Current Research on Microbe-Plastic Interactions in the Marine Environment

TLDR: The importance of eukaryotes in shaping theplastisphere, potential pathogens carried by plastics and the impact of the plastisphere on plastic transport and biogeochemical cycling are discussed.

Abstract: The microbial colonisers of plastics – the ‘plastisphere’ – can affect all interactions that plastics have with their surrounding environments. While only specifically characterised within the last 10 years, at the beginning of 2021 there were 140 primary research and 65 review articles that investigate at least one aspect of the plastisphere. We gathered information on the locations and methodologies used by each of the primary research articles, highlighting several aspects of plastisphere research that remain understudied: (i) the non-bacterial plastisphere constituents; (ii) the mechanisms used to degrade plastics by marine isolates or communities; (iii) the capacity for plastisphere members to be pathogenic or carry antimicrobial resistance genes; and (iv) meta-OMIC characterisations of the plastisphere. We have also summarised the topics covered by the existing plastisphere review articles, identifying areas that have received less attention to date – most of which are in line with the areas that have fewer primary research articles. Therefore, in addition to providing an overview of some fundamental topics such as biodegradation and community assembly, we discuss the importance of eukaryotes in shaping the plastisphere, potential pathogens carried by plastics and the impact of the plastisphere on plastic transport and biogeochemical cycling. Finally, we summarise the future directions suggested by the reviews that we have evaluated and suggest other key research questions.

Figures

Figure 3. The focus and topics covered of all plastisphere reviews that our literature search and subsequent filtering returned 5,18–20,151–211. The focus of the review is the over-arching topic that the review addresses, with

Figure 2. Cumulative number of plastisphere primary research articles published per year relating to different

Full text

Current Research on Microbe-Plastic Interactions in the
Marine Environment
Mira Latva
1
, Vinko Zadjelovic
1
& Robyn J. Wright
2
1
School of Life Sciences, University of Warwick, UK
2
Department of Pharmacology, Faculty of Medicine, Dalhousie University, Nova Scotia, Canada
robyn.wright@dal.ca
Abstract
The microbial colonisers of plastics – the ‘plastisphere’ – can affect all interactions that plastics have
with their surrounding environments. While only specifically characterised within the last 10 years, at
the beginning of 2021 there were 140 primary research and 65 review articles that investigate at least
one aspect of the plastisphere. We gathered information on the locations and methodologies used by
each of the primary research articles, highlighting several aspects of plastisphere research that remain
understudied: (i) the non-bacterial plastisphere constituents; (ii) the mechanisms used to degrade
plastics by marine isolates or communities; (iii) the capacity for plastisphere members to be
pathogenic or carry antimicrobial resistance genes; and (iv) meta-OMIC characterisations of the
plastisphere. We have also summarised the topics covered by the existing plastisphere review articles,
identifying areas that have received less attention to date – most of which are in line with the areas
that have fewer primary research articles. Therefore, in addition to providing an overview of some
fundamental topics such as biodegradation and community assembly, we discuss the importance of
eukaryotes in shaping the plastisphere, potential pathogens carried by plastics and the impact of the
plastisphere on plastic transport and biogeochemical cycling. Finally, we summarise the future
directions suggested by the reviews that we have evaluated and suggest other key research questions.
Keywords
Plastisphere; Plastic biofilms; Microbial communities; Marine plastic pollution; Plastic biodegradation
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0273.v1
© 2021 by the author(s). Distributed under a Creative Commons CC BY license.

1. Introduction
The quantity of plastic pollution entering the oceans annually is increasing year-on-year
1
, but the
ultimate fate and durability of plastics in the oceans are unknown
2
. With some studies suggesting a
persistence of hundreds of years
3
or fragmentation rates of only 1-5% per year
4
, this has led us to
look towards microbes for a solution to this problem. When plastics enter the environment, they are
rapidly covered by organic matter, known as the ecocorona
5
, and are colonised by microbes within
minutes
6
. These plastic-colonising microbes – bacteria, fungi and single-celled eukaryotes as well as
macro and other organisms – are collectively termed the ‘plastisphere’
7
. While marine plastics have
long been observed to have a colonising biofilm (e.g., Carpenter & Smith, 1972
8
), the first specific
characterisation of the plastisphere was published by Zettler et al. in 2013
7
, following a call for
research into microbial communities on plastics by Harrison et al. in 2011
9
. Some earlier studies
investigating biofilm formation in the marine environment did also include plastic materials (e.g.,
10
)
or investigated the biomass colonising plastics in relation to plastic degradation (e.g.,
11
), but the
specific focus on the taxonomic or functional characterisation of plastisphere communities has only
been within the last approximately eight years. A literature search carried out on 4
th
January 2021 for
the search terms “plastics plastisphere”, “plastics microbial community”, and “plastics microbial
degradation” yielded a total of 1069 unique results. This was supplemented with our own literature
collections and filtered manually to include only studies and reviews that fit the following criteria: (i)
available online by the beginning of 2021; (ii) examined at least one aspect of the colonisation or
degradation of surfaces in the marine environment; and (iii) the surfaces used included at least one
recalcitrant, petrochemical plastic. This resulted in 140 primary research articles (Figs. 1 and 2 and
Table S1) and 65 review articles (Fig. 3 and Table S2). Remarkably, 34 and 45% of the research articles
and reviews, respectively, published to date were made available online in 2020.
1.1. Focus of plastisphere studies and methods used
Each of the 140 plastisphere primary research articles returned by our literature search was reviewed
by one of the authors for details relating to the study topic, aims, experimental setup and methods
used and key findings (Figs. 1 and 2; full details can be found in Table S1). To date, most plastisphere
studies specifically focus (i.e., “study topic”; Fig. 1) on the characterisation of the community structure
of the colonising organisms (n=60), with a total of 46 or 77 studies (all study topics), that use
microscopy to visualise these microbial colonisers or sequence at least one microbial community
fraction, respectively. The majority of these use amplicon sequencing of marker genes (n=74) to
characterise either the prokaryotes (n=61), the eukaryotes (n=4) or both (n=9). Only three studies
returned by our literature search use metagenomic sequencing to characterise the plastisphere
12–14
(a fourth has since been published; Bhagwat et al.
15
) and one of these, Yang et al.
13
, re-analyses data
from Bryant et al.
12
, meaning that there are currently only three metagenomic datasets. The
remaining 80 studies were focussed on: (i) assessing the biodegradation of plastics by either
communities or isolates (n=30), with a total of 36 studies including at least one measure of
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0273.v1

biodegradation, for example, weight loss, spectroscopy or microscopy; (ii) antimicrobial resistance
(AMR), pathogenesis or harmful algal blooms (HAB; n=14), with a total of 20 studies including one
measure of AMR or pathogenicity, for example, polymerase chain reaction (PCR) for virulence or AMR
genes; (iii) vertical transport (n=9), with a total of 11 studies including some measure of vertical
transport, for example, an assessment of plastic buoyancy before and after colonisation or an
examination of plastic aggregation; or (iv) other (n=27), i.e., they were focussed on another topic, such
as assessing plastic concentrations in a given area but also included some information on or
characterisation of the plastisphere. As we previously noted
16
, there are relatively very few studies
conducted in the Southern Hemisphere (n=16), only five of which use sequencing to characterise
plastisphere communities, while there are a relatively large number of studies conducted around
Europe (n=63), particularly in the North, Baltic and Mediterranean Seas.
1.2. Focus of plastisphere reviews and topics covered
Each of the 65 review articles returned by our literature search was examined by one of the authors
for details on the focus of the review as well as the topics included (Fig. 3). Perhaps unsurprisingly,
given that the first specific plastisphere characterisation was only published in 2013
17
, the first
plastisphere-focussed review was published in 2015
18
. All reviews published prior to 2015 were
focussed on biodegradation (n=11) and the degradation of plastics by communities or consortia wasn’t
discussed in detail until 2017
19
. In total, there are 16, 40, 9 and 3 reviews focussed on the plastisphere,
plastic biodegradation, ecotoxicology of plastics or plastic-colonising pathogens, respectively. The first
ecotoxicology of plastics review that discussed the plastisphere was published in 2017
5
, the first
pathogen-focussed review was published in 2016
20
, and there are now 20 reviews that give an
overview of the plastisphere while there are 50 that give an overview of plastic biodegradation.
Interestingly, there are some research areas within both plastisphere and biodegradation topics that
have received more attention than others. For example, while 15 reviews discuss microbial
community assembly on plastics, only 6 discuss either horizontal or vertical transport that may be
mediated or affected by the plastisphere and only 8 or 8 reviews discuss colonising pathogens and
AMR or the methods used for plastisphere characterisation, respectively. Likewise, for degradation
topics, 37 and 38 reviews discuss plastic degradation by isolates and the pathways or enzymes used
for plastic degradation, respectively, but only one review discusses the rate at which degradation can
occur, while 22, 18 and 11 reviews discuss factors that limit the degradation of plastics, the methods
used to assess plastic degradation or plastic degradation by communities or consortia, respectively.
There were intermediate numbers of reviews that included sections on the contaminants and
additives of plastics (n=10) or plastic toxicity (n=19), while only four reviews discussed the role of
plastics or plastic degradation in biogeochemical cycling.
In the following sections, we give an overview of current knowledge on the plastisphere. In some
cases, we point the reader towards other reviews that have covered the topics in more detail. We
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0273.v1

summarise microbial community assemblages, the impact of eukaryotes on shaping the plastisphere,
plastic biodegradation by marine microbes, microbial pathogens within the plastisphere and the
combined effects of plastic pollution and plastisphere interactions on plastic transport and
biogeochemical cycling. Finally, we summarise current knowledge, as well as the future directions
suggested by the plastisphere reviews included here.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0273.v1

Figure 1. An overview of the sampling locations and methodologies used by all primary research articles that our literature search and subsequent filtering returned
4,6–8,10–
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Frequently asked questions

Q1. What have the authors contributed in "Current research on microbe-plastic interactions in the marine environment" ?

While only specifically characterised within the last 10 years, at the beginning of 2021 there were 140 primary research and 65 review articles that investigate at least one aspect of the plastisphere. The authors gathered information on the locations and methodologies used by each of the primary research articles, highlighting several aspects of plastisphere research that remain understudied: ( i ) the non-bacterial plastisphere constituents ; ( ii ) the mechanisms used to degrade plastics by marine isolates or communities ; ( iii ) the capacity for plastisphere members to be pathogenic or carry antimicrobial resistance genes ; and ( iv ) meta-OMIC characterisations of the plastisphere. The authors have also summarised the topics covered by the existing plastisphere review articles, identifying areas that have received less attention to date – most of which are in line with the areas that have fewer primary research articles. Therefore, in addition to providing an overview of some fundamental topics such as biodegradation and community assembly, the authors discuss the importance of eukaryotes in shaping the plastisphere, potential pathogens carried by plastics and the impact of the plastisphere on plastic transport and biogeochemical cycling. Finally, the authors summarise the future directions suggested by the reviews that they have evaluated and suggest other key research questions. 

Q2. What have the authors stated for future works in "Current research on microbe-plastic interactions in the marine environment" ?

The authors have therefore brought together some of the key themes that came up in the conclusions and future directions suggested by many of the reviews returned by their literature search ( Fig. 3 ), namely that future studies should: ( i ) use multi-OMIC and interdisciplinary approaches to understand the role of the plastisphere in plastic biodegradation as well as in determining the mechanisms and pathways used for biodegradation 19,151,157,161,164,170,172,175,178,180,181,183,184,188,189,191,194,196,198– 200,204–207 ; ( ii ) examine the impacts of plastic waste in concert with other ecological threats, such as climate change and antibiotic resistance 5,169,181,184 ; ( iii ) assess the consequences of the microbial communities on plastics on plastic toxicity and interactions with other organisms and their environments The authors hope that with the decreasing cost and therefore increasing availability of metagenomic sequencing, this approach will be used more extensively, and future studies will therefore include information on all taxonomic groups. Therefore, future research should aim to determine whether prokaryotic community composition differs if eukaryotes are excluded from the colonisation process 12. There are further key research questions brought about by both: ( a ) the criticisms within the microplastic research field that many studies use unrealistically high particle concentrations ( e. g., 354–357 ), while current levels of larger microplastics ( > 10 µm ) are predicted to inflict little ecological harm on the marine environment from an ecotoxicological perspective 358,359 ; and ( b ) the increasing number of studies examining the effects of plastics on biogeochemical cycling ( discussed in the previous section ). 

Q3. What are the key stages of biodegradation of polymeric hydrocarbons?

There are several key stages in the biodegradation of polymeric hydrocarbons: (i)biodeterioration – a decline in physicochemical properties mediated by microbial activity on the surface of the polymer 230; (ii) biofragmentation – a lytic process reducing polymer molecular weight 230 ; (iii) assimilation – integration of atoms resulting from the fragmentation process into biomass 230,in this case carbon uptake; and (iv) mineralisation – the process of carbon uptake and completetransformation into biomass, CO2/H2O in oxic environments and CO2/CH4/H2O in anoxic environments 151,210. 

Q4. What are the conditions in which microorganisms thrive?

In soiland compost, high nutrient availability, humidity and temperature are often conditions in which microorganisms thrive, positively impacting any potential biodegradation processes 240. 

Q5. What is the role of abiotic pre-oxidation in polyolefins?

for the potential degradation of polyolefins, it seemsessential to have abiotic pre-oxidation, introducing weak points for enzymatic attack; functionalgroups. 

Q6. What is the role of eukaryotic micro-organisms in biofilm?

Eukaryotic micro-organisms play several important roles in community regulation, dynamics and functioning in marine biofilms, ranging from photosynthesis and primary production (e.g., diatoms 7) to predation (e.g., predatory ciliates 7,88,127 and amoebae 150), parasitism (e.g., parasitic dinoflagellates and fungi 127) and other symbiotic interactions (e.g., ectosymbiotic bacteria on ciliates 7,127 or coralsymbiont dinoflagellates 12) (see review by Amaral-Zettler et al. 178). 

Q7. How many particles are predicted to increase in the environment?

their abundance in the environment is only predicted to increase 1, thereby having the potential to considerably and irreversibly 336,337 increase the amount of particles affecting microbial activities in the (otherwise largely oligotrophic) oceans 12,98,105. 

Q8. Why do plastics pose a greater risk to marine ecosystems?

however, compared with other marine particles, pose a greater risk to marine ecosystems because they are a durable and mostly (at least initially 321) buoyant anthropogenic pollutant. 

Q9. What are the common examples of polymers with no functional groups?

On the other hand, polymers with no evident functionalgroups are less susceptible to enzymatic attack (i.e., more recalcitrant polymers); some examples aretypical olefins such as PE and PP. 

Q10. What are the common bacterial groups found on marine biofilms?

diatom-associated bacterial groups have often been found dominating bacterial communities on marine biofilms, including those on plastics 16,46,85. 

Q11. What is the effect of sunlight on the weathering of polymers?

For instance, sunlight can influence the weathering of polymers by the action ofultraviolet (UV) (i.e., photooxidation; UV-A ~295-315 nm and UV-B ~315-400 nm) radiation and byheating (i.e., thermooxidation) which is mediated by the visible section of sunlight (400-760 nm) andinfrared radiation (760-2500 nm) 210. 

Q12. What is the term for the plastisphere?

These plastic-colonising microbes – bacteria, fungi and single-celled eukaryotes as well as macro and other organisms – are collectively termed the ‘plastisphere’ 

Q13. How many studies have been published that have investigated the effect of microbial biofouling?

To their knowledge, however, no studies have been published thatexperimentally investigated the effect of microbial biofouling on the buoyancy of individual, sub-millimetre low-density microplastics in either lab or field settings, likely due to the difficulties in thehandling, recovery and analysis of small particles while retaining natural exposure conditions, as noted by several studies 50,52,89,305,309. 

Q14. What are the potential ramifications of these hydrocarbons for degradation assessments?

The potential ramifications of these hydrocarbons for degradation assessments arediscussed further below, but further investigations are needed that experimentally address thedynamics of eukaryote–prokaryote interactions in the plastisphere and differentiate them from microbe–plastic interactions (see Bryant et al.12). 

Q15. what are the key themes in the conclusions and future directions of the literature search?

The authors have thereforebrought together some of the key themes that came up in the conclusions and future directionssuggested by many of the reviews returned by their literature search (Fig. 3), namely that future studiesshould:(i) use multi-OMIC and interdisciplinary approaches to understand the role of theplastisphere in plastic biodegradation as well as in determining the mechanisms and pathways used for biodegradation 19,151,157,161,164,170,172,175,178,180,181,183,184,188,189,191,194,196,198– 200,204–207;(ii) examine the impacts of plastic waste in concert with other ecological threats, such asclimate change and antibiotic resistance 5,169,181,184;(iii) assess the consequences of the microbial communities on plastics on plastic toxicity andinteractions with other organisms and their environments