Polystyrene microplastics increase microbial release of marine Chromophoric Dissolved Organic Matter in microcosm experiments

Luisa Galgani,Claudio Rossi,Anja Engel,Steven A Loiselle,Alessandro Donati

doi:10.1038/s41598-018-32805-4

Abstract

About 5 trillion plastic particles are present in our oceans, from the macro to the micro size. Like any other aquatic particulate, plastics and microplastics can create a micro-environment, within which microbial and chemical conditions differ significantly from the surrounding water. Despite the high and increasing abundance of microplastics in the ocean, their influence on the transformation and composition of marine organic matter is largely unknown. Chromophoric dissolved organic matter (CDOM) is the photo-reactive fraction of the marine dissolved organic matter (DOM) pool. Changes in CDOM quality and quantity have impacts on marine microbial dynamics and the underwater light environment. One major source of CDOM is produced by marine bacteria through their alteration of pre-existing DOM substrates. In a series of microcosm experiments in controlled marine conditions, we explored the impact of microplastics on the quality and quantity of microbial CDOM. In the presence of microplastics we observed an increased production of CDOM with changes in its molecular weight, which resulted from either an increased microbial CDOM production or an enhanced transformation of DOM from lower to higher molecular weight CDOM. Our results point to the possibility that marine microplastics act as localized hot spots for microbial activity, with the potential to influence marine carbon dynamics.

Highlights

The most abundant microplastics polymers found in marine surface waters are low-density polyolefines and polystyrene[6,7]
In experimental set up 2 (ES2), we focused on potential impacts of microplastics on dissolved organic matter (DOM) cycling with respect to photochemical processes with two experiments in series, first in the light and subsequently in the dark
For ES2, higher concentrations of microplastics (92000 particlesL−1) were used. In both experimental set up 1 (ES1) and ES2 changes in Chromophoric Dissolved Organic Matter (CDOM), dissolved organic carbon (DOC) and bacterial abundance were measured at fixed intervals over the experimental periods

Summary

Introduction

The most abundant microplastics polymers found in marine surface waters are low-density polyolefines (polyethylene, polypropylene) and polystyrene[6,7]. The first coating may facilitate the adhesion of other microorganisms towards the formation of a more complex biofilm[9], a matrix of self-produced extracellular organic matter that provides mechanical stability, mediates microbial adhesion to surfaces and allows for microbial metabolism by embedding extracellular enzymes[14] With their increased concentrations in the aquatic environment, microplastics may become a vector for carbon transport by favoring the marine biological pump[15]. About half of oceanic primary production is channeled into the dissolved phase as Dissolved Organic Matter (DOM), which is present in a continuum of sizes and reactivities, including colloidal phases Bacterial interaction with this pool creates activity hotspots that can host high microbial diversity and distinct biogeochemical processes[16]. While recent findings have shown that carbon leaching from plastics may stimulate microbial activity[22], by acting as a substrate for organic matter release microplastics would be interfering with organic matter cycling in the oceans

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Conclusion