Microplastics effects on marine microbial communities and their functioning

Microplastics pervade ocean ecosystems. Despite their effects on individuals or populations are well documented, the consequences of microplastics on ecosystem functioning, especially regarding lower trophic levels, are still largely unknown. Here we show how microplastics alter the structure and functioning of pelagic microbial ecosystems. Using experimental pelagic mesocosms, we found that microplastics indirectly affect marine productivity by changing the bacterial and phytoplankton assemblages. Specifically, the addition of microplastics increased phytoplankton biomass and shifted bacterial assemblages’ composition. Such changes altered the interactions between heterotrophic and autotrophic microbes and the cycling of ammonia in the water column, which ultimately benefited photosynthetic efficiency. The effects of microplastics on marine productivity were consistent for different microplastic types. This study demonstrates that microplastics affect bacteria and phytoplankton communities and influence marine productivity, which ultimately alters the functioning of the whole ocean ecosystem.

Methods
The dataset comes from an experimental mesocosm system. Six cylindrical nets (50 m3 each, 15 meters depth) were set in the surface waters of a coastal Mediterranean area (Gulf of Naples). The six mesocosms were divided into two groups: the first (M1, M2 and M3, hereafter No-Microplastics group) was only treated with nutrient fertilization, while the second (M4, M5 and M6, hereafter Microplastics group) with nutrient fertilization and microplastics addition. Five different polymers were used: polystyrene (PS, density 1.04-1.09 g cm-3), polyethylene (PE, 0.89-0.95 g cm-3), polypropylene (PP, 0.85-0.92 g cm-3), polyvinylchloride (PVC, 1.16-1.41 g cm-3) and polyethyleneterephthalate (PET, 1.34-1.41 g cm-3). We studied changes in phytoplankton productivity following the experimental addition of microplastics. Phytoplankton productivity was measured as the maximum quantum yield of primary photochemistry, which is the ratio between variable and maxima fluorescence (Fv/Fm) and reflects photosynthetic efficiency (Gorbunov and Falkowski 2022). To investigate the effects of microplastics on phytoplankton productivity mediated by bacterial and phytoplankton assemblages and their interaction, we measured three additional variables: 1) chlorophyll a concentration (mg·m-3), as a proxy of phytoplankton biomass, 2) the proportion of high (HNA) versus low (LNA) nucleic acid concentration (HNA/[HNA+LNA]) as a proxy of the bacterial community structure (see below), and 3) the concentration of ammonium (NH4+), a key element of the nitrogen cycle essential for both bacteria and phytoplankton. We use structural equation modesl to explore the direct and indirect effects of microplastics on marine microbial communities and their functioning.