Effects of biologically modified seawater on settling behaviour of microplastics

Microplastics (MPs) pose a serious threat to marine ecosystems. The interaction between MPs and surrounding environment depends to large extent on MPs settling dynamics. However, our knowledge about sinking of MPs in complex marine system is still insufficient. Here we show that sinking dynamics of MPs may completely change when seawater is biologically modified by the presence of exopolymers (EPSs) secreted by microorganisms during algal blooms. This is due to increased viscosity and modification of rheological properties of seawater induced by EPSs. To study these effects, we compared settling dynamics of spherical and cylindrical MPs made of PET, POM and PA in artificial seawater, salinity of 36, and in seawater with addition of xanthan gum as a proxy for EPSs. We performed rheological measurements of test liquids using oscillatory rheometer and settling experiments of MPs in a laboratory column using visualization and image analysis methods. Flow and oscillatory rheological tests revealed that upon exopolymers addition, a Newtonian seawater turned into shear-thinning viscoelastic liquid. The effects of modified seawater rheology were then observed in the behaviour of MPs sinking in the settling column. Stronger mechanical properties of biologically modified seawater induced MPs settling velocity decrease, while normal stresses caused orientation instabilities of non-spherical MPs compared to settling in seawater. Our results indicate that excessive presence of exopolymers secreted by microorganisms significantly change settling behaviour of MPs, which may have implications to the interaction of MPs with biotic and abiotic components of ecosystem. Also see: https://micro2022.sciencesconf.org/426670/document