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- Advances in Marine Antifouling Coatings and Technologies
- Novel Antifouling Coatings: A Multiconceptual Approach
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Advances in Marine Antifouling Coatings and Technologies
Marine biofouling is of major economic concern to all marine industries. The shipping trade is particularly alert to the development of new antifouling AF strategies, especially green AF paint as international regulations regarding the environmental impact of the compounds actually incorporated into the formulations are becoming more and more strict.
It is also recognised that vessels play an extensive role in invasive species propagation as ballast waters transport potentially threatening larvae. It is then crucial to develop new AF solutions combining advances in marine chemistry and topography, in addition to a knowledge of marine biofoulers, with respect to the marine environment.
This review presents the recent research progress made in the field of new non-toxic AF solutions new microtexturing of surfaces, foul-release coatings, and with a special emphasis on marine natural antifoulants as well as the perspectives for future research directions. Marine biofouling can be defined as the undesirable accumulation of microorganisms, algae and animals on submerged substrates leading to subsequent biodeterioration. This review focuses specifically on marine biofouling and its control using environmentally friendly AF technologies.
Depending on the geographical locations, the species involved fluctuate greatly accordingly to the environmental conditions salinity, temperature, nutrient levels, flow rates and the intensity of solar radiation. Moreover, the spawning season of the organisms and consequently the pressure of fouling vary significantly according to latitude and longitude: less fouling development in winter in temperate areas due to the reduction in day light hours and sea water temperature with the main spawning season being from spring to late summer; marine tropical and sub-tropical areas face few variations of water temperatures and light levels, resulting in high pressure of fouling throughout the year due to a continuous period of reproduction [ 2 ].
Biofouling can lead to significant increase in the cost of maritime transportation. The globalization of production and trade are concomitant as one cannot function without the other. The scale, volume and efficiency of the international trade all have continued to increase since the 70s [ 3 ]. Sailing across oceans, ships are confronted with significantly different environmental conditions from tropical waters to cold or temperate waters within a few days, leading for the need of active hull protection against a wide range of organisms.
The colonisation of hulls has been linked to two major environmental pollutions which are the emissions of gas CO 2 , CO, SO 2 and NOx into the atmosphere and the dissemination of potential alien species.
At a given time most vessels are relatively near shore, consequently the principal amount of gas emitted is along the coastline mainly in the Northern Hemisphere, along the West and East coast of the United States, in Northern Europe and in the North Pacific [ 3 , 4 ]. Reduction in NOx emissions motivated by air quality concerns will tend to reduce the net warming effect due to the tropospheric ozone and CH 4 concentrations.
If these NOx reductions are greater than the corresponding increases in CO 2 emissions, then the combined effect of NOx control could reduce the global warming impact of the international shipping [ 5 ]. Nowadays, ballast waters from large vessels are considered to be the dominant vector for international introductions of harmful invasive species.
In addition, recreational craft are now thought to be significant secondary vectors for their spread after an initial introduction, with for example anchor and anchor chains as potential site for attachment of alien species. Invasive alien species have the ability to colonise potential habitats different from their natural habitat, invade, outcompete natives and settle permanently in new environments.
They are widespread in the world and are known to affect biological diversity whether within or outside protected areas and to influence ecosystems, natural habitats and surrounding populations. All species that are non-indigenous to an ecosystem are potentially harmful, both to biodiversity and to social and economic interests [ 7 ]. The best-known and recorded examples are probably the zebra mussel introduction into the US waters and the comb jelly fish into the Black Sea [ 6 , 7 ]. AF coatings are necessary in order to avoid the colonisation of surfaces by biofoulers and consequently the high costs relative to transport delays, hull repairs, cleaning of desalination units and biocorrosion estimated at billion USD per year [ 1 ].
These chemicals were highly toxic for many aquatic organisms and have been proven to contaminate the food chain and to be persistent in the environment. Since the ban of TBT-based paints September , AFS Treaty [ 2 ] , new formulation have been developed containing high levels of copper and herbicides such as Irgarol , diuron, chlorothalonil, dichlorofuanid and zineb.
However, even if these paints claimed to be environmentally friendly when first put on the market, there are now evidences of a widespread of these compounds in many countries Europe, North America and Japan with significant concentrations in marinas and harbours [ 8 ]. In addition, it has been stated that bacteria which are in contacts with AF paints can develop rapidly resistance to biocides, especially in estuaries [ 9 , 10 ], where most of the boats and aquaculture structures are moored.
It is consequently important to actively continue the development of new biocides in order to be proactive regarding these resistance issues. The awakening of the global environmental awareness in the form of legislative measures has completely changed the way AF research is conducted nowadays.
Traditionally, the industry has developed biocidal products incrementally, generating safety data as market share grows and spreading the costs over several years. The regulatory authorities now require testing of new active substance before marketing authorisation [ 2 ]. The total costs have to be taken into account, for example not only preparing agreed protocols and placing studies but monitoring studies, analysis of the results, risk assessments based on exposure scenarios, dossier preparation, registration costs, task force participations, legal fees etc, as well as management activities of the directive and associated registration.
There is a real need for the continuous development of new non-toxic AF formulations. An ideal AF formulation would have the following properties: permit at least five years biofouling life cycle control, durable and resistant to damage, repairable, low maintenance, easy to apply, hydraulically smooth, compatible with existing anticorrosion coating, cost effective, non-toxic to non-target species, and, effective at port and sea [ 12 ].
An interesting and promising line of research is inspired by biomimetic solutions. Indeed, most marine organisms are prone to biofouling, and colonisation of their surfaces can lead to dramatic stress.
Organisms that settle on the body surface of other organisms are called the epibionts, at the opposite of the basibionts, which are the hosts. Epibiosis refers to the assemblage of epibionts on a basibiont. This complex association of species will affect the fitness of both the basibionts and the epibionts [ 13 ]. A better understanding of epibiosis and especially of its avoidance could help to design new AF solutions. Marine organisms have developed natural AF strategies which can be classified in four groups: chemical, physical, mechanical and behavioural [ 12 ].
The first three are of great interest for new AF developments and have been the basis of biotechnological research respectively on new microtexturing of surfaces, marine natural antifoulants, and foul-release coatings. They are many examples from natural fouling resistant organisms which can serve as a basis for new scientific investigations. Recently, particular attention has been paid to the physical defences of marine organisms, especially the surface topography of molluscan shells, crustose coralline algae, marine mammal and shark skin [ 14 ].
This complexity limits the effectiveness of surfaces to a restricted range of fouling organisms. Researchers are now developing multiple scales of topography with the goal of achieving broader deterrents effects [ 15 ]. Biomimetics models can enable an understanding of which microtextures have the best deterrence property.
The new specific surfaces developed should be more efficient than the actual synthetic microtextured surfaces. The two major difficulties preventing so far the commercialisation of microtextured surfaces are the price and the impractical use for large vessels.
This research area is very prolific and is progressing considerably through large consortium project such as the AMBIO project Advances Nanostructured Surfaces for the Control of Biofouling which aims at linking various scientific experts chemists, engineer and biologist with the aim of designing new wide range nano-structured coatings [ 16 ].
All marine sessile organisms use adhesive materials with temporary or permanent capabilities to attach to surfaces. Foul-release coatings have specific physical and chemical properties that affect the settlement pattern of specific biofoulers. Their efficiency varies according to the surface properties and the fracture mechanics [ 19 ]. The best anti-adhesive properties have been observed with the use of silicones as polymers [ 18 ], which on top have the advantage of being very durable.
However, even if it has been shown that foul release AF treatment can inhibit the development of macrofouling organisms, they do suffer from persisting colonisation by slime [ 20 ] adhering even on vessels at speeds over 50 knots and remaining unaffected by the turbulence effect [ 18 ]. The presence of this slime increases fuel consumption and consequently CO 2 emission. Moreover, another limitation is that foul-release coatings are efficient only when the speed of the ship produces the hydrodynamic shear needed for the loosely attached macrofouling organisms to fall off [ 21 ].
On static or slow-moving structures, the efficacy is limited to the initial stages of fouling which remain easy to remove [ 22 ]. This ability is linked to the production of secondary metabolites involved in the chemical defence [ 26 ]. These compounds could be used as active ingredients in AF formulations.
The discovery of naturally occurring bioactive agents is based on bioassay-guided fractionation and purification procedures. The choice of the test organisms for bioassays is crucial and has to be ecologically relevant.
In the previous years, most of the screening were conducted against Ulva intestinalis [ 27 ] and Balanus amphitrite [ 28 ].
But nowadays, the trend is to increase the number of organisms used in bioassays to draw a wider picture of the activity spectra of a specific compound and as well as of its mode of action [ 29 ].
Moreover, at different exposure levels, the same substance may be attractive, repellent, or even toxic, demonstrating the importance of always working with a range of concentrations [ 30 ]. Toxicity bioassays are used to determine acute short time exposure, typically 96 h or shorter and chronic effects long time exposure, from weeks to months, optimally ca. Toxicity bioassays are compulsory to get toxicological information of AF compounds.
For AF formulations, toxicity tests are usually performed towards a wide range of organisms including microalgae, Artemia sp. The active ingredients isolated and their performances against representative fouling organisms have been recently reviewed [ 25 ].
To date, purification of active products from marine organisms has yielded to around molecules with variable degrees of AF activities against a wide range of marine fouling organisms [ 25 ]. Discovery of new compounds has been improved through to the continuous advances in technical innovation increased NMR magnetic field strength, probe technology, MS bench top instruments, soft ionization and FT-MS allowing an increase in the number of newly discovered molecules while using less quantity for structural elucidation.
Moreover, the marine environment is rich in unexplored species estimated at 1—2 million that may have novel biosynthetic capabilities. Data analysis highlighted that AF activity is not driven by latitudinal trends, but rather by phylogenetic constraints [ 35 ]. Very promising compounds have been purified from microorganisms, macroalgae and sponges [ 25 ].
Thus, formoside and new triterpene glycosides were obtained from the sponge Eurylus formosus and did exhibit high and broad-spectrum activities towards bacteria, fungi, macroalgae and invertebrates [ 36 ]. Many other compounds have been purified from sponges but displayed activities against invertebrates settlement or microbial growth only. Regarding the investigation of macroalgal secondary metabolites for new AF compounds, most of the research has been focused towards Rhodophyceae and Phaeophyceae [ 25 ].
Species of the genus Laurencia have been extensively investigated for the production of secondary metabolites and are known to produce ca. Regarding AF activity, the best compound obtained from this genus is the elatol which is potent against marine bacteria, and invertebrates Balanus amphititre and Bugula neritina at low concentration [ 37 , 38 ].
Concerning Phaeophyceae, the most investigated genus are Bifurcaria and Sargassum [ 25 ]. Diterpenes displaying large spectra AF activities were isolated from Bifurcaria bifurcata [ 39 , 40 ].
Interesting compounds from Sargassum tennerimum were shown to interfere with larval settlement of Hydroides elegans and biofilm formation [ 41 ]. However, active compounds are quite often produced by the associated microflora on the surface or within the organisms , which offers a great advantage for the chemical industry as they can be grown in large volume for production of compounds. Most of the secondary metabolites are rapidly breakdown when released in the environment [ 43 ] and as a consequence their incorporation in paint a formulation is very challenging.
Release rate have to be carefully controlled in order to enhance the paint lifetimes. So far, the best method develop to counter this have been perform through microencapsulation of the bioactive MNPs [ 43 ].
When a lead compound is discovered from the laboratory screening, field assays and paints formulation require large quantities of MNPs and the difficulties of mass production becomes a serious constraint [ 25 ].
Various options are available for a sustainable production of MNPs: chemical synthesis, controlled harvesting, aquaculture, in vitro production, microbial fermentation and transgenic or enzymatic production. Despite the fact that all these technologies are available, the only MNPs that have been scaled up so far are for pharmaceutical applications [ 24 ] and not yet for AF formulations.
Controlled harvesting is an ideal solution when bioactive compounds are produced from unwanted biomass such as marine invasive species for examples [ 44 , 45 ]. Recently, it has been shown that the production of AF compounds by alien seaweeds may insure a more successful persistence in a new environment, especially when this defence appears more efficient than the ones developed by the native species [ 44 ].
Collecting organisms from the field for mass production is a cheap and convenient option but careful monitoring of the potential variation of extracts bioactivity must be carried out as it was stated that several macroalgae from temperate regions showed seasonal variation of the production of bioactive compounds [ 46 , 47 ], with a higher production in spring and summer.
However, when bioactive compounds are produced by the native flora, harvesting in large quantity may cause a negative ecological impact, and, in that case other solutions are preferably chosen. The production of chemicals, fuels, pharmaceuticals, flavours and fragrances is routinely performed with catalytic tools such as enzymes, inorganic and organic catalysts [ 48 ]. However, present state-of-the-art processes for synthesis of natural products are considered highly inefficient [ 49 ].
The exact build-up of functional groups within a complex molecule still represents a challenge [ 48 ]. Inspiration from biosynthetic pathways and the natural reactivity of functional groups have been used constructively in new approaches to chemical synthesis of MNPs without protecting groups [ 50 ].
When chemical synthesis is successful, the next step is the generation of synthetic chemical analogues of the originally isolated molecules, which gives rise to a complete family of active AF compounds and, often, candidates with a more suitable bioactive profile.
Novel Antifouling Coatings: A Multiconceptual Approach
Marine and Industrial Biofouling pp Cite as. The development of novel antifouling and foul release coatings must be considered in the context of business, government, and academic research. Existing antifouling technology is based upon the use of broad-spectrum biocides. Foul release technology is partially developed, has incompletely understood mechanisms and unknown long term fates and effects. Business is structured to register, generate, deliver, apply, and remove antifouling coatings based upon broad-spectrum biocides. Business is weak in biology and study of fates and effects beyond those required for registration. Government is structured to regulate, respond to, and support basic research.
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Marine biofouling is of major economic concern to all marine industries. The shipping trade is particularly alert to the development of new antifouling AF strategies, especially green AF paint as international regulations regarding the environmental impact of the compounds actually incorporated into the formulations are becoming more and more strict. It is also recognised that vessels play an extensive role in invasive species propagation as ballast waters transport potentially threatening larvae. It is then crucial to develop new AF solutions combining advances in marine chemistry and topography, in addition to a knowledge of marine biofoulers, with respect to the marine environment.
Part 1 Marine fouling organisms and their impact: The battle against marine biofouling: A historical review; Surface colonisation by marine organisms and its impact on antifouling research; Algae as marine fouling organisms: Adhesion damage and prevention; Bacterial adhesion and marine fouling; Understanding the biofouling of off-shore and deep-sea structures; The effects of marine biofouling on the performance of ocean-going vessels; The impact and control of biofouling in marine finfish aquaculture; Expected effect of climate change on fouling communities and its impact on antifouling research; Legislation affecting antifouling products. Part 2 Testing and development of antifouling coatings: Developing new marine antifouling substances: Learning from the pharmaceutical industry; Laboratory bioassays for screening marine antifouling compounds; Key issues in the formulation of marine antifouling paints; Modelling the design and optimization of chemically active marine antifouling coatings; High throughput methods for the design of fouling control coatings; Ageing tests and long-term performance of marine antifouling coatings; Testing the impact of biofilms on the performance of marine antifouling coatings. Part 3 Chemically active marine antifouling technologies: Tin-free self-polishing marine antifouling coatings; The use of copper as a biocide in marine antifouling paints; The use of broad-spectrum organic biocides in marine antifouling paints; Organic alternatives to copper in the control of marine biofouling; Natural marine products with antifouling activities; Enzyme-based solutions for marine antifouling coatings.
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