Low-Toxicity Diindol-3-ylmethanes as Potent Antifouling Compounds

Indole alkaloids from endophytic fungus Robillarda sessilis and their antibacterial activity

Three undescribed indole alkaloids, fusarindoles F and G (1 and 2), and chlamydosporin B (3), together with five known compounds (4-8) were isolated from Robillarda sessilis. Their structures were elucidated based on NMR, UV, HRESIMS, and ECD calculation. Fusarindole F (1) own unusual asymmetric bis-indole structure. Compounds 5, 6, 7 exhibited moderate antibacterial activity against methicillin-resistant Staphylococcus aureus with a MIC value of 12.5 μg/mL. According to molecular docking experiment, the target proteins of compound 7 against methicillin-resistant S. aureus may be ELANE, MAOB and STAT3.

Research Progress on New Environmentally Friendly Antifouling Coatings in Marine Settings: A Review

Any equipment submerged in the ocean will have its surface attacked by fouling organisms, which can cause serious damage. Traditional antifouling coatings contain heavy metal ions, which also have a detrimental effect on the marine ecological environment and cannot fulfill the needs of practical applications. As the awareness of environmental protection is increasing, new environmentally friendly and broad-spectrum antifouling coatings have become the current research hotspot in the field of marine antifouling. This review briefly outlines the formation process of biofouling and the fouling mechanism. Then, it describes the research progress of new environmentally friendly antifouling coatings in recent years, including fouling release antifouling coatings, photocatalytic antifouling coatings and natural antifouling agents derived from biomimetic strategies, micro/nanostructured antifouling materials and hydrogel antifouling coatings. Highlights include the mechanism of action of antimicrobial peptides and the means of preparation of modified surfaces. This category of antifouling materials has broad-spectrum antimicrobial activity and environmental friendliness and is expected to be a new type of marine antifouling coating with desirable antifouling functions. Finally, the future research directions of antifouling coatings are prospected, which are intended to provide a reference for the development of efficient, broad-spectrum and green marine antifouling coatings.

Discovery, Yield Improvement, and Application in Marine Coatings of Potent Antifouling Compounds Albofungins Targeting Multiple Fouling Organisms

Marine biofouling caused huge economic losses of maritime industries. We aim to develop high-efficient, less-toxic, and cost-effective antifoulants to solve the problems of biofouling. In this study, we described the antifouling compounds albofungin and its derivatives (albofungin A, chrestoxanthone A, and chloroalbofungin) isolated from the metabolites of bacterium Streptomyces chrestomyceticus BCC 24770, the construction of high-yield strains for albofungin production, and application of albofungin-based antifouling coatings. Results showed that these albofungins have potent antibiofilm activities against Gram-positive and Gram-negative bacteria and anti-macrofouling activities against larval settlement of major fouling organisms with low cytotoxicity. With the best antifouling activity and highest yield in bacterial culture, albofungin was subsequently incorporated with hydrolyzable and degradable copolymer to form antifouling coatings, which altered biofilm structures and prevented the settlement of macrofouling organisms in marine environments. Our results suggested that albofungins were promising antifouling compounds with potential application in marine environments.

Anti-Virulence Activity of 3,3′-Diindolylmethane (DIM): A Bioactive Cruciferous Phytochemical with Accelerated Wound Healing Benefits

Antimicrobial resistance is among the top global health problems with antibacterial resistance currently representing the major threat both in terms of occurrence and complexity. One reason current treatments of bacterial diseases are ineffective is the occurrence of protective and resistant biofilm structures. Phytochemicals are currently being reviewed for newer anti-virulence agents. In the present study, we aimed to investigate the anti-virulence activity of 3,3′-diindolylmethane (DIM), a bioactive cruciferous phytochemical. Using a series of in vitro assays on major Gram-negative pathogens, including transcriptomic analysis, and in vivo porcine wound studies as well as in silico experiments, we show that DIM has anti-biofilm activity. Following DIM treatment, our findings show that biofilm formation of two of the most prioritized bacterial pathogens Acinetobacter baumannii and Pseudomonas aeruginosa was inhibited respectively by 65% and 70%. Combining the antibiotic tobramycin with DIM enabled a high inhibition (94%) of P. aeruginosa biofilm. A DIM-based formulation, evaluated for its wound-healing efficacy on P. aeruginosa-infected wounds, showed a reduction in its bacterial bioburden, and wound size. RNA-seq was used to evaluate the molecular mechanism underlying the bacterial response to DIM. The gene expression profile encompassed shifts in virulence and biofilm-associated genes. A network regulation analysis showed the downregulation of 14 virulence-associated super-regulators. Quantitative real-time PCR verified and supported the transcriptomic results. Molecular docking and interaction profiling indicate that DIM can be accommodated in the autoinducer- or DNA-binding pockets of the virulence regulators making multiple non-covalent interactions with the key residues that are involved in ligand binding. DIM treatment prevented biofilm formation and destroyed existing biofilm without affecting microbial death rates. This study provides evidence for bacterial virulence attenuation by DIM.

Anti-Larval and Anti-Algal Natural Products from Marine Microorganisms as Sources of Anti-Biofilm Agents

Bacteria growing inside biofilms are more resistant to hostile environments, conventional antibiotics, and mechanical stresses than their planktonic counterparts. It is estimated that more than 80% of microbial infections in human patients are biofilm-based, and biofouling induced by the biofilms of some bacteria causes serious ecological and economic problems throughout the world. Therefore, exploring highly effective anti-biofilm compounds has become an urgent demand for the medical and marine industries. Marine microorganisms, a well-documented and prolific source of natural products, provide an array of structurally distinct secondary metabolites with diverse biological activities. However, up to date, only a handful of anti-biofilm natural products derived from marine microorganisms have been reported. Meanwhile, it is worth noting that some promising antifouling (AF) compounds from marine microbes, particularly those that inhibit settlement of fouling invertebrate larvae and algal spores, can be considered as potential anti-biofilm agents owing to the well-known knowledge of the correlations between biofilm formation and the biofouling process of fouling organisms. In this review, a total of 112 anti-biofilm, anti-larval, and anti-algal natural products from marine microbes and 26 of their synthetic analogues are highlighted from 2000 to 2021. These compounds are introduced based on their microbial origins, and then categorized into the following different structural groups: fatty acids, butenolides, terpenoids, steroids, phenols, phenyl ethers, polyketides, alkaloids, flavonoids, amines, nucleosides, and peptides. The preliminary structure-activity relationships (SAR) of some important compounds are also briefly discussed. Finally, current challenges and future research perspectives are proposed based on opinions from many previous reviews.

Methylene-bridged dimeric natural products involving one-carbon unit in biosynthesis

This review summarizes the methylene-bridged dimeric natural products involving one-carbon unit in biosynthesis, including their structures, biological activities, synthetic methods, and formation mechanisms.

Natural Indoles From the Bacterium Pseudovibrio denitrificans P81 Isolated From a Marine Sponge, Aaptos Species

A new natural indole, vibrindole B (1), together with known analogs, vibrindole A (2), trisindoline (3), norharmane (4), and 3-(hydroxyacetyl)indole (5), produced by the bacterium Pseudovibrio denitrificans P81, were isolated from a sponge, Aaptos species. The structures of indoles 1 to 5 were established by spectroscopic methods. The proposed biosynthetic pathway of 1 to 5 is also discussed, starting from tryptophan. Moreover, indoles 1 to 3 were found to exhibit cytotoxicity toward T24 tumor cells with IC50 values of 1.71 ± 0.11, 4.53 ± 0.14, and 2.26 ± 0.26 µM, respectively.

A Family of Nonribosomal Peptides Modulate Collective Behavior in Pseudovibrio Bacteria Isolated from Marine Sponges*

Although swarming motility and biofilms are opposed collective behaviors, both contribute to bacterial survival and host colonization. Pseudovibrio bacteria have attracted attention because they are part of the microbiome of healthy marine sponges. Two-thirds of Pseudovibrio genomes contain a member of a nonribosomal peptide synthetase-polyketide synthase gene cluster family, which is also found sporadically in Pseudomonas pathogens of insects and plants. After developing reverse genetics for Pseudovibrio, we isolated heptapeptides with an ureido linkage and related nonadepsipeptides we termed pseudovibriamides A and B, respectively. A combination of genetics and imaging mass spectrometry experiments showed heptapetides were excreted, promoting motility and reducing biofilm formation. In contrast to lipopeptides widely known to affect motility/biofilms, pseudovibriamides are not surfactants. Our results expand current knowledge on metabolites mediating bacterial collective behavior.

Marine Natural Products from Tunicates and Their Associated Microbes

Marine tunicates are identified as a potential source of marine natural products (MNPs), demonstrating a wide range of biological properties, like antimicrobial and anticancer activities. The symbiotic relationship between tunicates and specific microbial groups has revealed the acquisition of microbial compounds by tunicates for defensive purpose. For instance, yellow pigmented compounds, "tambjamines", produced by the tunicate, Sigillina signifera (Sluiter, 1909), primarily originated from their bacterial symbionts, which are involved in their chemical defense function, indicating the ecological role of symbiotic microbial association with tunicates. This review has garnered comprehensive literature on MNPs produced by tunicates and their symbiotic microbionts. Various sections covered in this review include tunicates' ecological functions, biological activities, such as antimicrobial, antitumor, and anticancer activities, metabolic origins, utilization of invasive tunicates, and research gaps. Apart from the literature content, 20 different chemical databases were explored to identify tunicates-derived MNPs. In addition, the management and exploitation of tunicate resources in the global oceans are detailed for their ecological and biotechnological implications.

Marine natural products as antifouling molecules - a mini-review (2014-2020)

In the present review, 182 antifouling (AF) natural products from marine microorganisms, algae and marine invertebrates reported from August 2014 to May 2020 are presented. Amongst these compounds, over half were isolated from marine-derived microorganisms, including 70 compounds from fungi and 31 compounds from bacteria. The structure-relationship of some of these compounds is also briefly discussed. Based on the work reported, a general workflow was drafted to refine the procedures for the commercialization of any novel AF compounds. Finally, butenolide, which is considered a potential environmentally friendly antifoulant, is used as a case study to show the procedures involved in AF compound work from the aspect of discovery, structure optimization, toxicity, stability, AF mechanism and coating incorporation, which highlight the current challenges and future perspectives in AF compound research.

Bacterial Tropone Natural Products and Derivatives: Overview of their Biosynthesis, Bioactivities, Ecological Role and Biotechnological Potential

Tropone natural products are non-benzene aromatic compounds of significant ecological and pharmaceutical interest. Herein, we highlight current knowledge on bacterial tropones and their derivatives such as tropolones, tropodithietic acid, and roseobacticides. Their unusual biosynthesis depends on a universal CoA-bound precursor featuring a seven-membered carbon ring as backbone, which is generated by a side reaction of the phenylacetic acid catabolic pathway. Enzymes encoded by separate gene clusters then further modify this key intermediate by oxidation, CoA-release, or incorporation of sulfur among other reactions. Tropones play important roles in the terrestrial and marine environment where they act as antibiotics, algaecides, or quorum sensing signals, while their bacterial producers are often involved in symbiotic interactions with plants and marine invertebrates (e. g., algae, corals, sponges, or mollusks). Because of their potent bioactivities and of slowly developing bacterial resistance, tropones and their derivatives hold great promise for biomedical or biotechnological applications, for instance as antibiotics in (shell)fish aquaculture.

Transcriptomic analysis of the mode of action of the candidate anti-fouling compound di(1H-indol-3-yl)methane (DIM) on a marine biofouling species, the bryozoan Bugula neritina

Di(1H-indol-3-yl)methane (DIM) was previously suggested to be an environmentally friendly antifouling compound, but it was also reported that the compound was highly stable in natural seawater. The present study reported that 3 h DIM treatments at 4 μg mL^-1 or higher concentration and 12 h DIM treatments at 2 μg mL^-1 or higher concentration induced significant larval mortality and metamorphic abnormality in the bryozoan Bugula neritina. The bioassay results correlated with the dose-dependent up-regulation of HSP family proteins, pro-apoptotic proteins, ubiquitination protein, and the dose-dependent down-regulation of anti-apoptotic genes and developmental genes. Unexpectedly, genes involved in fatty acid biosynthesis and protein synthesis were up-regulated in response to DIM treatment, but, in general, the effects of DIM on B. neritina larvae were comparable to that reported in human cancer cell lines. DIM also induced changes in steroid hormone biosynthesis genes in B. neritina larvae, leading to the concern that DIM might have long-term effects on marine lives. Overall, the present study suggested that application of DIM to the bryozoan larvae would trigger a major transcriptomic response, which might be linked to the observed larval mortality and abnormality. We suggest that application of DIM as an antifouling ingredient should be proceeded with great cautions.

Synthesis and evaluation of acrylate resins suspending indole derivative structure in the side chain for marine antifouling

A novel indole derivative (N-(1H-2-phenyl-indole-3-ylmethyl) acrylamide, NPI) synthesized by a Friedel-Crafts alkylation reaction was identified using IR spectroscopy, ¹H NMR, ¹³C NMR and elemental analysis. The inhibitory effect of this novel indole derivative on bacteria and marine algae was studied. The results showed that the inhibition ratios of the indole derivative against Escherichia coli and Staphylococcus aureus were 95.93% and 94.91%, respectively, and the indole derivative possessed prominent inhibitory activity against Phaeodactylum tricornutum, Nitzschia Closterium and Skeletonema costatum. These findings indicate that the indole derivative has high biological activity. Subsequently, the indole derivative was introduced to acrylate resins by free-radical polymerization. The resulting acrylate resins were subjected to self-polishing, anti-algal and antifouling test, the results of which indicated that acrylate resins containing the synthesized indole derivative could exhibit significant antifouling properties because of the combination of the biofouling resistance of the indole derivative and the self-polishing properties of acrylate. This work provides an academic foundation for studying environmentally friendly and highly efficient antifouling coatings.

High-performance liquid chromatography/electrospray ionisation mass spectrometric characterisation of metabolites produced by Pseudovibrio sp. W64, a marine sponge derived bacterium isolated from Irish waters

RATIONALE

In recent years, metabolites produced by Pseudovibrio species have gained scientific attention due to their potent antimicrobial activity. Recently, we also have assessed the antibacterial activities of Pseudovibrio sp. W64 isolates against Staphylococcus aureus, where only the dominant tropodithietic acid (TDA) was identified. However, characterisation of other metabolites is necessary as these metabolites may also serve as potent antimicrobial agents.

METHODS

Liquid chromatography/tandem mass spectrometry (LC/MS/MS), aided by accurate mass measurements, was employed to screen and characterise a range of metabolites produced by Pseudovibrio sp. W64 via assessment of ethyl acetate fractions generated from bacterial cultures.

RESULTS

Thirteen metabolites unique to the bacterial culture were detected and their chemical structures were assigned by MS/MS and accurate mass measurements. Among the thirteen metabolites, a methyl ester of TDA, a number of cholic acid derivatives, and amino diols and triols were characterised.

CONCLUSIONS

Pseudovibrio sp. W64 produces methylated TDA in addition to TDA, and metabolises lipids and amino acids in the cell-culture medium. To the best of our knowledge, this is the first report of methylated TDA, cholic acid and its various analogs, and sphinganine being detected in this Pseudovibrio strain. The data generated may help to better understand the biochemical processes and metabolism of bacterial strains towards discovery of antimicrobial agents from marine sources.

The Plant Alkaloid Camptothecin as a Novel Antifouling Compound for Marine Paints: Laboratory Bioassays and Field Trials

The extensive use of copper and booster biocides in antifouling (AF) paints has raised environmental concerns and the need to develop new AF agents. In the present study, 18 alkaloids derived from terrestrial plants were initially evaluated for AF activity using laboratory bioassays with the bryozoan Bugula neritina and the barnacle Balanus albicostatus. The results showed that 4 of the 18 alkaloids were effective in inhibiting larval settlement of B. neritina, with an EC₅₀ range of 6.18 to 43.11 μM, and 15 of the 18 alkaloids inhibited larval settlement of B. albicostatus, with EC₅₀ values ranging from 1.18 to 67.58 μM. Field trials that incorporated five alkaloids respectively into paints with 20% w/w indicated an in situ AF efficiency of evodiamine, strychnine, camptothecin (CPT), and cepharanthine, with the most potent compound being CPT, which also exhibited stronger AF efficiency than the commercial antifoulants cuprous oxide and zinc pyrithione in the field over a period of 12 months. Further field trials with different CPT concentrations (0.1 to 20% w/w) in the paints suggested a concentration-dependent AF performance in the natural environment, and the effective concentrations to significantly inhibit settlement of biofoulers in the field were ≥ 0.5% w/w (the efficiency of 0.5% w/w lasted for 2 months). Moreover, CPT toxicity against the crustacean Artemia salina, the planktonic microalgae Phaeodactylum tricornutum and Isochrysis galbana, was examined. The results showed that 24 h LC₅₀ of CPT against A. salina was 20.75 μM, and 96 h EC₅₀ (growth inhibition) values of CPT to P. tricornutum and I. galbana were 55.81 and 6.29 μM, respectively, indicating that CPT was comparatively less toxic than several commercial antifoulants previously reported. Our results suggest the novel potential application of CPT as an antifoulant.

Ecology and Biotechnological Potential of Bacteria Belonging to the Genus Pseudovibrio

Members of the genus Pseudovibrio have been isolated worldwide from a great variety of marine sources as both free-living and host-associated bacteria. So far, the available data depict a group of alphaproteobacteria characterized by a versatile metabolism, which allows them to use a variety of substrates to meet their carbon, nitrogen, sulfur, and phosphorous requirements. Additionally, Pseudovibrio-related bacteria have been shown to proliferate under extreme oligotrophic conditions, tolerate high heavy-metal concentrations, and metabolize potentially toxic compounds. Considering this versatility, it is not surprising that they have been detected from temperate to tropical regions and are often the most abundant isolates obtained from marine invertebrates. Such an association is particularly recurrent with marine sponges and corals, animals that play a key role in benthic marine systems. The data so far available indicate that these bacteria are mainly beneficial to the host, and besides being involved in major nutrient cycles, they could provide the host with both vitamins/cofactors and protection from potential pathogens via the synthesis of antimicrobial secondary metabolites. In fact, the biosynthetic abilities of Pseudovibrio spp. have been emerging in recent years, and both genomic and analytic studies have underlined how these organisms promise novel natural products of biotechnological value.

Review on Molecular Mechanisms of Antifouling Compounds: An Update since 2012

Better understanding of the mechanisms of antifouling compounds is recognized to be of high value in establishing sensitive biomarkers, allowing the targeted optimization of antifouling compounds and guaranteeing environmental safety. Despite vigorous efforts to find new antifouling compounds, information about the mechanisms of antifouling is still scarce. This review summarizes the progress into understanding the molecular mechanisms underlying antifouling activity since 2012. Non-toxic mechanisms aimed at specific targets, including inhibitors of transmembrane transport, quorum sensing inhibitors, neurotransmission blockers, adhesive production/release inhibitors and enzyme/protein inhibitors, are put forward for natural antifouling products or shelf-stable chemicals. Several molecular targets show good potential for use as biomarkers in future mechanistic screening, such as acetylcholine esterase for neurotransmission, phenoloxidase/tyrosinase for the formation of adhesive plaques, N-acyl homoserine lactone for quorum sensing and intracellular Ca²⁺ levels as second messenger. The studies on overall responses to challenges by antifoulants can be categorized as general targets, including protein expression/metabolic activity regulators, oxidative stress inducers, neurotransmission blockers, surface modifiers, biofilm inhibitors, adhesive production/release inhibitors and toxic killing. Given the current situation and the knowledge gaps regarding the development of alternative antifoulants, a basic workflow is proposed that covers the indispensable steps, including preliminary mechanism- or bioassay-guided screening, evaluation of environmental risks, field antifouling performance, clarification of antifouling mechanisms and the establishment of sensitive biomarkers, which are combined to construct a positive feedback loop.

Mini-Review: Antifouling Natural Products from Marine Microorganisms and Their Synthetic Analogs

Biofouling causes huge economic loss and generates serious ecological issues worldwide. Marine coatings incorporated with antifouling (AF) compounds are the most common practices to prevent biofouling. With a ban of organotins and an increase in the restrictions regarding the use of other AF alternatives, exploring effective and environmentally friendly AF compounds has become an urgent demand for marine coating industries. Marine microorganisms, which have the largest biodiversity, represent a rich and important source of bioactive compounds and have many medical and industrial applications. This review summarizes 89 natural products from marine microorganisms and 13 of their synthetic analogs with AF EC50 values ≤ 25 μg/mL from 1995 (the first report about marine microorganism-derived AF compounds) to April 2017. Some compounds with the EC50 values < 5 μg/mL and LC50/EC50 ratios > 50 are highlighted as potential AF compounds, and the preliminary analysis of structure-relationship (SAR) of these compounds is also discussed briefly. In the last part, current challenges and future research perspectives are proposed based on opinions from many previous reviews. To provide clear guidance for the readers, the AF compounds from microorganisms and their synthetic analogs in this review are categorized into ten types, including fatty acids, lactones, terpenes, steroids, benzenoids, phenyl ethers, polyketides, alkaloids, nucleosides and peptides. In addition to the major AF compounds which targets macro-foulers, this review also includes compounds with antibiofilm activity since micro-foulers also contribute significantly to the biofouling communities.

Thielavins W-Z₇, New Antifouling Thielavins from the Marine-Derived Fungus Thielavia sp. UST030930-004

Eleven new depsides—thielavins W–Z (1–4) and thielavins Z₁–Z₇ (5–11)—and also four known thielavins—A, H, J, and K (12–15)—were isolated from the ethyl acetate extract of a marine-derived fungal strain Thielavia sp UST030930-004. All of these compounds were evaluated for antifouling activity against cyprids of the barnacle Balanus (=Amphibalanus) amphitrite. The results showed that compounds 1–3 and 6–13 were active, with EC₅₀ values ranging from 2.95 ± 0.59 to 69.19 ± 9.51 μM, respectively. The inhibitive effect of compounds 1–3 and 7 was reversible. This is the first description of the antifouling activity of thielavins against barnacle cyprids.

Linking genomic responses of gonads with reproductive impairment in marine medaka (Oryzias melastigma) exposed chronically to the chemopreventive and antifouling agent, 3,3'-diindolylmethane (DIM)

3,3'-Diindolylmethane (DIM) has been promoted as an effective chemopreventive and antifouling additive. However, the concurrent risks or side effects of DIM are not fully understood, especially on tissues responsive to estrogen. Therefore, this study employed marine medaka (Oryzias melastigma) as a test model to evaluate relative safety and explore mechanisms of toxic action of DIM on development and function of gonad after chronic (28days) aqueous exposure to relatively low doses (0μg/L or 8.5μg/L). Integration of comprehensive toxicogenomic analysis at the transcriptome and proteome levels with apical endpoints, such as production of eggs and swimming performance of larvae, elucidated the molecular linkage in gonad from bottom up along the reproductive adverse outcome pathway. A series of sequential changes at the transcript and protein levels were linked to lesser fecundity and viability of larvae exposed to DIM. Anomalous production of vitellogenin (VTG) and eggshell proteins in testis confirmed the estrogenic potency of DIM. In the ovary, although storage of VTG was greater, lesser expressions of cathepsin enzymes blocked cleavage and incorporation of VTG into oocytes as yolk, which acted together with lower eggshell proteins to inhibit maturation of primary oocyte and thus contributed to impairment of fecundity. Overall, this study demonstrated that exposure to DIM impaired reproductive fitness. Diverse molecular initiating changes in gonads were linked to apical endpoints that could be used in assessment of risks posed by DIM on gametogenesis. In combination with chemical stability and potent endocrine disruption, the results of this study can inform decisions about the use of DIM either as chemopreventive or antifouling agent.

Antifouling phenyl ethers and other compounds from the invertebrates and their symbiotic fungi collected from the South China Sea

Marine organism-derived secondary metabolites are promising potential sources for discovering environmentally safe antifouling agents. In present study, 55 marine secondary metabolites and their synthesized derivatives were tested and evaluated for their antifouling activities and security. These compounds include 44 natural products isolated from marine invertebrates and their symbiotic microorganisms collected from the South China Sea and 11 structural modified products derived from the isolated compounds. The natural secondary metabolites, covering phenyl ether derivatives, terpenoids, 9, 11-secosteroids, anthraquinones, alkaloids, nucleoside derivatives and peptides, were isolated from two corals, one sponge and five symbiotic fungi. All of the isolated and synthesized compounds were tested for their antifouling activities against the cyprids of barnacle Balanus (Amphibalanus) amphitrite Darwin. Noticeably, five phenyl ether derivatives (9, 11, 13-15) exhibited potent anti-larval settlement activity with the EC50 values lower than 3.05 μM and the LC50/EC50 ratios higher than 15. The study of structure-activity relationship (SAR) revealed that the introduction of acetoxy groups and bromine atoms to phenyl ether derivatives could significantly improve their antifouling activities. This is the first report on the SAR of phenyl ether derivatives on antifouling activity against barnacle B. amphitrite. The polybrominated diphenyl ether derivative, 2, 4, 6, 2', 4', 6'-hexabromo-diorcinol (13), which displayed excellent antifouling activity, was considered as a promising candidate of environmentally friendly antifouling agents.