Buoyant microplastics in the marine environment

Despite growing plastic discharge into the environment, researchers have struggled to detect expected increases of marine plastic debris in sea surfaces, especially at sizes <5 mm. These “missing plastics” not only sparked discussions about final sinks for such debris, but also about our lack of adequate methods to find and quantify smaller size fractions (<1 mm), which all could be contributing to the observed gaps in the plastic budget. Deep-sea sediments have been suggested as the final sink for microplastics (generally particles < 5 mm). The meta-analysis I present in this thesis (Chapter 2) highlights that in open oceans, microplastic polymer types segregated in the water column according to their density. Lower density polymers, such as polypropylene and polyethylene, dominated sea surface samples but became less abundant through the water column, whereas denser polymers (i.e. polyesters and acrylics) were enriched with depth. The need for methods better suited to quantify small microplastics in environmental samples has been flagged. Chapter 3 of this thesis details the optimisation and implementation of a protocol that allows high throughput detection and automated quantification of small microplastic particles (20–1000 μm) using the dye Nile red, fluorescence microscopy and image analysis software. The preliminary application of this protocol showed a power-law increase of small microplastics (i.e. <1 mm) with decreasing particle size in coastal sea surface water. This finding suggests that part of the “missing” plastic fraction may have been missed due to the inefficiency of traditional methods to quantify the smaller fraction of microplastics. On sea surfaces, plastic debris is rapidly colonized by a diverse community of microorganisms, and speculation arose about microbes using such plastics as a carbon source. In Chapter 4, I show that weathered polyethylene became enriched by distinct genera within the biofilms, but only during early stages of colonization (i.e. after 2 days) in coastal marine water. Given the lack of persistent enrichment over time, common non-hydrolysable polymers might not serve as an important source of carbon for mature colonizing communities and these mainly persist by consuming labile photosynthate generated by primary producers. Overall, this thesis shows that buoyant plastics appear to be more prevalent on sea surfaces than earlier research had suggested, and that plastic biodegradation is likely limited to a minor process that occurs within the biofilm, but can be sped up when combined with abiotic weathering

Distribution of plastic polymer types in the marine environment ; a meta-analysis

Despite growing plastic discharge into the environment, researchers have struggled to detect expected increases of marine plastic debris in sea surfaces, sparking discussions about “missing plastics” and final sinks, which are hypothesized to be coastal and deep-sea sediments. While it holds true that the highest concentrations of plastic particles are found in these locations (103-104 particles m−3 in sediments vs. 0.1–1 particles m−3 in the water column), our meta-analysis also highlights that in open oceans, microplastic polymer types segregated in the water column according to their density. Lower density polymers, such as polypropylene and polyethylene, dominated sea surface samples (25% and 42%, respectively) but decreased in abundance through the water column (3% and 2% in the deep-sea, respectively), whereas only denser polymers (i.e. polyesters and acrylics) were enriched with depth (5% in surface seawater vs. 77% in deep-sea locations). Our meta-analysis demonstrates that some of the most abundant and recalcitrant manufactured plastics are more persistent in the sea surface than previously anticipated and that further research is required to determine the ultimate fate of these polymers as current knowledge does not support the deep sea as the final sink for all polymer types

A mechanistic understanding of polyethylene biodegradation by the marine bacterium Alcanivorax

Polyethylene (PE) is one of the most recalcitrant carbon-based synthetic materials produced and, currently, the most ubiquitous plastic pollutant found in nature. Over time, combined abiotic and biotic processes are thought to eventually breakdown PE. Despite limited evidence of biological PE degradation and speculation that hydrocarbon-degrading bacteria found within the plastisphere is an indication of biodegradation, there is no clear mechanistic understanding of the process. Here, using high-throughput proteomics, we investigated the molecular processes that take place in the hydrocarbon-degrading marine bacterium Alcanivorax sp. 24 when grown in the presence of low density PE (LDPE). As well as efficiently utilising and assimilating the leachate of weathered LDPE, the bacterium was able to reduce the molecular weight distribution (M from 122 to 83 kg/mol) and overall mass of pristine LDPE films (0.9 % after 34 days of incubation). Most interestingly, Alcanivorax acquired the isotopic signature of the pristine plastic and induced an extensive array of metabolic pathways for aliphatic compound degradation. Presumably, the primary biodegradation of LDPE by Alcanivorax sp. 24 is possible via the production of extracellular reactive oxygen species as observed both by the material's surface oxidation and the measurement of superoxide in the culture with LDPE. Our findings confirm that hydrocarbon-biodegrading bacteria within the plastisphere may in fact have a role in degrading PE

Independence of microplastic ingestion from environmental load in the round goby (Neogobius melanostomus) from the Rhine river using high quality standards

Rivers play a crucial role in collecting and transporting microplastics. Nonetheless, the degree to which microplastic pollution of freshwaters affects its biota remains understudied. Sampling of wild fishes has so far demonstrated that microplastic ingestion occurs commonly across species with alternate feeding modes, as well as in different environmental compartments. Due to the exploratory nature of many preceding studies, drawing insight about factors driving microplastic ingestion has remained difficult. It continues unknown for instance, what the importance of varying environmental microplastic concentrations is to predict ingestion rates in fish from those areas. Here we show that ingestion rates of microplastic particles (>300 μm) in the benthic round goby from the Rhine river were negligible (1 particle in 417 fish). Among the 535 visually selected putative microplastic fragments, stringent data processing steps to reduce the number of false positives during reference library searches, revealed the importance of taking such steps into account in comparison with other data processing routines. Our observations remained consistent, despite having collected fish from a strongly polluted site of the lower Rhine, which served as contrast to a significantly cleaner site upstream. These results demonstrate that higher environmental microplastic concentrations are not necessarily mirrored by higher ingestion rates in a given fish species

Marine Plastic Debris: A New Surface for Microbial Colonization

Plastics become rapidly colonized by microbes when released into marine environments. This microbial community-the Plastisphere-has recently sparked a multitude of scientific inquiries and generated a breadth of knowledge, which we bring together in this review. Besides providing a better understanding of community composition and biofilm development in marine ecosystems, we critically discuss current research on plastic biodegradation and the identification of potentially pathogenic "hitchhikers" in the Plastisphere. The Plastisphere is at the interface between the plastic and its surrounding milieu, and thus drives every interaction that this synthetic material has with its environment, from ecotoxicity and new links in marine food webs to the fate of the plastics in the water column. We conclude that research so far has not shown Plastisphere communities to starkly differ from microbial communities on other inert surfaces, which is particularly true for mature biofilm assemblages. Furthermore, despite progress that has been made in this field, we recognize that it is time to take research on plastic-Plastisphere-environment interactions a step further by identifying present gaps in our knowledge and offering our perspective on key aspects to be addressed by future studies: (I) better physical characterization of marine biofilms, (II) inclusion of relevant controls, (III) study of different successional stages, (IV) use of environmentally relevant concentrations of biofouled microplastics, and (V) prioritization of gaining a mechanistic and functional understanding of Plastisphere communities

ONSITE Design-driven field studies for safer demanding marine operations

ONSITE is a research project that builds knowledge on how to implement human-centered design processes within ship design. The project focuses specifically on a field study methodology and the transfer of knowledge between human-centered design and engineering disciplines.

Marine Plastic Debris: A New Surface for Microbial Colonization

Plastics become rapidly colonized by microbes when released into marine environments. This microbial community—the Plastisphere—has recently sparked a multitude of scientific inquiries and generated a breadth of knowledge, which we bring together in this review. Besides providing a better understanding of community composition and biofilm development in marine ecosystems, we critically discuss current research on plastic biodegradation and the identification of potentially pathogenic “hitchhikers” in the Plastisphere. The Plastisphere is at the interface between the plastic and its surrounding milieu, and thus drives every interaction that this synthetic material has with its environment, from ecotoxicity and new links in marine food webs to the fate of the plastics in the water column. We conclude that research so far has not shown Plastisphere communities to starkly differ from microbial communities on other inert surfaces, which is particularly true for mature biofilm assemblages. Furthermore, despite progress that has been made in this field, we recognize that it is time to take research on plastic–Plastisphere–environment interactions a step further by identifying present gaps in our knowledge and offering our perspective on key aspects to be addressed by future studies: (I) better physical characterization of marine biofilms, (II) inclusion of relevant controls, (III) study of different successional stages, (IV) use of environmentally relevant concentrations of biofouled microplastics, and (V) prioritization of gaining a mechanistic and functional understanding of Plastisphere communities.R.J.W. was supported by an MIBTP PhD scholarship (BB/M01116X/1) and Waitrose & Partners as part of the Association of Commonwealth Universities Blue Charter Programme. G.E.-C. was supported by a NERC CENTA PhD scholarship. V.Z. was supported by CONICYT-BECAS CHILE/Doctorado Becas Chile en el Extranjero, Folio 72160583 and NERC research project NE/S005501/1. M.L. was supported by an MIBTP PhD scholarship (BB/M01116X/1). J.C.-O. was supported by the NERC Independent Research Fellowship NE/K009044/1, Ramón y Cajal contract RYC-2017-22452 (funded by the Ministry of Science, Innovation and Universities, the National Agency of Research, and the European Social Fund) and MINECO project PID2019-109509RB-I00 (FEDER cofunding)

Microplastics in fecal samples of whale sharks (Rhincodon typus) and from surface water in the Philippines

Marine plastic abundance has increased over the past 60 years and microplastics (< 5 mm) constitute a primary component of such litter. Filter-feeding megafauna, such as the whale shark, might be particularly affected by microplastic pollution as their feeding mode requires filtration of up to thousands of cubic meters of water. In addition, the habitat range of whale sharks intersects with several recognized microplastic pollution hotspots, among which is the Coral Triangle. Direct evidence for microplastic ingestion in whale sharks however, has not yet been presented. Here we show that whale shark scat collected in the Philippines from 2012 to 2019 contained a mean of 2.8 microplastics g − 1 . Contrary to our expectations, the microplastic concentration in the scat remained consistent from 2012 to 2019. Water samples from the study site in 2019 indicated that the local microplastic pollution (5.83 particles m − 3 ) was higher than in surface waters in other whale shark habitats, but well below other pollution hot-spots found in Southeast Asia and China (range: 100-4100 particles m − 3 ). With the predicted growth in plastic use, leading to increased plastic marine pollution, whale sharks are expected to become more exposed to this form of pollution. To what extent microplastic ingestion impacts the overall health status of this endangered species remains an open question