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

New Brusatol Derivatives as Anti-Settlement Agents Against Barnacles, Targeting HSP90: Design, Synthesis, Biological Evaluation, and Molecular Docking Investigations

The increasing challenge of marine biofouling, mainly due to barnacle settlement, necessitates the development of effective antifoulants with minimal environmental toxicity. In this study, fifteen derivatives of brusatol were synthesized and characterized using 13C-NMR, 1H-NMR, and mass spectrometry. All the semi-synthesized compounds obtained using the Multi-Target-Directed Ligand (MTDL) strategy, when evaluated as anti-settlement agents against barnacles, showed promising activity. Compound 3 exhibited the highest anti-settlement capacity, with an EC50 value of 0.1475 μg/mL, an LC50/EC50 ratio of 42.2922 (>15 indicating low toxicity), and a resuscitation rate of 71.11%, while it showed no significant phenotypic differences in the zebrafish embryos after treatment for 48 h. The toxicity screening of zebrafish also demonstrated the low ecotoxicity of the selected compounds. Furthermore, homology modeling of the HSP90 structure was performed based on related protein sequences in barnacles. Subsequently, molecular docking studies were conducted on HSP90 using these newly synthesized derivatives. Molecular docking analyses showed that most activated derivatives displayed low binding energies with HSP90, aligning well with the biological results. They were found to interact with key residues in the binding site, specifically ARG243, TYR101, and LEU73. These computational findings are anticipated to aid in predicting the enzyme targets of the tested inhibitors and their potential interactions, thus facilitating the design of novel antifoulants in future research endeavors.

A Chemical Toolbox to Unveil Synthetic Nature-Inspired Antifouling (NIAF) Compounds

The current scenario of antifouling (AF) strategies to prevent the natural process of marine biofouling is based in the use of antifouling paints containing different active ingredients, believed to be harmful to the marine environment. Compounds called booster biocides are being used with copper as an alternative to the traditionally used tributyltin (TBT); however, some of them were recently found to accumulate in coastal waters at levels that are deleterious for marine organisms. More ecological alternatives were pursued, some of them based on the marine organism mechanisms' production of specialized metabolites with AF activity. However, despite the investment in research on AF natural products and their synthetic analogues, many studies showed that natural AF alternatives do not perform as well as the traditional metal-based ones. In the search for AF agents with better performance and to understand which molecular motifs were responsible for the AF activity of natural compounds, synthetic analogues were produced and investigated for structure-AF activity relationship studies. This review is a comprehensive compilation of AF compounds synthesized in the last two decades with highlights on the data concerning their structure-activity relationship, providing a chemical toolbox for researchers to develop efficient nature-inspired AF agents.

Understanding the Artemia Salina (Brine Shrimp) Test: Pharmacological Significance and Global Impact

BACKGROUND

The microplate benchtop brine shrimp test (BST) has been widely used for screening and bio-guided isolation of many active compounds, including natural products. Although the interpretation given to the results appears dissimilar, our findings suggest a correlation between positive results with a specific mechanism of action.

OBJECTIVE

This study aimed to evaluate drugs belonging to fifteen pharmacological categories having diverse mechanisms of action and carry out a bibliometric analysis of over 700 citations related to microwell BST.

METHODS

Test compounds were evaluated in a serial dilution on the microwell BST using healthy nauplii of Artemia salina and after 24 hrs of exposition, the number of alive and dead nauplii was determined, and the LC50 was estimated. A metric study regarding the citations of the BST miniaturized method, sorted by type of documents cited, contributing country, and interpretation of results was conducted on 706 selected citations found in Google Scholar.

RESULTS

Out of 206 drugs tested belonging to fifteen pharmacological categories, twenty-six showed LC50 values <100 μM, most of them belonging to the category of antineoplastic drugs; compounds with different therapeutical uses were found to be cytotoxic as well. A bibliometric analysis showed 706 documents citing the miniaturized BST; 78% of them belonged to academic laboratories from developing countries located on all continents, 63% interpreted their results as cytotoxic activity and 35% indicated general toxicity assessment.

CONCLUSION

BST is a simple, affordable, benchtop assay, capable of detecting cytotoxic drugs with specific mechanisms of action, such as protein synthesis inhibition, antimitotic, DNA binding, topoisomerase I inhibitors, and caspases cascade interfering drugs. The microwell BST is a technique that is used worldwide for the bio-guided isolation of cytotoxic compounds from different sources.

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.

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.

The Sponge-Associated Fungus Eurotium chevalieri MUT 2316 and its Bioactive Molecules: Potential Applications in the Field of Antifouling

The need for new environmentally friendly antifouling and the observation that many marine organisms have developed strategies to keep their surface free of epibionts has stimulated the search for marine natural compounds with antifouling activities. Sponges and in particular fungi associated with them represent one of the most appropriate sources of defence molecules and could represent a promising biomass for the supply of new antifouling compounds. The objective of this work was therefore to evaluate the antifouling potency of 7 compounds isolated from the sponge derived fungus Eurotium chevalieri MUT 2316. The assessment of their activity targeted the inhibition of the adhesion and/or growth of selected marine bacteria (5) and microalgae (5), as well as the inhibition of the mussel's byssus thread formation (tyrosinase activity). The 7 compounds showed bioactivity, with various levels of selectivity for species. Cyclo-L-Trp-L-Ala was the most promising active compound, and led to the inhibition, at very low concentrations (0.001 μg ml^-1 in 61.5% of cases), of adhesion and growth of all the microalgae, of selected bacteria, and towards the inhibition of tyrosinase. Promising results were also obtained for echinulin, neoechinulin A, dihydroauroglaucin and flavoglaucin, respectively, leading to inhibition of adhesion and/or growth of 9, 7, 8 and 8 microfouling species at various concentrations.

Biodiversity and antifouling activity of fungi associated with two soft corals from the South China Sea

Bacteria in corals have been studied in detail in the past decades. However, the biodiversity and bioactivity of fungi in corals are still poorly understood. This study investigated the biodiversity and antifouling activity of fungi in soft corals Cladiella krempfi and Sarcophyton tortuosum from the South China Sea. A high diverse and abundant fungal community was found in the two soft corals. Furthermore, five isolates shared 83-95% similarity with their closest relatives, indicating that they might be novel species in genera Phaeoshaeria and Mucor. In addition, approximately 50% of the representative isolates exhibited distinct antifouling activity. In particular, isolates Fungal sp. SCAU132 and Fungal sp. SCAU133 displayed very strong antifouling activity against Bugula neritina, suggesting they can provide a potential resource for further investigation on isolation of novel antifouling metabolites. To our knowledge, this study is the first report to investigate the biodiversity and antifouling activity of fungi in C. krempfi and S. tortuosum.

Antifouling performance and mechanism of elastic graphene-silicone rubber composite membranes

Composite coatings have attracted great attention as an eco-friendly and economic solution to prevent ship hulls from biofouling. Inspired by the unstable surfaces of marine organisms with antifouling properties, this study describes the preparation of graphene-silicone rubber composite membranes. The membranes are characterized by a low surface energy and an adjustable elastic modulus, and these properties are conducive to preventing biofouling. Bacterial attachment was tested under both quasi-static and hydrodynamic conditions, and one rigid polystyrene sheet was used as the control group to verify the antifouling effects of unstable surfaces. The polystyrene sheet and the elastic membranes showed similar antifouling performance under quasi-static conditions. However, under hydrodynamic conditions, the elastic membranes showed better antifouling performance than the rigid polystyrene sheet. The results obtained using a laser-displacement sensor showed that micron-scale deformations were present on the elastic surface, and a mechanical model was employed to verify this conclusion. This study first confirmed the antifouling effects of the unstable surface, and proposed a model to reveal the antifouling mechanism of the unstable surface. According to the bacterial attachment test, a new generation membrane was made showing antifouling capacity with just 0.36 wt% graphene included during the fabrication of the membrane. This study provided a deeper insight into the antifouling mechanism of the elastic surface, and the membrane (0.36 wt%) may be promising for practical applications.

Host-microbe interactions in octocoral holobionts - recent advances and perspectives

Octocorals are one of the most ubiquitous benthic organisms in marine ecosystems from the shallow tropics to the Antarctic deep sea, providing habitat for numerous organisms as well as ecosystem services for humans. In contrast to the holobionts of reef-building scleractinian corals, the holobionts of octocorals have received relatively little attention, despite the devastating effects of disease outbreaks on many populations. Recent advances have shown that octocorals possess remarkably stable bacterial communities on geographical and temporal scales as well as under environmental stress. This may be the result of their high capacity to regulate their microbiome through the production of antimicrobial and quorum-sensing interfering compounds. Despite decades of research relating to octocoral-microbe interactions, a synthesis of this expanding field has not been conducted to date. We therefore provide an urgently needed review on our current knowledge about octocoral holobionts. Specifically, we briefly introduce the ecological role of octocorals and the concept of holobiont before providing detailed overviews of (I) the symbiosis between octocorals and the algal symbiont Symbiodinium; (II) the main fungal, viral, and bacterial taxa associated with octocorals; (III) the dominance of the microbial assemblages by a few microbial species, the stability of these associations, and their evolutionary history with the host organism; (IV) octocoral diseases; (V) how octocorals use their immune system to fight pathogens; (VI) microbiome regulation by the octocoral and its associated microbes; and (VII) the discovery of natural products with microbiome regulatory activities. Finally, we present our perspectives on how the field of octocoral research should move forward, and the recognition that these organisms may be suitable model organisms to study coral-microbe symbioses.

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.

Diphenyl ethers from a marine-derived isolate of Aspergillus sp. CUGB-F046

One new diphenyl ether, diorcinol K (1), along with three known compounds, diorcinols D (2), F (3) and I (4) were isolated from the fermentation media of a marine-derived fungus Aspergillus sp. CUGB-F046 which was isolated from a sediment sample collected from the Bohai Sea, China. Their structures were elucidated by detailed spectroscopic methods. Compounds 1, 2 and 4 displayed significant antibacterial activities against Staphylococcus aureus and methicillin-resistant S. aureus with MIC values of 3.125, 6.25 and 6.25 μg/mL, respectively.