Polymer specific or core plastisphere? Using microscopy and DNA based analyses to assess microbial colonization of microplastics in the Caribbean Sea

Microplastics, plastic particles less than 5 mm, are emerging marine pollutants that act as a novel matrix for diverse assemblages of prokaryotic and eukaryotic organisms to colonize and form biofilms, or “plastispheres”. The communities growing on the microplastics may ultimately determine their degradation, deposition and utilization by the food web, but the factors that shape their composition and development, such as environment, plastic type and exposure time, are still insufficiently understood. We carried out a controlled 6-week long in situ incubation experiment of six common plastic polymers: polyethylene terephthalate (PETE, #1), high-density polyethylene (HDPE, #2), polyvinyl chloride (PVC, #3), low-density polyethylene (LDPE, #4), polypropylene (PP, #5), and polystyrene (PS, #6), in Bocas del Toro, Panama. Sequencing analyses of the V4 and V4-V5 regions of the 16S and 18S RNA genes, respectively, show that prokaryotic and eukaryotic communities of the plastisphere were not shaped by polymer specific specialists, but were instead driven by time exposed to the environment. Conversely, when analyzing the diatom communities that colonized the plastics using scanning electron microscopy, we observed that this group exhibited polymer preference by the end of the six-week incubation period. Results also showed that microplastics could serve as a vector for both pathogenic and toxigenic eukaryotes; organisms that could be transported on the plastics from the coastal bays into the open ocean via currents or uptake by zooplankton and fish, or possibly affect benthic communities by sinking to the seafloor. Our research shows that a complementary approach that includes both DNA-based and microscopy-based analyses is necessary to comprehensively characterize plastispheres and to understand the factors that aid in their development.