Plastic particles in the ocean are typically covered with microbial biofilms, but it remains unclear whether distinct microbial communities colonize different polymer types. In this study, we analyzed microbial communities forming biofilms on floating microplastics in a bay of the island of Elba in the Mediterranean Sea. Raman spectroscopy revealed that the plastic particles mainly comprised polyethylene (PE), polypropylene (PP), and polystyrene (PS) of which polyethylene and polypropylene particles were typically brittle and featured cracks. Fluorescence in situ hybridization and imaging by high-resolution microscopy revealed dense microbial biofilms on the polymer surfaces. Amplicon sequencing of the 16S rRNA gene showed that the bacterial communities on all plastic types consisted mainly of the orders Flavobacteriales, Rhodobacterales, Cytophagales, Rickettsiales, Alteromonadales, Chitinophagales, and Oceanospirillales. We found significant differences in the biofilm community composition on PE compared with PP and PS (on OTU and order level), which shows that different microbial communities colonize specific polymer types. Furthermore, the sequencing data also revealed a higher relative abundance of archaeal sequences on PS in comparison with PE or PP. We furthermore found a high occurrence, up to 17% of all sequences, of different hydrocarbon-degrading bacteria on all investigated plastic types. However, their functioning in the plastic-associated biofilm and potential role in plastic degradation needs further assessment.

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<p>Ocean plastic debris poses a large threat to the marine environment. Millions of tons of plastic end up in the ocean each year and the Mediterranean Sea is one of the most plastic polluted sea. Ocean plastic particles are typically covered with microbial biofilms, but it remains unclear if different polymer types are colonized by different communities. Knowledge in this aspect strengthens our understanding if microbes purely use plastic debris as attachment surface or if they may even contribute to the degradation of plastic. To gain a better understanding of the composition and structure of biofilms on micro plastic particles (MP) in the Mediterranean Sea, we analyzed microbial community covering floating MP in a bay/marina (Marina di Campo) on the island of Elba. MPs were collected with a plankton net (mesh size 50µm), fixed for fluorescence microscopy and stored for subsequent DNA extraction, and identification of the polymer with Raman spectroscopy. The particles were mainly comprised of polyethylene (PE), polypropylene (PP) and polystyrene (PS) and were often brittle and with cracks (PE, PP) and showed visual signs of biofouling (PE, PP, PS). Fluorescence in situ hybridization and imaging by high resolution confocal laser scanning microscopy of single MPs revealed high densities of colonization by microbes. 16S rRNA gene amplicon sequencing (Illumina Miseq) revealed higher abundance of archaeal sequences on PS (up to 29% of the reads) in comparison to PE or PP (up to 3% of the reads).  The bacterial community in the biofilms on each of the three plastic types consisted mainly of the orders Flavobacteriales, Rickettsiales, Alteromonadales, Cytophagales, Rhodobacterales and Oceanospirillales. Furthermore, we found significant difference in the community composition of biofilms on PE compared to PP and PS but not between PP and PS. The indicator species on PE were Calditrichales, detected at 10 times higher sequence abundance on PE than on PP and PS, as well as several uncultured orders. This study sheds light on preferential microbial attachment and biofilm formation on microplastic particles, yet it remains to be revealed, whether and which of these may contribute to plastic degradation.</p>

<p>Marine plastic pollution has increased exponentially since the start of mass production in the 1950´s and the negative impacts of marine plastic debris (MPD) on marine life are general acknowledged. Typically, MPD is overgrown by diverse biofilms comprised of prokaryote and eukaryotes, but it is mostly unresolved whether polymers are colonized by opportunist that attach to any hard surface, or if different polymers attract specific communities. This question is further complicated by the fact that floating MPD is subjected to UV-induced photo-oxidation, which results in polymer degradation, i.e. the release of smaller and more bioavailable daughter products, and also causes changes in the polymer’s surface properties. If weathered surfaces are more prone to colonization than pristine ones and whether communities on these surfaces are different is unknown. In consonance, whether colonizers interact with the different polymers, e.g. degrade it, or are just ‘hitching a ride’ is ambiguous. To solve this complex problem we investigated the initial colonization of pristine plastics and the influence of photo-oxidation on community succession. We incubated five different polymer types (PE, PP, PET, PS and Nylon; one set UV pre-treated, one set pristine), in shallow coastal waters of the Caribbean island St. Eustatius at a depth of 5m. Multivariant-analyses to compare day 1 and day 6 revealed that the microbial community changed over time, which is a typical feature during colonisation. Communities of day 1 and day 6 were also analysed separately to assess the influences of UV pre-treatment and polymer type during separate stages of biofilm development. On day 1, UV pre-treated foils attracted a different community than non-pre-treated foils, while there was no statistical difference in community composition between the five polymer types. In contrast, on day 6, the influence of UV treatment on community composition was no longer significant, while different polymer types supported different communities. These results show that the community is dynamic in the initial stage of colonization of polymers. The effects of UV pre-treatment and polymer type indicate that colonizers are not purely opportunistic. With more in-depth analysis on OTU and/or order level, we aim to answer the following questions: 1) What is the main driver for community succession 2) Are there polymer-specific members of the community 3) Do the different polymer types select for communities that might utilise the polymer or its UV-degradation products for energy gain and/or growth.</p>