Influence of shear and turbulence fluctuations on biofilm growth: A key issue for ocean microplastics*

This study investigates biofilm growth under turbulent flows with the aim of understanding biofouling on microplastics (MPs) in oceanic environments. By increasing particle stickiness, biofilms promote MP aggregation and sinking; therefore, a thorough understanding of this multi-scale process is crucial to improve predictions of the MPs fate. We conducted a series of laboratory experiments using an oscillating-grid system to promote biofilm growth under homogeneous-isotropic turbulence with a grid Reynolds number between 305 and 2220. Biofilm formed in all cases in a days-scale then the biomass on the pieces was carefully measured and interpreted using a one-dimensional model, coupling the biofilm metabolism (Michaelis-Menten kinetics) with the turbulent diffusion of the surrounding bulk liquid. The biomass initially grows with turbulence intensity following an increase of the Sherwood number (i.e., enhanced availability of nutrients); then, the growth rate saturates due to: (i) uptake-limited kinetics; (ii) biofilm erosion due to shear forces. Furthermore, a subset of plastic pieces were analyzed with a scanning electron microscope, revealing that turbulence also affects the microscopic configuration of biofilm clusters, increasing their compactness with the turbulent fluctuations. These results contribute not only to our fundamental understanding of biofilms under flow, but can also inform global models of MP transport in marine environments.