Effects of Three Antifouling Biocides on Marine Biofilm-Forming Bacteria: Highlighting the Need to Monitor Resistance Development When Reducing Active Compound Concentrations

Environmental concerns regarding the ecotoxicological effects of antifouling biocides have led to the development of products targeting biofilm-forming bacteria. However, the potential for sublethal biocide exposure to promote bacterial adaptation and increase the risk of resistance development poses a potential threat to marine ecosystems and human health, yet it remains insufficiently understood. Although this study focuses on conventional antifouling biocides, the findings are intended to inform the future development and evaluation of both conventional and environmentally friendly antifouling technologies by highlighting the importance of assessing resistance induction at sublethal concentrations. In this study, the effects of three representative antifouling biocides on marine bacterial growth and bacterial adhesion were investigated. Sea-Nine 211 (DCOIT), copper sulphate (CuSO4), and tributyltin oxide (TBTO; included as a historical reference compound due to its environmental persistence) were tested at four concentrations (0.01, 0.1, 1.0, and 10 µg/mL) against six marine biofilm-forming bacteria: Vibrio proteolyticus, V. aestuarianus, V. harveyi, V. natriegens, Shewanella putrefaciens and Pseudoalteromonas elyakovii. The results showed that Sea-Nine 211 exhibited a strong antibacterial effect at 10 µg/mL against all tested species except V. harveyi, whereas at the lowest concentration it promoted bacterial adhesion in V. proteolyticus. In contrast, TBTO and CuSO4 showed limited antibacterial activity and increased microbial adhesion at the three lowest concentrations tested. These findings demonstrate that antifouling biocides can induce distinct responses depending on the concentration, ranging from growth inhibition to enhanced bacterial adhesion. Given that reducing biocide release has been proposed as a strategy to mitigate environmental impacts, our results highlight two potential challenges: (i) reduced antifouling efficacy at sublethal concentrations and (ii) an increased risk of bacterial adaptation associated with enhanced adhesion. To support future monitoring and resistance risk assessment, we propose a conceptual Resistance Risk Index (RRI) framework that could contribute to the sustainable management of antifouling agents while accounting for local environmental conditions.