Vertical and seasonal variations in biofilm formation on plastic substrates in coastal waters of the Black Sea

Plastic contamination of the marine environment is an increasing concern worldwide. Therefore, it is important to understand the kinetics of biofilms on plastics to study their behavior, fate, and transport pathways in the ocean. In this study, the vertical and seasonal variations in biofouling formation on transparent polyethylene terephthalate (PET) plastic fragments in the Southwest Crimea coastal waters of the Black Sea were investigated. Biofilms were identified in the transient light as 'dark spots' on the plastic surface, for which the numbers, size, and area were measured using specialized software. The rate of biofouling in the surface water layer was lower than those found in the middle and near-bottom water column, which could be due to a damaging effect of turbulent mixing on the biofilm. The highest rates of biofouling and diverse community were observed during the summer. The epibiotic assembly was represented by diatoms (11 taxa), dinoflagellates (3 taxa), green algae, filamentous cyanobacteria, small flagellates, and ciliates. Significant differences between the biofouling rates observed in different seasons made it difficult to estimate the period of time the plastic substrate has been in the marine environment. It was proposed to use the green alga Phycopeltis arundinacea (Montgn) De Tender et al., 2015 as a bioindicator to study the age of the biofouling community. Discoid thalli were identified at all stages of colonization of the plastic fragments in different seasons. Results obtained in this study demonstrate that biofouling organisms may be good model organisms in revealing age of biofilm formation and longevity of plastic debris in the ocean. Consequently, it is proposed that such biofouling organisms could be used as target species to monitor the biodegradation of plastic debris.