Microplastic-associated microbes: variability and trends in the Bay of Naples (Italy)

Raffaella Casotti1, Vincenzo Donnarumma1, Fabio D'Agostino2, Roberta Piredda3, Erik Zettler4, Carola Murano5, Anna Palumbo5, Augusto Passarelli3, Linda Amaral Zettler6 and Alvaro Adame Rodriguez3, (1)Stazione Zoologica Anton Dohrn, EMI Department, Naples, Italy, (2)CNR, ISMAR Capo Granitola and Mazara del Vallo, Italy, Italy, (3)STAZIONE ZOOLOGICA A. DOHRN, Napoli, Italy, (4)NIOZ, Royal Netherlands Institute for Sea Research and Utrecht University, Den Burg, Netherlands, (5)Stazione Zoologica Anton Dohrn, BEOM Department, Naples, Italy, (6)NIOZ, Royal Institute for Sea Research and University of Amsterdam, Texel and Amsterdam, Netherlands
The “Plastisphere” harbors a rich collection of microbes, as well as offers support to macroorganisms such as invertebrates like microscopic animals. We present here an investigation of the microbial plastisphere associated with microplastic pieces collected at 3 sites in the Gulf of Naples (Italy) in January and July 2018. Plastic pieces were counted and characterized for their chemical composition by Fourier-transform infrared spectroscopy (FT-IR) in ATR mode. The microbial community attached was analyzed by DNA amplicon Illumina sequencing of 16S rRNA gene hypervariable regions V4 and V5 and quantified using Scanning Electron Microscopy (SEM).

Average floating microplastic concentrations (pieces per m3) were higher in July (3.13 ± SE 1.08) than in January (1.39 ± SE 0.12), although not significantly different. Polyethylene was the dominant polymer at both dates. July samples presented significantly (p<0.001) more diatoms per mm2 (386 ± SD 389) than January samples (227 ± SD 507) as to be expected for a bloom period. Conversely, January samples showed more bacteria (8905 ± SD 14392) per mm2, than July ones (average 7264 ± SD 12776) per mm2, although not significant. Diatoms were represented by genera such as Cocconeis, Amphora, Mastogloia, among others. Bacterial DNA sequencing showed that the bacterial community attached to microplastics was different from the free-living one, with few overlapping OTUs. Bacteroidetes dominated the free-living and Proteobacteria the attached bacteria. In addition, different polymers had different bacterial communities, with Bacteroidetes phylum being more abundant on polyethylene and Firmicutes on polystyrene, suggesting that the substrate has a role in selecting bacterial species.

In general, our data highlight a specific community harbored by microplastics in seawater and suggest that plastic pieces represent a selective habitat for microbes, whose community composition is affected by polymer structure, seasonal environmental factors, circulation and time of residency in the water.