A review of plastic waste biodegradation

With more and more plastics being employed in human lives and increasing pressure being placed on capacities available for plastic waste disposal, the need for biodegradable plastics and biodegradation of plastic wastes has assumed increasing importance in the last few years. This review looks at the technological advancement made in the development of more easily biodegradable plastics and the biodegradation of conventional plastics by microorganisms. Additives, such as pro-oxidants and starch, are applied in synthetic materials to modify and make plastics biodegradable. Recent research has shown that thermoplastics derived from polyolefins, traditionally considered resistant to biodegradation in ambient environment, are biodegraded following photo-degradation and chemical degradation. Thermoset plastics, such as aliphatic polyester and polyester polyurethane, are easily attacked by microorganisms directly because of the potential hydrolytic cleavage of ester or urethane bonds in their structures. Some microorganisms have been isolated to utilize polyurethane as a sole source of carbon and nitrogen source. Aliphatic-aromatic copolyesters have active commercial applications because of their good mechanical properties and biodegradability. Reviewing published and ongoing studies on plastic biodegradation, this paper attempts to make conclusions on potentially viable methods to reduce impacts of plastic waste on the environment.     

A Review of Plastic Waste Biodegradation

ABSTRACT

With more and more plastics being employed in human lives and increasing pressure being placed on capacities available for plastic waste disposal, the need for biodegradable plastics and biodegradation of plastic wastes has assumed increasing importance in the last few years. This review looks at the technological advancement made in the development of more easily biodegradable plastics and the biodegradation of conventional plastics by microorganisms. Additives, such as pro-oxidants and starch, are applied in synthetic materials to modify and make plastics biodegradable. Recent research has shown that thermoplastics derived from polyolefins, traditionally considered resistant to biodegradation in ambient environment, are biodegraded following photo-degradation and chemical degradation. Thermoset plastics, such as aliphatic polyester and polyester polyurethane, are easily attacked by microorganisms directly because of the potential hydrolytic cleavage of ester or urethane bonds in their structures. Some microorganisms have been isolated to utilize polyurethane as a sole source of carbon and nitrogen source. Aliphatic-aromatic copolyesters have active commercial applications because of their good mechanical properties and biodegradability. Reviewing published and ongoing studies on plastic biodegradation, this paper attempts to make conclusions on potentially viable methods to reduce impacts of plastic waste on the environment.     

塑料添加剂向生态环境中的释放与迁移研究进展

塑料废弃物,尤其是粒径小于5 mm的微塑料造成的环境污染问题已引起全球的普遍关注。塑料制品在生产过程中常使用多种添加剂,以提高聚合物的性能并延长其使用寿命。然而,在废弃塑料制品的回收及自然老化过程中,这些添加剂会不断释放出来,对生态环境的安全与人类的健康产生威胁。综述了近年来国内外塑料添加剂的使用情况及其向生态环境释放与迁移等方面的研究进展,具体包括常用塑料添加剂的种类、废弃物塑料回收和塑料老化过程中添加剂向生态环境中的释放与迁移及机制等。未来需要更加关注绿色塑料添加剂的研发、废弃塑料回收工艺的改进以及关于塑料添加剂的释放、在各类环境介质中的迁移转化以及在生态系统各个圈层间的相互作用方面的系统性的研究,并构建相应的迁移模型评估塑料添加剂产生的生态风险。     

Insight in to the phylogeny of polyhydroxyalkanoate biosynthesis: horizontal gene transfer

Polyhydroxyalkanoates (PHAs) are gaining more and more importance the world over due to their structural diversity and close analogy to plastics. Their biodegradability makes them extremely desirable substitutes for synthetic plastics. PHAs are produced in organisms under certain stress conditions. Here, we investigated 253 sequenced (completely and unfinished) genomes for the diversity and phylogenetics of the PHA biosynthesis. Discrepancies in the phylogenetic trees for phaA, phaB and phaC genes of the PHA biosynthesis have led to the suggestion that horizontal gene transfer (HGT) may be a major contributor for its evolution. Twenty four organisms belonging to diverse taxa were found to be involved in HGT. Among these, Bacillus cereus ATCC 14579 and Xanthomonas axonopodis pv. citri str. 306 seem to have acquired all the three genes through HGT events and have not been characterized so far as PHA producers. This study also revealed certain potential organisms such as Streptomyces coelicolor A3(2), Staphylococcus epidermidis ATCC 12228, Brucella suis 1330, Burkholderia sp., DSMZ 9242 and Leptospira interrogans serovar lai str. 56601, which can be transformed into novel PHA producers through recombinant DNA technology.     

Engineered biosynthesis of biodegradable polymers

Advances in science and technology have resulted in the rapid development of biobased plastics and the major drivers for this expansion are rising environmental concerns of plastic pollution and the depletion of fossil-fuels. This paper presents a broad view on the recent developments of three promising biobased plastics, polylactic acid (PLA), polyhydroxyalkanoate (PHA) and polybutylene succinate (PBS), well known for their biodegradability. The article discusses the natural and recombinant host organisms used for fermentative production of monomers, alternative carbon feedstocks that have been used to lower production cost, different metabolic engineering strategies used to improve product titers, various fermentation technologies employed to increase productivities and finally, the different downstream processes used for recovery and purification of the monomers and polymers.     

The importance of avoiding chemical contamination for a successful cultivation of marine organisms

1. Using 6 phytoplankton species and/or the copepodEuterpina acutifrons or larvae of the sea urchinArbacia lixula the potential inhibitory effects of chemicals released from some 70 different materials (mainly plastics) have been tested. In addition, the effects of 6 detergents have been examined. 2. Several materials, such as natural rubbers and polyvinyl chlorides, are highly toxic and should never be used when experimenting with living marine organisms. 3. Teflon (Algoflon), Perspex, Polyethylene, Tygon, Polypropylene, Polycarbonates (Makrolon) and Polyester (Gabraster) have been shown to be non-toxic and are, therefore, suitable for use in cultivation of marine organisms. Some materials had slightly negative effects on the organisms tested and should, therefore, be used only if no alternatives are available. 4. Some suggestions are advanced on how to construct non-toxic samplers and laboratory equipment used for experiments with marine organisms.     

Food or just a free ride? A meta-analysis reveals the global diversity of the Plastisphere

It is now indisputable that plastics are ubiquitous and problematic in ecosystems globally. Many suggestions have been made about the role that biofilms colonizing plastics in the environment—termed the “Plastisphere”—may play in the transportation and ecological impact of these plastics. By collecting and re-analyzing all raw 16S rRNA gene sequencing and metadata from 2,229 samples within 35 studies, we have performed the first meta-analysis of the Plastisphere in marine, freshwater, other aquatic (e.g., brackish or aquaculture) and terrestrial environments. We show that random forest models can be trained to differentiate between groupings of environmental factors as well as aspects of study design, but—crucially—also between plastics when compared with control biofilms and between different plastic types and community successional stages. Our meta-analysis confirms that potentially biodegrading Plastisphere members, the hydrocarbonoclastic Oceanospirillales and Alteromonadales are consistently more abundant in plastic than control biofilm samples across multiple studies and environments. This indicates the predilection of these organisms for plastics and confirms the urgent need for their ability to biodegrade plastics to be comprehensively tested. We also identified key knowledge gaps that should be addressed by future studies.Subject terms: Microbial ecology, Soil microbiology, Water microbiology, Microbial ecology, Microbiome     

Agar overlay method to measure adherence of Staphylococcus epidermidis to four plastic surfaces

The comparative adherence of seven strains of Staphylococcus epidermidis to plastic surfaces was measured by an agar overlay technique in which adherent organisms were counted by their ability to form colonies under an agar overlay. The degree of adherence to plastics decreased in the order polystyrene, polyvinyl chloride, Silastic, and polytetrafluoroethylene.     

Agar overlay method to measure adherence of Staphylococcus epidermidis to four plastic surfaces.

The comparative adherence of seven strains of Staphylococcus epidermidis to plastic surfaces was measured by an agar overlay technique in which adherent organisms were counted by their ability to form colonies under an agar overlay. The degree of adherence to plastics decreased in the order polystyrene, polyvinyl chloride, Silastic, and polytetrafluoroethylene.     

Production of biodegradable plastics from activated sludge generated from a food processing industrial wastewater treatment plant

Most of the excess sludge from a wastewater treatment plant (60%) is disposed by landfill. As a resource utilization of excess sludge, the production of biodegradable plastics using the sludge has been proposed. Storage polymers in bacterial cells can be extracted and used as biodegradable plastics. However, widespread applications have been limited by high production cost. In the present study, activated sludge bacteria in a conventional wastewater treatment system were induced, by controlling the carbon: nitrogen ratio to accumulate storage polymers. Polymer yield increased to a maximum 33% of biomass (w/w) when the C/N ratio was increased from 24 to 144, where as specific growth yield decreased with increasing C/N ratio. The conditions which are required for the maximum polymer accumulation were optimized and are discussed.     

Effects of adsorbed organic and primary fouling films on bryozoan settlement

Marine primary fouling films, which consist of molecular organic and microbial components, have been reported to facilitate colonization of immersed surfaces by marine fouling organisms. Larvae of the cosmopolitan fouling bryozoan Bugula neritina (Linnaeus) were offered various substrata for attachment and metamorphosis. The materials were offered (a) after detergent washing, (b) after sorption of dissolved organic molecular films, and (c) after formation of primary films consisting of both microbial and adsorbed organic material. Wettability of the substrata by sea water was determined by contact angle measurements for each substratum. On washed substrata, attachment was favored with contact angles greater than ≈45° (cos contact angle <0.7). Adsorbed surface films had no effect on the low settlement of larvae on glass and high settlement on plastics. Microbial primary films, however, made glass attractive and plastics unattractive. These settlement preference changes did not correlate with the changes in wettability observed on these substrata. Dispersion of larvae over the settlement surface was random except on wettable surfaces coated with bacterial films, where settlement was strongly clustered (contagious).     

Marine plastic litter as an artificial hard bottom fouling ground

20 fouling organisms were observed on plastic litter dredged from the Elbe estuary during July 1990; 60% of the species were typical sessile hard bottom organisms. Most individuals found on this artificial hard bottom were barnacles (Balanus crenatus, Elminius modestus), the musselMytilus edulis and the polychaeteLanice conchilega. All individuals were juveniles which had settled only recently on the plastics. The earliest settlers were not much older than 4–8 weeks.     

A catchment‐scale perspective of plastic pollution

Plastic pollution is distributed across the globe, but compared with marine environments, there is only rudimentary understanding of the distribution and effects of plastics in other ecosystems. Here, we review the transport and effects of plastics across terrestrial, freshwater and marine environments. We focus on hydrological catchments as well‐defined landscape units that provide an integrating scale at which plastic pollution can be investigated and managed. Diverse processes are responsible for the observed ubiquity of plastic pollution, but sources, fluxes and sinks in river catchments are poorly quantified. Early indications are that rivers are hotspots of plastic pollution, supporting some of the highest recorded concentrations. River systems are also likely pivotal conduits for plastic transport among the terrestrial, floodplain, riparian, benthic and transitional ecosystems with which they connect. Although ecological effects of micro‐ and nanoplastics might arise through a variety of physical and chemical mechanisms, consensus and understanding of their nature, severity and scale are restricted. Furthermore, while individual‐level effects are often graphically represented in public media, knowledge of the extent and severity of the impacts of plastic at population, community and ecosystem levels is limited. Given the potential social, ecological and economic consequences, we call for more comprehensive investigations of plastic pollution in ecosystems to guide effective management action and risk assessment. This is reliant on (a) expanding research to quantify sources, sinks, fluxes and fates of plastics in catchments and transitional waters both independently as a major transport routes to marine ecosystems, (b) improving environmentally relevant dose–response relationships for different organisms and effect pathways, (c) scaling up from studies on individual organisms to populations and ecosystems, where individual effects are shown to cause harm and; (d) improving biomonitoring through developing ecologically relevant metrics based on contemporary plastic research.     

生物降解聚烯烃类塑料研究进展

聚烯烃类塑料是一类以C–C键为骨架的高分子材料,被广泛应用于日常生活的各个领域。由于具有稳定的化学性质并且难以被环境中的微生物快速降解,聚烯烃塑料废弃物在全球范围内持续积累,造成了严重的环境污染及生态危机。近年来,利用生物方法降解聚烯烃类塑料引起了研究人员的广泛关注。自然界丰富的微生物资源为生物降解聚烯烃类塑料废弃物提供了可能,已经有一些对聚烯烃塑料具有降解能力的微生物被陆续报道。本文总结了聚烯烃类塑料生物降解资源及生物降解机制的研究进展,提出了目前聚烯烃类塑料生物降解过程存在的问题,并对未来的研究方向进行了展望。     

Accumulation and fragmentation of plastic debris in global environments

One of the most ubiquitous and long-lasting recent changes to the surface of our planet is the accumulation and fragmentation of plastics. Within just a few decades since mass production of plastic products commenced in the 1950s, plastic debris has accumulated in terrestrial environments, in the open ocean, on shorelines of even the most remote islands and in the deep sea. Annual clean-up operations, costing millions of pounds sterling, are now organized in many countries and on every continent. Here we document global plastics production and the accumulation of plastic waste. While plastics typically constitute approximately 10 per cent of discarded waste, they represent a much greater proportion of the debris accumulating on shorelines.Mega- and macro-plastics have accumulated in the highest densities in the Northern Hemisphere, adjacent to urban centres, in enclosed seas and at water convergences (fronts). We report lower densities on remote island shores, on the continental shelf seabed and the lowest densities (but still a documented presence) in the deep sea and Southern Ocean. The longevity of plastic is estimated to be hundreds to thousands of years, but is likely to be far longer in deep sea and non-surface polar environments. Plastic debris poses considerable threat by choking and starving wildlife, distributing non-native and potentially harmful organisms, absorbing toxic chemicals and degrading to micro-plastics that may subsequently be ingested. Well-established annual surveys on coasts and at sea have shown that trends in mega- and macro-plastic accumulation rates are no longer uniformly increasing: rather stable, increasing and decreasing trends have all been reported. The average size of plastic particles in the environment seems to be decreasing, and the abundance and global distribution of micro-plastic fragments have increased over the last few decades. However, the environmental consequences of such microscopic debris are still poorly understood.     

An updated overview on the regulatory circuits of polyhydroxyalkanoates synthesis

Polyhydroxyalkanoates (PHA) are a promising and sustainable alternative to the petroleum-based synthetic plastics. Regulation of PHA synthesis is receiving considerable importance as engineering the regulatory factors might help developing strains with improved PHA-producing abilities. PHA synthesis is dedicatedly regulated by a number of regulatory networks. They tightly control the PHA content, granule size and their distribution in cells. Most PHA-accumulating microorganisms have multiple regulatory networks that impart a combined effect on PHA metabolism. Among them, several factors ranging from global to specific regulators, have been identified and characterized till now. This review is an attempt to categorically summarize the diverse regulatory circuits that operate in some important PHA-producing microorganisms. However, in several organisms, the detailed mechanisms involved in the regulation of PHA synthesis is not well-explored and hence further research is needed. The information presented in this review might help researcher to identify the prevailing research gaps in PHA regulation.     

An overview of lipid metabolism in yeasts and its impact on biotechnological processes

High energy prices, depletion of crude oil supplies, and price imbalance created by the increasing demand of plant oils or animal fat for biodiesel and specific lipid derivatives such as lubricants, adhesives, and plastics have given rise to heated debates on land-use practices and to environmental concerns about oil production strategies. However, commercialization of microbial oils with similar composition and energy value to plant and animal oils could have many advantages, such as being non-competitive with food, having shorter process cycle and being independent of season and climate factors. This review focuses on the ongoing research on different oleaginous yeasts producing high added value lipids and on the prospects of such microbial oils to be used in different biotechnological processes and applications. It covers the basic biochemical mechanisms of lipid synthesis and accumulation in these organisms, along with the latest insights on the metabolic processes involved. The key elements of lipid accumulation, the mechanisms suspected to confer the oleaginous character of the cell, and the potential metabolic routes enhancing lipid production are also extensively discussed.     

Bioproduction of polyhydroxyalkanoates from bacteria: a metabolic approach

The advent of molecular biological techniques and a developing environmental awareness initiated a renewed scientific interest in Polyhydroxyalkanoates (PHAs) and the biosynthetic machinery for PHA metabolism has been the area of research over the last two decades. PHAs are polyesters of hydroxyalkanoates synthesized by numerous bacterial species with atleast five different PHA biosynthetic pathways. These are accumulated as an intracellular carbon and energy storage material. This diversity, in combination with genetic and molecular engineering has opened up this area for development of optimum PHA producing organisms. Even though PHAs have been recognized as a good candidate for biodegradable plastics, their industrial application is limited owing to high production cost. The classical microbiology and modern molecular biology have been brought together to decipher the intricacies of PHA metabolism both for production purposes and for the unraveling of the natural role of PHA. This review provides an overview of the different PHA biosynthetic systems, the enzymes involved in PHA biosynthesis and there genetic background followed by a detailed summation of how this natural diversity is being used to develop commercially attractive recombinant process for large scale production of PHAs.     

Transport and release of chemicals from plastics to the environment and to wildlife

Plastics debris in the marine environment, including resin pellets, fragments and microscopic plastic fragments, contain organic contaminants, including polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons, petroleum hydrocarbons, organochlorine pesticides (2,2′-bis(p-chlorophenyl)-1,1,1-trichloroethane, hexachlorinated hexanes), polybrominated diphenylethers, alkylphenols and bisphenol A, at concentrations from sub ng g^–1 to µg g^–1. Some of these compounds are added during plastics manufacture, while others adsorb from the surrounding seawater. Concentrations of hydrophobic contaminants adsorbed on plastics showed distinct spatial variations reflecting global pollution patterns. Model calculations and experimental observations consistently show that polyethylene accumulates more organic contaminants than other plastics such as polypropylene and polyvinyl chloride. Both a mathematical model using equilibrium partitioning and experimental data have demonstrated the transfer of contaminants from plastic to organisms. A feeding experiment indicated that PCBs could transfer from contaminated plastics to streaked shearwater chicks. Plasticizers, other plastics additives and constitutional monomers also present potential threats in terrestrial environments because they can leach from waste disposal sites into groundwater and/or surface waters. Leaching and degradation of plasticizers and polymers are complex phenomena dependent on environmental conditions in the landfill and the chemical properties of each additive. Bisphenol A concentrations in leachates from municipal waste disposal sites in tropical Asia ranged from sub µg l^–1 to mg l^–1 and were correlated with the level of economic development.