Organic matter production and recycling in marine biofilm developing on common and new plastics

To face the recent pandemic and comply with international legislation, new plastic objects (surgical masks, nitrile gloves, compostable plastics) have been produced, with a significant increase of their input into the marine environment, together with other common plastics. Given that floating plastic provides a suitable surface for settlement of micro-organism, biofilm accretion was studied in laboratory experiments. The characteristics of biofilm in terms of organic matter production and recycling were evaluated under natural and forced conditions, some of them resembling anthropogenic-affected states (eutrophication) and others environmental variability (darkness and oligotrophy). Under natural conditions, the different plastics, due to their structure and composition, hosted different biofilms. Thicker biofilm was observed on surgical mask and compostable plastic (organic carbon maxima of 35.0 ± 4.7 μg cm^-2 and 4.3 ± 0.8 μg cm^-2, respectively). Compostable plastic hosted a higher carbohydrate quantity than polyethylene terephthalate, polystyrene and nitrile (on average 8.0 ± 0.8 μg cm^-2 vs 3.6 ± 1.6 μg cm^-2 for the others). The multi-layer structure of masks and the composition of compostable plastic were the main factors responsible for these differences. Polystyrene and nitrile hosted a higher photoautotrophic biomass, with chlorophyll-a maxima higher than 50 μg cm^-2 vs values lower than 10 μg cm^-2 for compostable plastic. Inhibition of photosynthetic activity (darkness) allowed a greater biofilm mass, which in natural aphotic zone, may enhance the sinking of plastics. The large availability of carbon (eutrophication) allowed thicker biofilms, providing seawater of additional organic matter load. These biofilms could protect pathogenic organisms, especially on disposable protection equipment, allowing a larger spreading.