On the surface interaction between Cylindrotheca fusiformis and aerospace aluminum alloys

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Microbes, and the biofilms they form on surfaces, generally have a negative impact on performance. Well­-known examples are the increase of drag in ships and pipes, and the accelerated corrosion of metallic structures known as MIC. Among microbes, diatoms form a large unicellular algae group known to play a relevant role in the first stages of biofilm formation. Diatoms are well-­known for their species­-dependent silica exoskeletons, but they also produce, as other microbes, extracellular polymeric substances (EPSs) that may offer opportunities for the development of novel bio­based surface treatments. In this work we studied the interaction between a marine diatom species named Cylindrotheca fusiformis and aerospace aluminum alloys as a first step towards novel surface protection treatments of aircraft structures.

Two routes were followed to study the interaction of C. fusiformis with aluminum alloys after studying diatom population growth. The first route focused on the biofilm formation on aluminum 99.5%, AA20204-­T3 and AA7075­-T6 to study the effect of surface composition. The effect of the surface preparation (degreased vs. polished) was then studied with AA2024­T3 substrates. The second route focused on the study of diatoms’ motility on aluminum 99.5% and AA2024-­T3 with various surface pretreatments. In this work, a number of characterization techniques such as FTIR, SEM/EDS, Raman spectroscopy and image correlation techniques were used.

It was found that C. fusiformis forms well­-adherent biofilms on all aluminum substrates, independently of the surface composition and surface pre­treatment. Nevertheless, the biofilms appeared to be most homogeneous on AA7075­T6, while the EPS layer was more homogeneous on aluminum 99.5%
and AA2024­T3. The motility studies showed that C. fusiformis lowers its surface motility rapidly when on aluminum 99.5% until cells stop being motile. On AA2024­T3, cells maintain their motility for longer periods of time. The motility dependency on surface chemistry is hypothesized to come from speciation dependent aluminum toxicity. Post­mortem chemical analysis of the exposed surfaces showed traces of organic material in the form of proteins and carbohydrates attributed to displacement trails. The results confirm that monocultures of C. fusiformis are able to generate biofilms on aerospace aluminum alloys and release water­ insoluble organic matter on the surface. The promising results here obtained pave the way for more dedicated research to understand the relationships between surface chemistry and topology, and diatom motility and biofilm formation and composition.