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