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. 2021 Jun 16:12:673553.
doi: 10.3389/fmicb.2021.673553. eCollection 2021.

Microbial Communities on Plastic Polymers in the Mediterranean Sea

Annika Vaksmaa  1 , Katrin Knittel  2 , Alejandro Abdala Asbun  1 , Maaike Goudriaan  1 , Andreas Ellrott  2 , Harry J Witte  1 , Ina Vollmer  3 , Florian Meirer  3 , Christian Lott  4 , Miriam Weber  4 , Julia C Engelmann  1 , Helge Niemann  1   5
Affiliations

Microbial Communities on Plastic Polymers in the Mediterranean Sea

Annika Vaksmaa et al. Front Microbiol. .

Abstract

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.

Keywords: biofilms; hydrocarbon degrading bacteria; marine plastic debris; microbial community; plastic polymer.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

FIGURE 1
FIGURE 1
Classification and distribution of 40 plastic particles. (A–C) Identification of polymers was achieved with Raman spectroscopy, representative Raman spectra are shown. (D) Most particles were identified as PE followed by PP and PS, and 5% of particles remained unidentified (polyethylene in red, polypropylene in blue, polystyrene in green, and unidentified particles in gray).
FIGURE 2
FIGURE 2
(A) Confocal laser scanning (A,B,D–F) and superresolution-structured illumination micrographs (C) of microbial communities on MPD. The images (A,C–F) were scanned as z-stack with a 63X/1.4 objective lens. For image (B), a 10X/0.3 objective lens was used. Images (D–F) were visualized as maximum-intensity projection with the following stack sizes: (D) 12.6 μm, (E) 14 μm, and (F) 18.5 μm. (A) Confocal laser scanning micrograph of a section of a plastic thread visualized as three-dimensional reconstruction. The micrograph shows DAPI-stained cells in blue, Eukarya hybridized with probe EUK516 in red, Archaea hybridized with probe Arch915 in yellow, and Bacteria hybridized with (EUB I) in green on the surface of the plastic particle. (B) Confocal laser scanning overview image showing the surface topography of a sheet-like PE microplastic particle as maximum-intensity projection of a 261-μm-thick z-stack, with prokaryotic and eukaryotic cells stained with DAPI (note that single microbes appear as bright white-blue spheres and rods while the plastic emits a relatively strong blue autofluorescence signal). (C) Superresolution-structured illumination micrograph of SYBR green-stained cells on the same particle as in (B) shown as a three-dimensional reconstruction close up [note that the field of view is not the same as in (B)]. (D) DAPI-stained cells (blue), Eukarya hybridized with probe EUK516 (red), Archaea hybridized with probe Arch915 (yellow), and Bacteria hybridized with probe EUB338-I (green; the polymer of this plastic piece remained undetermined). (E) Verrucomicrobia were identified by probe EUB338-III (shown in red) on a PE plastic piece. Numerous other microbes (probe EUB338-I, green; DAPI, blue) were detected. (F) DAPI-stained cells (blue) and Eukarya hybridized with probe EUK516 (shown in purple) on an unidentified MPD.
FIGURE 3
FIGURE 3
(A) Top 10 dominant orders (percentage of total 16S rRNA gene read counts) per plastic type (PE, polyethylene; PP, polypropylene; PS, polystyrene). Error bars depict the standard deviation, and only the upper bar is shown. (B) Venn diagram showing the unique and shared number and percentage of the 100 most abundant orders of each plastic type.
FIGURE 4
FIGURE 4
Non-metric multidimensional scaling (NMDS) of microbial communities on OTU level based on 16S rRNA gene sequencing from biofilms colonizing polyethylene, polystyrene, and polypropylene plastic polymers. nMDS on order level yielded similar results (Supplementary Table 1).
FIGURE 5
FIGURE 5
Bacterial genera with members involved in hydrocarbon or oil degradation. Their abundance is presented per plastic type. Sizes of the circles represent rounded percentage of 16S rRNA gene sequence reads assigned to the different taxa.
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