## 1 Introduction  
The accumulation on wet surfaces (natural or artificial) of undesirable microorganisms (bacteria, diatoms and spores of microalgae) is commonly known as fouling or biofouling [1]. Biofouling causes problems for microalgae farming in open culture systems (raceways) [2], due to pipes and valves clogging [3] and components corrosion. To prevent them it is vital to investigate and develop antibiofouling coatings of the surfaces in contact with the fluid.  
Such active coatings, capable of preventing the adhesion of such microorganisms, could be based on the use of amphiphilic polymers [4] such as polyvinyl alcohol (PVA). PVA is a biodegradable polymer soluble in water, and available in different degrees of hydrolysis, with potential as an antibiofouling nanoparticles (NPs) carrier. Its low toxicity [5] ability to form thin films [6; 7; 8; 9; 10] and stabilize nanoparticles [11] are known. However, aqueous media can produce PVA swelling and dissolve it, consequently producing open structures and lowering the continuity of its coatings [12]. Therefore, it is necessary to balance the hydrophilic and hydrophobic properties by controlled cross-linking [13] using multifunctional compounds, such as dialdehydes [14], dicarboxylic acids [15] and dianhydrides [16]. Chemical crosslinking is a versatile method to improve mechanical, thermal and chemical stability of polymers [17]. Glutaraldehyde (GA) is an attractive PVA crosslinking agent,since its reaction with the PVA's hydroxyl groups [18; 19] is easily promoted by a mineral acid, such as HCl [20].  
Metallic nanoparticles, such as copper [21] and silver, incorporated into polymeric matrices could be considered a new hybrid coating material given their potential advantages, but the latter must be proven. Copper shows an excellent antimicrobial activity against a wide range of microorganisms [22; 23]. Silver nanoparticles (AgNPs) and silver-doped formulations are also interesting when working in biological environments, since they are considered safer and less toxic than other heavy metal-based formulations. In addition, the application of nano-silver on surfaces is known to be very versatile, allowing wide flexibility of use [24; 25; 26]. It has been demonstrated that the incorporation of metal oxide nanoparticles, such as ZnO, CuO and TiO\({}_{2}\)[27; 28; 29] in polymer matrices has improved the antibiofouling properties of some coatings. However, according to recent studies, the antibiofouling power of TiO\({}_{2}\) nanoparticles decreases markedly on substrates with a rough and porous surface. This surface morphology promotes the retention of water and nutrients, favoring the growth of biomass and reducing the oxides inhibitory effect [30; 31].  
In this work, the antibiofouling capacity of a thin film coat, based on PVA crosslinked with GA, doped with metal and metal oxides nanoparticles was applied by spin coating. Silver and metal oxide nanoparticles (ZnO, CuO and TiO\({}_{2}\)) were synthesized by a seed and growth technique [32] and a sol-gel method [33] and added to the polymer matrix. The effect of the combined incorporation of nanosized Ag and TiO\({}_{2}\) was also studied.  
The effect of these different coatings was investigated by immersing the coated substrates in a laboratory-scale raceway with a _Coalastrella_ microalgae culture. The antibiofouling capacity of each coating was evaluated through image analysis after different immersion times (15 and 30 days) [34; 35].