Surface properties influence marine biofilm rheology, with implications for ship drag.

Marine biofilms on ship hulls increase frictional drag, which has economic and environmental consequences. It is hypothesised that biofilm mechanics, such as viscoelasticity, play a critical role in biofilm-associated drag, yet is a poorly studied area. The current study aimed to rheologically characterise ship-relevant marine biofilms. To combat marine biofilms on ship hulls, fouling-control coatings are often applied; therefore, the effect of different surfaces on marine biofilm mechanics was also investigated. Three surfaces were tested: a non-biocidal, chemically inert foul-release coating (FRC), an inert primer (ACP) and inert PVC. Physical properties of biofilms were explored using Optical Coherence Tomography (OCT) and a parallel-plate rheometer was used for rheological testing. Image analysis revealed differences in the thickness, roughness, and percent coverage between the different biofilms. Rheological testing showed that marine biofilms, grown on FRC and ACP acted as viscoelastic materials, although there were differences. FRC biofilms had a lower shear modulus, a higher viscosity, and a higher yield stress than the ACP biofilms, suggesting that the FRC biofilms were more readily deformable but potentially more robust. The results confirmed that surface treatment influences the structural and mechanical properties of ship-relevant marine biofilms, which could have implications for drag. A better understanding of how different surface treatments affect marine biofilm rheology is required to improve our knowledge on biofilm fluid-structure interactions and to better inform the coating industry of strategies to control biofilm formation and reduce drag.

Coupled CFD-DEM modeling to predict how EPS affects bacterial biofilm deformation, recovery and detachment under flow conditions.

The deformation and detachment of bacterial biofilm are related to the structural and mechanical properties of the biofilm itself. Extracellular polymeric substances (EPS) play an important role on keeping the mechanical stability of biofilms. The understanding of biofilm mechanics and detachment can help to reveal biofilm survival mechanisms under fluid shear and provide insight about what flows might be needed to remove biofilm in a cleaning cycle or for a ship to remove biofilms. However, how the EPS may affect biofilm mechanics and its deformation in flow conditions remains elusive. To address this, a coupled computational fluid dynamic- discrete element method (CFD-DEM) model was developed. The mechanisms of biofilm detachment, such as erosion and sloughing have been revealed by imposing hydrodynamic fluid flow at different velocities and loading rates. The model, which also allows adjustment of the proportion of different functional groups of microorganisms in the biofilm, enables the study of the contribution of EPS toward biofilm resistance to fluid shear stress. Furthermore, the stress-strain curves during biofilm deformation have been captured by loading and unloading fluid shear stress to study the viscoelastic properties of the biofilm. Our predicted emergent viscoelastic properties of biofilms were consistent with relevant experimental measurements.

A rapid benchtop method to assess biofilm on marine fouling control coatings.

A rapid benchtop method to measure the torque associated with minidiscs rotating in water using a sensitive analytical rheometer has been used to monitor the drag caused by marine fouling on coated discs. The method was calibrated using sandpaper surfaces of known roughness. Minidiscs coated with commercial fouling control coatings, plus an inactive control, were exposed in an estuarine harbour. After 176 days the drag on the fouling control-coated discs, expressed as a moment coefficient, was between 73% and 90% less than the drag on the control coating. The method has potential use as a screen for novel antifouling and drag reducing coatings and surfaces. Roughness functions derived using Granville's indirect similarity law are similar to patterns found in the general hydrodynamics literature, and so rotational minidisc results can be considered with reference to other fouling drag datasets.Supplemental data for this article is available online at https://doi.org/10.1080/08927014.2021.1929937 .

Comparative study of biofilm formation on biocidal antifouling and fouling-release coatings using next-generation DNA sequencing.

The bacterial and eukaryotic communities forming biofilms on six different antifouling coatings, three biocidal and three fouling-release, on boards statically submerged in a marine environment were studied using next-generation sequencing. Sequenced amplicons of bacterial 16S ribosomal DNA and eukaryotic ribosomal DNA internal transcribed spacer were assigned taxonomy by comparison to reference databases and relative abundances were calculated. Differences in species composition, bacterial and eukaryotic, and relative abundance were observed between the biofilms on the various coatings; the main difference was between coating type, biocidal compared to fouling-release. Species composition and relative abundance also changed through time. Thus, it was possible to group replicate samples by coating and time point, indicating that there are fundamental and reproducible differences in biofilms assemblages. The routine use of next-generation sequencing to assess biofilm formation will allow evaluation of the efficacy of various commercial coatings and the identification of targets for novel formulations.

A computational model for microbial colonization of an antifouling surface.

Biofouling of marine surfaces such as ship hulls is a major industrial problem. Antifouling (AF) paints delay the onset of biofouling by releasing biocidal chemicals. We present a computational model for microbial colonization of a biocide-releasing AF surface. Our model accounts for random arrival from the ocean of microorganisms with different biocide resistance levels, biocide-dependent proliferation or killing, and a transition to a biofilm state. Our computer simulations support a picture in which biocide-resistant microorganisms initially form a loosely attached layer that eventually transitions to a growing biofilm. Once the growing biofilm is established, immigrating microorganisms are shielded from the biocide, allowing more biocide-susceptible strains to proliferate. In our model, colonization of the AF surface is highly stochastic. The waiting time before the biofilm establishes is exponentially distributed, suggesting a Poisson process. The waiting time depends exponentially on both the concentration of biocide at the surface and the rate of arrival of resistant microorganisms from the ocean. Taken together our results suggest that biofouling of AF surfaces may be intrinsically stochastic and hence unpredictable, but immigration of more biocide-resistant species, as well as the biological transition to biofilm physiology, may be important factors controlling the time to biofilm establishment.

Marine biofilms on different fouling control coating types reveal differences in microbial community composition and abundance.

Marine biofouling imposes serious environmental and economic impacts on marine applications, especially in the shipping industry. To combat biofouling, protective coatings are applied on vessel hulls which are divided into two major groups: biocidal and non-toxic fouling release. The current study aimed to explore the effect of coating type on microbial biofilm community profiles to better understand the differences between the communities developed on fouling control biocidal antifouling and biocidal-free coatings. Biocidal (Intersmooth® 7460HS SPC), fouling release (Intersleek® 900), and inert surfaces were deployed in the marine environment for 4 months, and the biofilms that developed on these surfaces were investigated using Illumina NGS sequencing, targeting the prokaryotic 16S rRNA gene. The results confirmed differences in the community profiles between coating types. The biocidal coating supported communities dominated by Alphaproteobacteria (Loktanella, Sphingorhabdus, Erythrobacter) and Bacteroidetes (Gilvibacter), while other taxa, such as Portibacter and Sva0996 marine group, proliferated on the fouling-release surface. Knowledge of these marine biofilm components on fouling control coatings will serve as a guide for future investigations of marine microfouling as well as informing the coatings industry of potential microbial targets for robust coating formulations.