Biofouling in the marine environment refers to an unwanted build-up of marine organisms on subsea surfaces including harbor docks, hulls of ships and offshore installations. The first stage of marine fouling occurs as a microbial biofilm which forms via the aggregation of bacterial, algal, and fungal cells. This biofilm provides a favorable substrate for the larval settlement of larger organisms such as mussels, barnacles and hard corals which accumulate to uncontrollable extents, causing issues for the maritime industries. Since the ban of tributyltin (TBT) in 2008 by the International Maritime Organisation, alternative antifouling agents have been used such as algaecides and copper-based coatings. Recent studies are showing that these can accumulate in the marine environment and have toxic effects against non-target species. Marine microbes and invertebrates are known to be prolific producers of bioactive molecules, including antifouling active compounds. These compounds are often produced by marine organisms as a means of chemical defense to deter predators and prevent fouling of their own surfaces, making them a promising source of new antifouling agents. This article discusses the effects of biofouling on the maritime industries, the environmental dangers of currently used antifouling compounds and why natural products from marine organisms could be a source of environmentally friendly antifouling agents.

Marine biofouling is the undesired accumulation of organic molecules, microorganisms, macroalgae, marine invertebrates, and their by-products on submerged surfaces. It is a serious challenge for marine vessels and the oil, gas, and renewable energy industries, as biofouling can cause economic losses for these industries. Natural products have been an abundant source of therapeutics since the start of civilisation. Their use as novel anti-fouling agents is a promising approach for replacing currently used, harmful anti-fouling agents. Anthraquinones (AQs) have been used for centuries in the food, pharmaceutical, cosmetics, and paint industries. Citreorosein and emodin are typical additives used in the anti-fouling paint industry to help improve the global problem of biofouling. This study is based on our previous study, in which we presented the promising activity of structurally related anthraquinone compounds against biofilm-forming marine bacteria. To help uncover the anti-fouling potential of other AQ-related structures, 2194 compounds from the COCONUT natural products database were analysed. Molecular docking analysis was performed to assess the binding strength of these compounds to the LuxP protein in Vibrio carchariae. The LuxP protein is a vital binding protein responsible for the movements of autoinducers within the quorum sensing system; hence, interrupting the process at an early stage could be an effective strategy. Seventy-six AQ structures were found to be highly docked, and eight of these structures were used in structure-based pharmacophore modelling, resulting in six unique pharmacophore features.

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.