Conditions for a Microbial Consortium for the Biological

Degradation of Plastic Polymers

Xinyue Liu

and Finn Stirling

Wuhan Britain China School, Wuhan, China

The University of Cambridge, U.K.

Keywords: Biodegradation, Polymer, Plastic, Consortium, Microbiology.

Abstract: Plastic pollution is one of the most serious environmental problems the world faces. Conventional

approaches to plastic waste, such as landfilling, incineration and recycling are not sufficient to maintain an

equilibrium between plastic production and decomposition. Since plastics can provide an energy source for

microbes, biotic degradation could contribute a solution towards excessive plastic waste. Over the last 30

years, hundreds of species have been described to degrade one or more of the six most common polymer

types. By combining multiple types of species to form consortias, the efficiency of degradation can be

synergistically increased. In this study, we consider the factors that would contribute to a microbial

consortium, and what combinations of species would most effectively degrade plastic polymers. Abiotic

factors such as temperature, oxygen level, pH, UV exposure, moisture level, carbon availability and

abundance of trace elements are considered. The origin of individual microbes is considered with regard to

their compatibility. The usefulness and shortcomings of reported degradation efficiencies is discussed. A

theoretical consortium is recommended, taking into account individual strain decomposition efficiency and

the comprehensiveness of plastic degradation.

1 INTRODUCTION

Plastic pollution is one of the greatest environmental

concerns of our time, adversely affecting most

biomes and wildlife across the globe. The growth in

volume of plastic production alongside the longevity

of most plastic polymers have both contributed to

this issue. Annual plastic production has increased

rapidly since 1940, with global plastic production

reaching 365 million tons in 2019. The most

common plastics polymers, which account for about

80% of worldwide plastic production, are

polyethylene (PE), polypropylene (PP), polyvinyl

chloride (PVC), polystyrene (PS), polyethylene

terephthalate (PET) and polyurethane (PU). It has

been predicted that in the next decade, if there are no

improvements to manage the pollution, 99 million

tons of plastic waste will end up in the environment

by 2030.

Due to its longevity in nature, and its propensity

to be used as a disposable item, the output of plastic

https://orcid.org/0000-0001-5123-0402

https://orcid.org/0000-0002-5960-4429

polymers vastly outpaces its natural degradation.

Plastic can take from 20 to 500 years to decompose,

depending on its structure and chemical

composition. The longevity of plastic is estimated to

be hundreds to thousands of years, and is likely to be

even longer in deep sea and non-surface polar

environments.

2 METHODS

Conventional approaches to plastic waste, including

landfilling, incineration and recycling, all come with

their own drawbacks. Land filling can lead to

contamination of earth's surface and result in

anaerobic production of methane gas which

contributes to climate change. Furthermore, because

of the large footprint required by landfills, the

habitat of animals may be affected and displaced. In

addition, landfilling produces leachate that pollutes

the surrounding water and soil. The incineration of

waste plastic material produces toxic gases, which

can result in human health complications such as

lung disease and carcinomas. During the process of

198

Liu, X. and Stirling, F.

Conditions for a Microbial Consortium for the Biological Degradation of Plastic Polymers.

DOI: 10.5220/0011195700003443

In Proceedings of the 4th International Conference on Biomedical Engineering and Bioinformatics (ICBEB 2022), pages 198-203

ISBN: 978-989-758-595-1

 2022 by SCITEPRESS – Science and Technology Publications, Lda. All r ights reserved

burning plastic, dangerous chemicals like

hydrochloric acid, sulfur dioxide, dioxins, furans and

heavy metals can be discharged. These kinds of

emissions are known to be carcinogenic and can

cause respiratory diseases. The burning of

polystyrene polymers from food packaging releases

styrene which can be absorbed through the skin and

the lungs. When it comes to plastic recycling, there

are several problems that need to be addressed. First,

it generates economically low yields, with

complications such as the removal of dyes, fillers

and other additives increasing the cost of recycling.

Second, individual polymer types require their own

specific pathways to be recycled, therefore plastic

waste needs to be sorted before efficient processing

can take place. These factors make recycling

suboptimal for many countries (Suchismita

Satapathy 2017).

The abiotic degradation of plastics takes place

through four main pathways, oxidation, photolysis,

hydrolysis and mechanical shearing (Figure 1). The

mechanical shearing of polymers is influenced by a

wide variety of environmental factors such as

weathering and freeze thaw cycles. Plastic waste

gradually experiences cracking, surface erosion,

abrasion, and breakdown to mesoplastics (~5–20

mm), large microplastics (~1–5 mm), small

microplastics (~20–999 μm), and nanoplastic (<1

μm) sized pieces. In the presence of water,

hydrolysis of those plastics with heteroatoms in their

backbone causes cleavage of ester bonds, allowing

the rapid breakdown of PET or PU to their monomer

constituents. Exposing polymers to UV radiation

causes photodegradation, and is considered the

primary method of degradation for discarded plastic

pollution. UV radiation provides the energy to break

C-H or C-C bonds, forming a highly reactive radical

group. Alternatively, UV radiation can lead to the

dechlorination of a C-Cl bond in PVC, releasing

chlorine. The first step of polymer oxidation is free

radical formation, often initiated by UV mediated

breaking of C-C and C-H bonds. The carbon

centered free radicals that are generated can then

react with free oxygen in the environment, leading to

the formation of peroxy radicals; or react with the

polymer, causing the formation of more carbon

centered free radicals. In an effort to quench the free

radicals, this often results in chain linking and chain

scission reactions in the polymer backbone.

Figure 1: Four major pathways of abiotic polymer

degradation. “R” represents a polymer chain.

Plastics present a poor food source for most

organisms, but there are those that have evolved the

cellular machinery required. Plastic polymers

synthesised from fossil fuels are a relatively new

addition to world ecosystems, and generally

represent an inefficient reservoir of energy. As such,

there has not been the opportunity for biology to

evolve an effective means of degrading them. The

millions of tons of plastic that ends up in the sea,

landfills and other environments has created a

reservoir of potential energy for microbial life. The

microbial degradation of plastic plays a not

insignificant role in polymer degradation, with over

400 species having been identified to date. The

general biodegradation mechanism for polymer

breakdown can be summarised in five steps;

secretion, adsorption, release, uptake and

degradation (Figure 2). Enzymes such as PETase,

Cutinase, or perhaps alkane hydroxylases are

excreted by the cell into the extracellular. These

enzymes will then adsorb to the hydrophobic

polymer surface, a process facilitated by the

formation of biofilms. The enzymes secreted can

then bind to their potential substrates, the polymers,

and interact with them. Enzymes can successfully

break down the large polymers into smaller

segments or monomers which are then released.

These smaller fragments can then be uptaken into

the cell through specific importers. Once in the

cytoplasm, additional specific enzymes may be

required to enact modification before the polymer

fragments can be incorporated into cellular

metabolism.

Conditions for a Microbial Consortium for the Biological Degradation of Plastic Polymers

199

Figure 2: Proposed mechanism for microbial led

biodegradation of polymers.

Microorganisms will often exist in nature in

established consortiums, allowing for powerful

benefits to all its members, including the capacity to

degrade substances. Cultures with multiple species

have better communal properties like robustness and

division of labor. Various components are required

for the biological breakdown of plastics (Figure 2),

and dividing their production over multiple species

could lead to synergistic effects. Robustness is the

trait of microbes which enables them to survive in

varieties of unstable environments. Such a property

is presented in lichens, a composite organism which

can include bacteria, fungi, and algae. When algae

and fungi are alone, they both suffer from oxidative

damage during desiccation. But when in a

consortium, both enable up-regulation of protective

systems. Without the contact with fungus, the algae

can tolerate only low levels of light, and their

photoprotective systems are not upregulated; in the

absence of algae, the glutathione-based antioxidant

system of fungus is slow and invalid. There are

already some successful applications of consortium

in the field of plastic degradation. Individual

bacterial strains were isolated from various faecal

samples, and tested both individually and in

consortium for their capacity to degrade

polyethylene or polyethylene and polypropylene.

Individually, degradation rates ranged from 15.5%

to 29% over the course of the assay, but in

consortiums of four they achieved 75% efficiency of

degradation for polyethylene and 56% for

polypropylene.

3 DISCUSSIONS

In January 2021, Gambarini et. al. published an

extensive and theoretically comprehensive database

describing every microbial strain that has been

reported to degrade any type of plastic polymer to

date. This database is continually updated as new

publications come out. For this work, we have

analysed all species from this database that degrade

one or more of the 6 predominant types of “non-

biodegradable” plastics; PE, PP, PVC, PS, PET or

PU.

Incorporating organisms across the different

kingdoms into a consortium will likely allow for a

wider degradation coverage of polymer types and

additives. Since different types of organisms with

varying enzymes and chemical substances have the

ability of degrading different parts of the plastic,

they are able to decompose larger proportions of

plastic if they can work as a consortium. As a

comparison, if a human wants to share a banana with

a chimpanzee, they are only able to consume the

banana pulp; if the human can share the banana with

ants which can consume the skin, the whole banana

may be decomposed in a rapid fashion. The

degradation of plastic can be divided into several

steps, such as degrading long chains and small

chains, or the backbone of a polymer vs the

monomers released, or a hydrophobic bacteria that

facilitates interactions between the polymer and

other organisms. Different kinds of organisms can

be responsible for each part. For example, some

insects have been identified that are able to chew

plastic debris into smaller parts, assisting with

mechanical shearing, before the bacteria in their gut

break down the plastic enzymatically.

Microbial strains isolated from a similar

environmental location have a higher chance of

successfully cultivating in a single, compatible,

minimal media. When considering a consortium,

organisms isolated from the same kind of

environment are likely to have a higher

compatibility than organisms collected from

disparate environments. For example, Samples

collected from water (particularly ocean water) are

unlikely, though not impossible, to be compatible

with samples from soil. In addition, an organism's

capacity to grow in a minimal medium with a

limited carbon source will likely contribute to its

efficiency at degrading plastic polymers. With

plastic polymers generally being a poor source of

energy, if an alternative energy source is present

then it will be consumed preferentially first.

Therefore, if an organism has been shown to grow

successfully in a minimal medium with plastic

polymers as the sole carbon source, this would be a

positive sign. The growth environment should also

contain the essential trace elements, such as alkali

metals, transition metals or phosphates. Since there

is little experimental evidence observing

ICBEB 2022 - The International Conference on Biomedical Engineering and Bioinformatics

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consortiums of polymer degrading microbes

growing together, it is hard to say which strains will

be compatible so it is worth trying out a variety of

combinations.

For a consortium to be effective, all organisms

must be able to successfully grow at equivalent

temperatures, with higher temperatures being

preferable for thermodynamic optimisation. The

higher the temperature the organism can survive at,

more likely that faster degradation will occur. The

thermal compatibility of different organisms is

heavily dependent on the environmental niche that

organism was isolated from, and its thermophilic

potential.

Additional environmental factors to be

considered are pH and oxygen level. A significant

part of polymer degradation is hydrolysis (Figure 3),

which can be influenced by the pH value. In an

acidic environment, the carbonyl group of an ester

bond will be protonated, allowing for an easier

nucleophilic attack. This process is particularly

relevant for polymers with heteroatoms in their

backbones, such as PET and PU (Fig 1). Oxygen is

also an important consideration for the degradation

of plastic, playing a role in both abiotic and biotic

degradation. For abiotic degradation, oxidation and

incorporation of oxygen into the polymer structure

both require the presence of oxygen. Biotically, for

most microbes, oxygen is preferential or even

necessary for survival. It is also worth noting that

the vast majority of polymer degradation studies

have been carried out in an aerobic environment, as

experimental convenience favours this condition. It

is possible that anaerobic metabolism could offer

alternative pathways.

Figure 3: The proportion of waste, non-fibrous plastic

mass attributed to the most common forms of plastic, from

2015. MT= million tons.

Reported degradation efficiency gives an

indication of the potential of an organism, but comes

with the considerable caveat that degradation studies

are far from standardized, or even reliable.

Theoretically, we would choose bacteria that have a

higher degradation rate over a shorter period of time,

for a more efficient breakdown of material.

However, many reported strains have no data

supporting their degradation rate, and those that do

differ vastly in the substrates used or the

methodology carried out. In addition, different

studies vary in their pretreatment of the plastic

substrate used with thermal, acidic UV or no

treatment employed. It can be difficult to determine

if an observed weight loss or CO₂ capture is due to

the degradation of the polymer additives in the

plastic. Moreover, degradation rates are often

reported as a single value after a time course, and

therefore it cannot be determined if degradation will

continue at the same rate or if an asymptote is

reached.

To create the optimal conditions for the

biodegradation of polymers, a variety of potentially

incompatible environmental factors will need to be

considered and balanced. These factors include

temperature, oxygen level, pH, UV exposure,

moisture level, carbon availability and abundance of

trace elements. In order to control the temperature,

heating elements or ventilation can be used to

maintain the temperature within an optimum range.

To prevent an oxygen gradient forming, mechanical

rotation may be necessary, although this may

become prohibitively expensive on larger scales. pH

will need to be monitored and kept within

biologically acceptable limits using additives. UV

levels are expensive to control artificially therefore

natural light will be a more viable method. Moisture

levels can be controlled using the addition of water.

The consortium should be provided with the

necessary trace elements generally required for

microbial growth, and should not be provided with

an alternative carbon source such as sugars or fats

that would be consumed preferentially over the

polymers. It is important to keep in mind that

exposure to sunlight as a UV source will provide

complications when trying to maintain an optimum

temperature and moisture level, therefore a balance

must be struck.

Genera that occur more frequently could be more

reliable and compatible, with multiple related

species all reported with degradation capacity. When

strains capable of degrading polymers are analysed,

some genera occur much more frequently than

others. The three most common genera are

Aspergillus, Bacillus, and Pseudomonas. There are

16, 18 and 19 species for each, reported across 18,

18 and 20 publications, respectively. Species that

originate from the same genus will have closer

genotypes, which means that their habitat and

environmental needs for trace elements will be more

compatible, so being able to select multiple species

from the same genus could prove advantageous. It

Conditions for a Microbial Consortium for the Biological Degradation of Plastic Polymers

201

could be argued that because these genera are found

more frequently to degrade plastics, they are the

genera which are best adapted for it. However, it is

more likely that laboratory based screening has a

strong bias for strains that thrive in standard

laboratory conditions.

Several species have been identified that have

been observed to degrade multiple types of polymer,

which could prove advantageous when considering a

consortium. Only three types of bacteria or fungi

have the ability to break down three or more types of

plastic Bacillus cereus, Bacillus gottheilii, and

Phanerochaete chrysosporium. Bacillus cereus and

Phanerochaete chrysosporium have each been

observed to degrade three types of polymer (PE,

PET and PS and PE, PP and PVC respectively),

while Bacillus gottheilii degrades 4 types of polymer

(PE, PP, PS and PET) If all these species can be

utilized, then with the help of just these three

species, five of the six main plastics can be broken

down. Since two of them are from the same genus,

they are likely to be compatible and both of them are

isolated from soil/sediment environments. With a

smaller number of species, biotic degradation and

the use of consortia could be more practical for

large-scale use. However, since related research is

still limited, some organisms that can break down

multiple plastics may not have been observed to

yet.Although reported degradation rates are not

always reliable or comparable, there are some

species with such a high reported rate that they are

worth additional consideration. For example, four

species that have a reported weight loss for PE

above 50% are Penicillium chrysogenum,

Penicillium oxalicum, Microbacterium paraoxydans,

and Pseudomonas aeruginosa. The two fungal

species, P. chrysogenum and P. oxalicum, were

found to degrade 55 % and 59 % of a PE sheet over

a 90 day period. P. aeruginosa and M.

paraoxydansare bacteria found to degrade PE with a

50.5% and 61% weight loss recorded after 60 days

at room temperature. These four species have all

been isolated from soil samples, and have an optimal

growth temperature of around 28 °C and have

similar growth conditions, so it would be reasonable

to have them in the same consortium. However, it is

important to remember that methodologies for

degradation rates vary greatly between publications,

making exact comparisons and conclusions hard to

draw. Nevertheless, it is probably worth considering

these four species for a consortium that is required to

degrade PE.

In order to select the right organisms for a

consortium, compatibility, efficiency and

degradation comprehensiveness need to be

considered. As previously mentioned, B. cereus and

B. gottheilii are both soil microbes capable of

degrading multiple types of polymer. Ideonella

sakaiensis is soil microbe whose PET degrading

activity is well characterized, as is Acinetobacter

baumannii. The soil fungus Aspergillus flavus could

be considered for the decomposition of PU. High

levels of PE degradation could be covered by one or

more of the soil microbes P. chrysogenum, P.

oxalicum, M. paraoxydans, or P. aeruginosa. By

using these organisms the six predominant forms of

plastic (PE, PP, PU, PS, PVC and PET) can be

degraded by a range of organisms all capable of

growing in a similar environment.

4 CONCLUSIONS

If implemented correctly, and at a significant scale,

biodegradation of plastic waste through microbial

consortiums could present an efficient, economical

and environmentally sound response to the world’s

ever increasing plastic waste crisis. However, we do

not recommend that this approach be taken on as an

alternative to reducing the current polymer

production levels, rather as a method for reducing

the waste that already exists.

ACKNOWLEDGEMENTS

If any, should be placed before the references

section without numbering.

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