biodeterioration, biofragmenta- tion and assimilation [66]. At the time environmental abiotic degradation occurs vias Mechanical, Chemical, Photocat- alytic Thermal and Ozone-induced degradation [67]. Biodeterioration It all begins when the plastic material is exposed to the environment (where abiotic degradation factors join the process), then, bacteria start to settle down into the plastic surface to form a biofilm or the so called “Plastisphere” [68]. Microbial activity of consortia (i.e. protein and enzymatic activity) causes deterioration of physical, mechanical and chemical properties of the polymer, leading to cracking of surfaces, formation of oligomers, monomers, as well as http://dx.doi.org/10.36956/sms.v3i2.432 21 Sustainable Marine Structures | Volume 03 | Issue 02 | July 2021 Distributed under creative commons license 4.0 carbon and nitrogen sources [66]. Biofragmentation In order to reach polymer assimilation, microorganisms have to break polymer bonds for cellular absorption of ol- igomers and monomers, since polymer chemical structure is too big and complex to be directly absorbed by micro- organisms [69]. This goal is reached through the secretion of polymer-specific enzymes and free radical generation [66]. Action of extracellular enzymes on a polymer is gen- erally defined as the concept of depolymerization [70]. Bio- fragmentation could lead to microplastic formation if the plastic media is not assimilated yet by microorganisms [64]. Assimilation This stage is defined for the real absorption of the plas- tic atoms into the microbial cell; providing essential needs such as energy, electrons and elementary sources like carbon, nitrogen and phosphorus. Microorganisms are able to sustain and reproduce at the time they produce energy via aerobic respiration, anaerobic respiration or fermentation [66]. As a result of polymer cleavage, monomer/oligomer absorption and metabolic processation; microbes can re- lease mineral molecules, contributing to natural biogeo- chemical processes, as well as organic molecules, which some could be ecotoxic threats under certain conditions and degrees [66]. Mineralization differs by the presence of carbon dioxide and water under aerobic conditions, and methane and carbon dioxide for anaerobic [67]. Some byproducts laid by microbial metabolic activity through the biodegradation process could have potential use for other technological or industrial uses and applica- tions [71]. The chemical reactions resulting from plastic biodegra- dation through aerobic and anaerobic respiration are illus- trated by Equation 1 and Equation 2. C plastic + O2  CO2 + H2O + C residual + Biomass Equation 1. Aerobic microbial biodegradation of plas- tic [72]. C plastic  CH4 + CO2 + H2O + C residual + Biomass Equation 2. Anaerobic microbial biodegradation of plastic [72]. Aerobic microbial biodegradation of plastics (Equation 1) is performed by the use of oxygen as electron acceptor, breaking down organic chemicals into smaller organic compounds or monomers. Carbon dioxide and water are excreted as byproducts of the cleavage. Meanwhile, in an- aerobic microbial biodegradation (Equation 2), microbes set nitrate, sulphate, iron, manganese and carbon dioxide as electron acceptors due to lack of oxygen for the cleav- age and formation of smaller compounds [72]. All these stages of the general biodegradation process of plastic through microbial activity are illustrated in Figure 1. Figure 1. General biodegradation process by microorgan- isms. The three stages of biodeterioration are illustrated as detailed previously in this review. Note that the Chem, Mech, Photo, °C and O3 symbols represent Chemical degradation, Mechanical degradation, photodegradation, temperature and Ozone, as they are abiotic drivers for plastic degradation on the environment. 6. Conclusions Some remarkable features about the state of the art is that bacterial strains come mostly from the bias to the hu- man environment, rather than natural ecosystems. Waste and trash disposal sites, recycling sites, city soil samples and laboratory or purchased strains are the most common origins for plastic degrading bacteria. Otherwise, if natural environments are also well covered, most research papers focus on marine environments, mainly in seawater expo- sure experiments, leaving a great research gap and an op- portunity for studies on coastal and wetland environments, considering that these ecosystems have great biodiversity. Microbial biodegradation of plastic is known to be an environmentally friendly, cheap and acceptable way for plastic waste treatment [29], so waste management actions should pay some attention to these potential opportunities. This knowledge could be applied into technological developments for bioremediation or biomitigation of plas- tic polluted ecosystems. As well as integrate to municipal waste management plans, which even today are not well designed nor applied to most urban and rural locations. Incursions into new strategies and solutions to plas- tic pollution will cause positive impacts on the world's ecosystems and human population, with the participation of every government level, as well as corporations and non-governmental organizations. Some other positive impacts of development and action on this subject rely on the achievement of United Nation´s 2030 Sustainable De- velopment Goals. In such objectives as Good Health and well-being, clean water and sanitation, climate action, life below water, life on land and partnerships for the goals. Industrial companies could benefit from economic incen- http://dx.doi.org/10.36956/sms.v3i2.432 22 Sustainable Marine Structures | Volume 03 | Issue 02 | July 2021 Distributed under creative commons license 4.0 tives, positive publicity and product mark-up as a result of extending their value chain responsibility by contributing into plastic-pollution mitigation initiatives. 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