The spectroscopic detection and bacterial colonisation of synthetic microplastics in coastal marine sediments

Synthetic microplastics (≤5-mm fragments) are anthropogenic contaminants that are rapidly accumulating in coastal environments worldwide. The distribution, abundance, ecological impacts and fate of these pollutants are poorly understood. In this study, a novel reflectance micro-FT-IR spectroscopy method was developed to detect microplastics in sediments from 17 sites (UK), using polyethylene (PE) as the model polymer. Additionally, a 14-day laboratory microcosm experiment was used to characterise bacterial succession on low-density polyethylene (LDPE) fragments over time and across three sediment types (ranging from sand to silt) from the Humber Estuary, UK. In contrast with ATR-FT-IR measurements, micro-FT-IR measurements in reflectance mode were susceptible to refractive error when analysing irregularly shaped PE fragments. However, molecular mapping by reflectance micro-FT-IR spectroscopy successfully detected PE within polymer-spiked sediments and in a non-spiked sediment retentate. Moreover, reflectance micro-FT-IR spectra of PE were consistent across all 17 sampling sites. Bacteria were found to rapidly colonise LDPE fragments, with bacterial community structure and diversity differing significantly from those in bulk sediments, as demonstrated by scanning electron microscopy, T-RFLP analysis and 16S rRNA gene sequencing. The composition of LDPE-colonising assemblages within different sediment types increasingly converged over time, with 16S rRNA gene sequencing analysis identifying site-specific populations of the genera Arcobacter (Epsilonproteobacteria) and Colwellia (Gammaproteobacteria) as dominant members (up to 93% of sequences) of the plastic-associated communities after 14 days of exposure. Log-fold increases in the relative abundance of LDPE-associated bacteria occurred within 7 days of exposure with bacterial abundance differing significantly across sediment types, as shown by Q-PCR amplification of 16S rRNA genes. Attachment of bacterial cells and specifically of Colwellia spp. onto LDPE surfaces was demonstrated by CARD-FISH analysis. These results provide a foundation to both developing improved spectroscopy methods to detect microplastics, and characterising ecological interactions between microorganisms and microplastic debris within marine sediments.