The plastic-associated microorganisms of the North Pacific Gyre

•Microorganisms mediate processes affecting the fate and impacts of marine plastic.•North Pacific Gyre (NPG) plastics were examined with scanning-electron microscopy.•Bacillus bacteria and pennate diatoms dominated the NPG plastic fouling community.•Bacterial abundance was patchily distributed but increased on foamed polystyrene.•Diatom abundance increased on rough surfaces and at sites with high plastic density. Microorganisms likely mediate processes affecting the fate and impacts of marine plastic pollution, including degradation, chemical adsorption, and colonization or ingestion by macroorganisms. We investigated the relationship between plastic-associated microorganism communities and factors such as location, temperature, salinity, plankton abundance, plastic concentration, item size, surface roughness, and polymer type. Small plastic items from the surface of the North Pacific Gyre in 2011 were examined using scanning electron microscopy. Bacillus bacteria (mean 1664±247 individuals mm−2) and pennate diatoms (1097±154mm−2) were most abundant, with coccoid bacteria, centric diatoms, dinoflagellates, coccolithophores, and radiolarians present. Bacterial abundance was patchy, but increased on foamed polystyrene. Diatom abundance increased on items with rough surfaces and at sites with high plastic concentrations. Morphotype richness increased slightly on larger fragments, and a biogeographic transition occurred between pennate diatom groups. Better characterizing this community will aid in understanding how it interacts with plastic pollution.