Microplastics alter the functioning of marine microbial ecosystems

Abstract

Microplastics pervade ocean ecosystems. Despite their effects on individuals or populations are well documented, the consequences of microplastics on ecosystem functioning 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.

Keywords: bacteria; ecosystem functioning; microplastics; oceans; phytoplankton.

FIGURE 1

Mesocosm experimental design. (a) Schema of the six mesocosms. Blue and red cylindrical nets correspond to control treatments and treatments to which microplastics were added, respectively. Microbial communities composed of bacteria and phytoplankton were sampled within each net. Different particle colors within microplastic treatments indicate different types of microplastic. (b) Experimental location (Gulf of Naples, Mediterranean Sea). (c) View of the six mesocosms (two groups of three) from the sea surface. (d) Underwater picture of a three mesocosms group.

FIGURE 2

The effect of microplastics on several aspects of the structure and functioning of marine microbial communities. Plots show linear regressions between microplastic concentrations and structural and functional variables. Bacterial composition is represented by the proportion of high (HNA) versus low (LNA) nucleic acid concentration (HNA/[HNA + LNA]) as a proxy of the bacterial community structure (a). Chlorophyll a concentration (mg·m⁻³) was used as a proxy of phytoplankton biomass (b). Ammonia concentration (NH₄ ⁺), a key element ofthe nitrogen cycle essential for bacteria and phytoplankton and used in photosynthesis (c). Productivity is represented by photosynthetic efficiency, measured as the ratio between variable and maxima fluorescence (F _v /F _m) (d).

FIGURE 3

Structural equation model exploring the effects of microplastics on marine productivity. Productivity is represented by photosynthetic efficiency, which is measured as F _v /F _m. Black and red solid arrows denote positive and negative associations, respectively. Dashed paths indicate no detectable influence of the driver (p ≥ .05). Fisher's C = 3.738; df = 6; p = .712; AIC_c = 14.796. Numbers in boxes represent the standardized coefficient of each path. R ²s are reported as the conditional R ² based on the variance of both the fixed and random effects. Individual models for different microplastic types are presented in Table 1.

FIGURE 4

Microplastics' (MPS) effects on light conditions and β‐glucosidase activity. (a) Light conditions (daily light irradiance) with and without microplastics measured as anomalies of the daily light irradiance at 14 h00 (local time) (Mann–Whitney test, N = 8, p = .0009). (b) Daily β‐glucosidase activity with and without microplastics (Mann–Whitney test, N = 6; p = .005).