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Settling of microplastics in mucus-rich water column: The role of biologically modified rheology of seawater
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Laboratory experiments showed that naturally occurring exopolymers secreted by marine algae and bacteria convert seawater into a non-Newtonian fluid that slows the sinking of microplastic particles, altering how and where they accumulate in the ocean water column. Understanding these biologically driven settling dynamics is important for predicting microplastic distribution and exposure risk for marine organisms at different depths.
Non-buoyant microplastics (MPs) sink through the marine water column, adversely affecting the ecosystem. The manner in which MPs influence the water environment depends to a large extent on their settling dynamics, driven by their properties and the physio-chemical characteristics of water column. However, some properties of seawater remain elusive, limiting our ability to fully explain the sinking processes of MPs. One of the gaps in our understanding relates to the elevated content of exopolymers (EPSs) secreted by algae and bacteria, which locally transform seawater into a non-Newtonian liquid, altering the hydrodynamics of particle transport. In this study, we present a series of lab-scale experiments on the dynamics of isometric (spheres and irregular particles) and anisometric (disks, rods, and blades) MPs settling in artificial seawater with the addition of polysaccharides. We find that upon the appearance of EPSs in seawater, the sinking velocity of MPs diminishes and may fluctuate, the orientation pattern changes in a non-intuitive way, and MPs may tumble. As measured in rheological tests, these consequences result from seawater gaining viscoelastic and shear-thinning properties. Our findings raise concerns that mucus-rich seawater may favor the aggregation of MPs with organic matter, interaction with biota, and biofouling, which can affect the biogeochemistry of the marine ecosystem. Based on these findings, we recommend that seawater rheology, modified by excessive amounts of EPSs during algal blooms, should be considered in biogeochemical and microplastic transport models.
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Effects of biologically modified seawater on settling behaviour of microplastics
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Researchers found that during algal blooms, exopolymers secreted by microorganisms significantly increased seawater viscosity and changed how microplastic particles sank through the water column. This finding means that the sinking behavior of microplastics in real ocean conditions — especially during bloom events — is far more complex than laboratory tests with clean seawater suggest.
Effects of rheologically stratified seawater during algal bloom on sinking dynamics of microplastics
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This study investigated how algal blooms affect the sinking behavior of microplastics in seawater by examining the role of exopolymers secreted by microorganisms during bloom events, which make the water more viscous and gel-like. In bloom-affected, rheologically modified seawater, microplastics sank differently than in normal conditions, with some particles being retarded and others accelerated depending on their properties. Understanding these dynamics is important because algal blooms are becoming more frequent globally, and how microplastics sink determines how long they remain in the water column where marine life is exposed to them.
Viscoelastic and shear-thinning effects of aqueous exopolymer solution on disk and sphere settling
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Using a gel-like polymer to mimic natural aquatic conditions, researchers found that the gel's non-Newtonian properties significantly slow the settling speed of particles compared to plain water. This suggests microplastics in natural waters rich in organic matter may sink more slowly than lab studies in clean water would predict.
Effect of exopolymer gels on the viscoelasticity of mucus-rich saltwater and settling dynamics of particles
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Researchers investigated how exopolymer gels produced during algal blooms affect the rheological properties of mucus-rich saltwater and the settling dynamics of particles, using xanthan gum as a model exopolymer in sodium chloride solutions. They found that colloidal exopolymers created a stronger internal structure than particulate gels and increased drag coefficients on sinking particles up to sixfold, suggesting colloidal exopolymers retard particle settling in marine environments more effectively.
Phytoplankton Exopolymers Enhance Adhesion of Microplastic Particles to Submersed Surfaces
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Researchers found that exopolymers secreted by phytoplankton — sticky substances produced naturally in seawater — significantly enhance the adhesion of microplastic particles to submerged surfaces. This suggests that phytoplankton activity plays an important role in how microplastics accumulate in marine environments.
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