Sinking of microbial-associated microplastics in natural waters.

Thu Ha Nguyen,Fiona H M Tang,Federico Maggi,Matthäus Bäbler

doi:10.1371/journal.pone.0228209

Abstract

Degraded plastic debris has been found in nearly all waters within and nearby urban developments as well as in the open oceans. Natural removal of suspended microplastics (MPs) by deposition is often limited by their excess buoyancy relative to water, but this can change with the attachment of biological matter. The extent to which the attached biological ballast affects MP dynamics is still not well characterised. Here, we experimentally demonstrate using a novel OMCEC (Optical Measurement of CEll colonisation) system that the biological fraction of MP aggregates has substantial control over their size, shape and, most importantly, their settling velocity. Polyurethane MP aggregates made of 80% biological ballast had an average size almost twice of those containing 5% biological ballast, and sank about two times slower. Based on our experiments, we introduce a settling velocity equation that accounts for different biological content as well as the irregular fractal structure of MP aggregates. This equation can capture the settling velocity of both virgin MPs and microbial-associated MP aggregates in our experiment with 7% error and can be used as a preliminary tool to estimate the vertical transport of MP aggregates made of different polymers and types of microbial ballast.

Highlights

The global plastic production has reached 348 million tonnes in 2017 [1] for household, retail, and industrial demands
The attached biological fraction has a well defined linear impact on the MP aggregate density ρ: for any MP density, the biological ballast can bring ρ close to the ambient fluid density ρf, or cause ρ to exceed or fall below ρf
Our experiment and analyses showed that the correlation between fb and MP aggregates architecture was nonlinear, with MP aggregates containing about 60% to 80% of biological ballast having the largest size, the most irregular shape, and the lowest fractal dimension

Summary

Introduction

The global plastic production has reached 348 million tonnes in 2017 [1] for household, retail, and industrial demands. Plastic production and use have not been accompanied by efficient plastic waste management, and 1 to 4% of this plastic amount reaches the ocean every year [2, 3]. Microplastics (MPs) with size smaller than 5 mm is the most common plastic debris found in marine habitats [4]. MPs contain toxic additives like phthalate, organotin, nonylphenol, polybrominated diphenyl ethers (PBDE), and triclosan, which can be released if degraded chemically or biologically [6, 7]. MPs have the peculiarity to sorb and be the carrier agent of anthropogenic pollutants such as dichloro-diphenyl-trichloroethane

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Conclusion