Anthraquinones (AQs) are chemical scaffolds that have been used both naturally and synthetically for centuries in the food, pharmaceutical, cosmetic and paint industries. AQs, such as citreorosein and emodin, are common additives in antifouling paints which help prevent the global issue of biofouling. To determine the antifouling potential of a family of structurally related compounds nineteen AQs (1–19), were tested for their microbial growth and biofilm adhesion inhibition activity against three marine biofilm forming bacteria, Vibrio carchariae, Pseudoalteromonas elyakovii and Shewanella putrefaciens. More than three-quarters of the tested AQ compounds exhibited activity against both V. carchariae and P. elyakovii at 10 μg/ml whilst exhibiting low antimicrobial effects. The most active compounds (1, 5, 6, 7, 9, 10, 14, 15, 18, 19) were tested for their minimum inhibitory concentrations (MICs) which highlighted that all the tested compounds were highly effective at inhibiting the biofilm growth of P. elyakovii at a very low concentration of 0.001 μg/ml. The variability in MIC for inhibiting the biofilm growth of V. carchariae was assessed by analysing the structure-activity relationships (SARs) between the AQ compounds, and the key structural features leading to improved biofilm growth inhibition activity are reported. Molecular docking analysis was also performed to assess whether interruption of quorum sensing in V. carchariae could be a possible mode of action for the antifouling activity observed.

The syntheses, spectroscopic data and crystal structures of (H₂NCOCH₂CH₂-C)(R₂NCS₂-S,S^′)₃Sn (1), (H₂NCOCH₂CH₂-C,O)₂SnCl₂ (triclinic polymorph) (2), (H₂NCOCH₂CH₂-C,O)(2-H₂NCOC₆H₄S-O,S)SnCl₂ (3) and Sn(C₁₁H₉N₂O₂)Cl₃ (4) are reported. The tin atom in compound 1 (R = Et) is coordinated by three chelating dithiocarbamate anions and a C-bound non-chelating amidoethyl ligand to generate a SnS₆C pentagonal-bipyramidal coordination polyhedron. Compound 2, which features SnC₂O₂Cl₂ octahedra, was crystallised from mixed solvents (ethanol and water) and complements the two known monoclinic forms. Compound 3 arose unexpectedly due to ligand disproportionation of the tin starting material and a “trans-esterification” reaction of the starting ligand: distorted SnCSO₂Cl₂ octahedra are seen in the crystal structure. Compound 4 arose from another ligand disproportionation reaction and features neutral complex molecules with N₂OCl₃ donor sets coordinating to the octahedral tin atoms.

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.