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Plastic in the Ocean: Microbial Transformation of an ‘Unconventional’ Carbon Substrate

Periodic Reporting for period 2 - VORTEX (Plastic in the Ocean: Microbial Transformation of an ‘Unconventional’ Carbon Substrate)

Reporting period: 2019-12-01 to 2021-05-31

Large quantities of plastics are released from the terrestrial to the marine realm, and the impacts of plastics to the marine environment are problematic. The severity of this problem is gaining momentum because the demand for plastic, and coupled to this, the release of plastic debris to the ocean is probably increasing. The vast majority of plastics are of a petrochemical origin and derived from the polymerisation of monomers resulting in synthetic organic polymers.Their versatile properties have boosted mass production to meet the rapidly developing demand of plastics for a broad variety of applications.Nowadays almost all aspects of daily life involve plastics: transport, clothing, construction and packaging materials. As a result of waste mismanagement and littering, accumulation of plastic debris in the natural environment, including the ocean, is widespread and scientific concern and public awareness related to the plastic pollution are growing. In the last decades, investigations have focused on microplastics (particle size: 1µm – 5mm) and, more recently, nanoplastics (particle size: <1µm) and their potential impact on the ocean environment. Micro and nanoplastics can originate from primary industrial sources or from the degradation of macroplastics (particle size: >5mm). The latter is facilitated by different types of physical, chemical and biological processes, which can lead to the fragmentation and degradation of plastics.Because of their small size, microplastics and nanoplastics become bioavailable and can bioaccumulate, while current evidence on biomagnification across marine food webs is ambiguous. Micro and nanoplastic can have negative effects on biota (e.g. by causing inflammation, oxidative stress and disruption of hormone signalling). In addition, plastics often contain additives (e.g. softeners, flame retardants) that may be incorporated and can lead to potentially negative effects for the host organism. It thus seems likely that organisms higher up in the food chain are impacted by plastic pollution, too.
It is unknown how long plastic waste remains in the environment. The durability of plastic entails slow degradation and it was believed that plastics may persists in the environment for an extended period of time, possibly exceeding centuries or even millennia. Yet, floating plastics not only fragment in the marine environment but are also degraded through photoxidation, which, in combination with microbial degradation, might severely shorten the lifetime of PMD. Furthermore, plastics are typically rich in chemical energy making polymer oxidation reactions exergonic.Plastic degradation is thus, in principal, a worthwhile strategy for microorganisms to obtain energy and/or carbon. Earlier works could show that a diversity of microbes colonise PMD and some evidence exists that these not only comprise opportunistic communities, but that the different polymers might select for a specific, plastic-related community. Other studies found found terrestrial microbes that seemingly degrade some plastic polymers directly. Yet, it is at present debated if and in how far such metabolic traits exist in the marine environment and/or substantially contribute to plastic removal from the ocean environment.
A substantial problem of ongoing research is to clearly link apparent plastic degradation to microbial action and to determine the velocity of plastic degradation. The overarching goal of the ERC project VORTEX is to assess marine degradation of important plastics by applying innovate stable isotope assays in tandem with lipidomics, NGS-based microbial diversity and functional gene analyses. VORTEX comprises three major objectives and work packages with clear links between them: (i) to estimate the potential for- and kinetics of the microbial degradation of the most relevant plastics in the ocean, (ii) to identify/quantify key microbes mediating degradation and (iii) to determine boundary conditions promoting/hampering degradation and to identify potential pathways of degradation.
The first phase of the project was dedicated to assembling the project team. VORTEX comprises now 3 PhD students, 1 postdoc, an associated guest postdoc and a senior scientist (the PI). In addition, several conducted their BSc or MSc project in ~VORTEX. A second major task in the project's begin was to acquire 13C-labelled polymers, some of which had to be custom synthesised. The labelled polymers are for several of the main scientific tasks in VORTEX (tracing labelled polymer into microbial degradation products and biomass).
Further activities in the first year of VORTEX included design and building of field equipment, ie. platforms with 13C-labelled polymers that can be colonised by environmental microbes. These were installed in contrasting ocean environments (Svalbard, Arctic; North Sea, temperate climate; Elba, subtropical climate; St. Eustasia, tropical climate); most of these are retrieved by now and are stored for analysis. One platform was installed on a mooring in the North Pacific gyre (one of of the trash vortexes), but this platform, together with the entire mooring was lost. At costal sites, we also installed experiments to monitor initial colonisation, these were subsampled on a daily base. From several colonisation experiments, we extracted DNA and conducted community analyses to determine potential unique microbial community patterns.
In addition, we also carried out several cruises to the Wadden Sea, North Sea and North Atlantic to recover floating macro/micro plastics as well as nano plastics and to conduct incubation experiments. We also isolated microbes from plastic associated biofilms and tested their ability to degrade plastic and are currently investigating the gene expression (transcriptome reconstruction) of one organism. Finally, we developed stable isotope probing techniques to determine rates of plastic degradation.
The COVID pandemic was substantially hampering our activities as travel and work restrictions had to be faced. As a fall back strategy, we put more efforts into literature review works. As a result of this, one review paper has been published, one peer reviewed bookchapter has been accepted and three further papers are in prep.
IN addition to the scientific work, VORTEX was also very active in public outreach activities: team members are frequently interviewed by the local, national and international media to comment on questions related to marine plastic pollution and potential microbial degradation of plastic. A BBC Camera team followed a cruise led by the project leader. The results of the film shooting constitute a substantial part of the 2h documentary that was aired in Jul. 2020 on BBC4 (the ocean autopsy).
In addition to our 'regular' work on plastic degradation by microbes, we also started to investigate coupled photooxidation and microbial degradation. We furthermore teamed up with colleagues from Utrecht University and developed a method to identify and quantify nano plastic in the ocean environment.

In the remaining period of the project, I expect to conduct one more research cruise to the atlantic, to recover remaining field experiments and to work up genetic data. The 3 PhD projects will also finish in this period and several publications are expected to result from the project's work.
artist impression on the fate of plastic in the ocean