*Article* **How do Graphene Composite Surfaces Affect the Development and Structure of Marine Cyanobacterial Biofilms?**

**Maria J. Romeu 1,2, Luciana C. Gomes 1,2, Francisca Sousa-Cardoso 1,2, João Morais 3, Vítor Vasconcelos 3,4, Kathryn A. Whitehead 5, Manuel F. R. Pereira 2,6, Olívia S. G. P. Soares 2,6 and Filipe J. Mergulhão 1,2,\***


**Abstract:** The progress of nanotechnology has prompted the development of novel marine antifouling coatings. In this study, the influence of a pristine graphene nanoplatelet (GNP)-modified surface in cyanobacterial biofilm formation was evaluated over a long-term assay using an in vitro platform which mimics the hydrodynamic conditions that prevail in real marine environments. Surface characterization by Optical Profilometry and Scanning Electron Microscopy has shown that the main difference between GNP incorporated into a commercially used epoxy resin (GNP composite) and both control surfaces (glass and epoxy resin) was related to roughness and topography, where the GNP composite had a roughness value about 1000 times higher than control surfaces. The results showed that, after 7 weeks, the GNP composite reduced the biofilm wet weight (by 44%), biofilm thickness (by 54%), biovolume (by 82%), and surface coverage (by 64%) of cyanobacterial biofilms compared to the epoxy resin. Likewise, the GNP-modified surface delayed cyanobacterial biofilm development, modulated biofilm structure to a less porous arrangement over time, and showed a higher antifouling effect at the biofilm maturation stage. Overall, this nanocomposite seems to have the potential to be used as a long-term antifouling material in marine applications. Moreover, this multifactorial study was crucial to understanding the interactions between surface properties and cyanobacterial biofilm development and architecture over time.

**Keywords:** antifouling surface; biofilm architecture; cyanobacterial biofilm; graphene; marine biofouling
