DOI: 10.5061/dryad.z612jm6h2
title: Microplastics effects on marine microbial communities and their functioning

description: 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.
description: 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.
description: This repository includes the data and code used in Montoya et al
 (to be submitted to L&O Letters). More specifically: Dataset.xlsx
 includes data on MPS, bacteria and phytoplankton data, environmental
 variables, and ocean productivity ‘Microplastics code’ refers to the
 programming code used for the statistical analyses and plot construction
 using R software. Data and code are provided in Excel and text file
 format, respectively.