More than 4,000 marine animal and plant species, all of them sessile, i.e. fixed in one place, have so far been identified in biofouling. The "fouling community" mainly consists of algae and barnacles, molluscs and bryozoans, polychaetes, tubeworms, tunicates, mussels and hydrozoans. Fouling species settle on almost all suitable surfaces of submerged objects. These can be of natural origin (stones, hard soils, reefs) or man-made structures (e.g. ship’s sides, drilling platforms, quay walls, navigation marks or aquaculture facilities). Biofouling sometimes even accumulates on the body surfaces of living marine organisms such as turtles or whales. These colonies are called epibioses if the relationship is not parasitic. Animal fouling organisms typically have complex life cycles with a planktonic larval phase during which the hard structures are colonized followed by a sessile (benthonic) juvenile and adult phase.
Microorganisms play a central role in the development of marine biofouling since they can both inhibit and promote the colonisation and later metamorphosis of planktonic larval stages of invertebrates and algae spores. The establishment of such communities follows a characteristic pattern which was described in a publication of the Woods Hole Oceanographic Institution in 1952. According to this, biofouling ecosystems develop in four stages:
- The first step before the actual colonization is the formation of an "adhesive film" on the submerged surface. Van der Waals forces ensure that the surface is coated with a film of organic polymers in just a few minutes.
- In the following 24 hours bacteria and diatoms adhere to this basic layer and initiate the formation of a biofilm.
- After about a week sufficient nutrients are concentrated in this layer and enable the secondary colonization of the biofilm with macroalgae and protozoa.
- In the following 2 to 3 weeks, "tertiary colonizers" ("macro fouling") attach themselves to the biofilm. They include tunicates as well as sessile molluscs and cnidarians.
"Hard fouling" presents a particular problem. It mainly includes barnacles, polychaetes, bryozoans and similar organisms that form solid lime structures that are very difficult to remove. The economic consequences of biofouling are considerable. According to rough estimates more than 5.7 billion US dollars are spent annually worldwide to prevent or remove biofouling. Without rigorous control measures the damage would be incalculable. Biofouling blocks pipelines and cooling systems, adds to the weight of floating objects and increases flow resistance. The maintenance and servicing downtimes required to remove the growth cost billions. Although biofouling occurs everywhere it is – from an economic point of view – particularly serious for shipping. It can damage hulls and propulsion systems, it reduces manoeuvrability and travelling speed, which in turn increases fuel consumption and thus costs and is, on top of that, more damaging to the environment and climate. Tests in the flow channel have shown that a 5% increase in flow resistance caused by biofouling increases a ship's fuel consumption by 17% and CO2-, NOx- and SO2 emissions by 14%.
Biofouling also affects marine aquaculture. It clogs up the meshes of the nets, which endangers water exchange with the environment and the O2 supply to the fish. Floating installations, lines and boundary buoys become heavier and therefore sink deeper into the water; greater cleaning effort increases production costs and also requires a lot of time. Mussel cultures suffer twice as much. On the one hand, because the growth can cut off the filter feeders from their food (some fouling organisms are even filter feeders themselves which makes them direct competitors for the cultivated mussels). On the other hand, the mussels have to be thoroughly cleaned before they can be marketed, which requires additional effort and adds to costs.
Biocide-containing paint coatings inhibit growth
However, biofouling not only causes economic losses, but also involves ecological risks, because an uncontrollable mix of aquatic species –including potentially invasive species – "travels" in the growth on a ship’s sides and can spread worldwide. This explains the dramatic increase in social, economic and environmental pressures to develop more effective strategies for controlling biofouling.
In purely technical industrial plants such as pipelines or cooling systems rigorous methods are often used to kill growth organisms… for example biodispersive substances or regular chlorination, heat shocks or energy pulses. This is not possible in natural environments, however, and so "biocides" are used (Greek bios - life, Latin caedere - kill). Even in extremely low concentrations these are highly toxic to growth organisms. To achieve a long-term effect they are incorporated into antifouling coatings. In ancient times, boat builders used to coat the sides of ships with paint mixtures of pitch and copper oxide or arsenic, oil and sulphur to prevent fouling and keep away wood-boring crustaceans. In the 18th century, the British navy had keels and hulls coated with copper sheeting which was an expensive but effective method of protection against fouling. When the first ships with steel hulls appeared, however, copper plating was no longer appropriate due to the galvanic corrosion between copper and iron in salt water.
Around 1860, an antifouling paint was first tested in Liverpool which contained an active ingredient designed to protect the surface from colonisation by biofouling organisms. Until the middle of the 20th century, antifouling paints were mostly based on copper oxides. The decisive breakthrough did not occur until the 1960s with self-polishing paints that released the toxin only slowly and in a controlled manner. The success of these antifoulants was mainly due to the biocides used, which were usually based on organotin compounds. Tributyltin-oxide (TBT), which offered very effective protection of ship surfaces from fouling growth, became particularly important. At the peak of TBT paints, approximately 70% of the global shipping fleet was coated with this antifoulant. And so the shock was all the greater when it was realised that TBT not only prevented biofouling but also damaged marine life as a whole. Even the tiniest amounts of this biotoxin in the nanogram range can have serious consequences for marine ecosystems. TBT has been described as the "most toxic pollutant" ever deliberately released by humans into the oceans. As a result, the International Maritime Organization (IMO) banned the use of TBT as a biocide for ships in October 2001.
After the end of the organotin toxin era interest in copper as an active ingredient increased again. Paint manufacturers developed special coatings with epoxide-based honeycomb matrices and microchannels through which the enclosed copper biocides were gradually released to combat fouling. Modern adhesives made it possible to apply copper alloys to steel ships without causing galvanic corrosion. However, since copper is only effective against animal fouling, further biocides had to be added to the paints to prevent the growth of silica and other algae. The realisation that copper can also have undesirable effects on the environment has further intensified the search for alternative antifoulants. Researchers around the world are now looking for coatings that contain less toxic biocides but are still at least as effective as the banned TBT. This necessitates different competences. Colour developers today work closely with marine and molecular biologists, chemists and ecotoxicologists, often across national borders. Examples of this are international research programmes such as AMBIO in EU countries or the Biofouling Control Coating project of the US Department of Marine Research (ONR).
Less aggressive methods for alternative antifouling
Apart from the removal of fouling with brushes and high-pressure cleaners – which does not have a preventive effect but usually only takes place when the growth has already taken over – biocides are likely to remain the mainstay of antifouling strategies for the time being. But there are also some completely new ideas and approaches for less toxic fouling control. Some concepts are primarily aimed at preventing the colonisation of the microorganisms that create the initial biofilm on surfaces. Other new, very promising possibilities affect the growth of the organisms that appear later on in the process. For example, experiments are currently underway to prevent the colonisation of barnacles (which with their calcareous walls and base plates probably cause the greatest fouling problems) with medetomidine, a pharmacologically effective, less toxic compound from the group of imidazoles that can be synthesised on an industrial scale and has long been used as a sedative and painkiller in veterinary and human medicine.
Whereas previous strategies were usually based on creating a toxic surface in order to prevent the settlement of larvae, the fundamental idea behind medetomidin is quite different. Here the researchers start with neurobiological processes and try to modify the behaviour of the cypris larvae of barnacles (Cirripedia). Since physiological processes are mostly communicated by neurotransmitters, pharmacological instruments such as medetomidine can be used effectively for antifouling purposes. Originally, the researchers were working on another idea. Since the cement secretion of the Cypris larva is controlled by dopamine – another neurotransmitter – they wanted to block the corresponding receptor. However, a very high concentration of the antagonist would have been necessary to put a complete stop to cement secretion. It seemed just as effective (an also simpler to implement) to try to prevent the larvae from reaching the surface in the first place. When colonising a surface, the Cypris larvae are dependent on their own mobility; any disturbance of the process drastically limits their ability to reach the target surface, cling to it and settle there.
Cypris larvae approaching a surface that contains medetomidine are no longer able to control their swimming behaviour and move in the desired direction. Researchers suspect that medetomidin blocks the octopamine receptors in the larval nervous system, prevents the transmission of signals, and thus disrupts the coordination of swimming movements. Field trials have shown that medetomidin is already effective in very small amounts. Minimal concentrations of 0.025 weight per cent in antifouling coatings were sufficient to completely inhibit the colonization of barnacles. Although tests have not yet been completed preliminary results suggest that medetomidine has no endocrine effects, has a low lethality, and does not lead to bioaccumulation in the organism. This makes the substance a promising candidate that could replace copper in marine antifouling paints. However, medetomidine must be combined with co-biocides because it is not effective against plant growth.
Smooth surfaces impede fouling adhesion
Non-toxic mechanical strategies against biofouling include coatings and materials with smooth and slippery surfaces on which organisms cannot settle. Some are inspired by living animals such as sharks and dolphins whose skin is not, or at least extremely rarely, overgrown. Such "non-stick" coatings are mostly based on nanotechnologies or organic polymers with additional "antimicrobial" properties. A variant of the non-toxic antifouling coatings is based on hydrophobic surfaces with extremely low friction. Such smooth layers, which prevent the adhesion of larger microorganisms, usually consist of fluoropolymers or silicone. Although these substances are ecologically "neutral" they are mechanically not very stable. For this reason, polydimethylsiloxane (PDMS), which consists of silicon and oxygen atoms, is currently more commonly used for self-polishing coatings. However, the cleaning effect of PDMS depends on the flow strength at the coated surface; it is only fully effective at speeds above 20 knots. For stationary objects such as quay sides, anchored aquaculture facilities or ships with long port lay times, PDMS is therefore not an effective protection.
A new kind of approach to non-toxic antifouling is coatings with hydrophilic properties that effectively prevent the adhesion of bacteria and the formation of biofilms. However, these technologies are still in development (e.g. at ONR) and are not yet freely available.
In addition to these "passive" processes, there are also "active", energy-intensive methods to prevent fouling. Pulsed laser irradiation, for example, can be used against diatom growth. Plasma pulse technology is also very effective against bivalves, killing organisms within milliseconds. Some LED manufacturers have developed UVC devices (250-280 nm) which can detect and even prevent the formation of biofouling. The detection of beginning fouling is already possible in the early colonization phase because microorganisms possess intracellular fluorophores that fluoresce when stimulated by UV light. This is mainly due to the amino acids tyrosine, phenylalanine and tryptophan. Strong radiation in the UVC range deactivates the DNA of microorganisms and thus prevents the formation of a biofilm which is a prerequisite for the development of the fouling community. The extent to which this non-contact, non-chemical method can also be used as an antifouling solution for larger objects remains to be seen.
Ultrasound, too, has similar effects and is being tested on small to medium-sized boats as an alternative to antifouling on a paint and coating basis. Ultrasound also kills or denatures microalgae and microorganisms which are still at the beginning of the formation sequence of fouling communities. In pipe systems and cooling systems simple heat treatments often prevent the colonisation of growth organisms, especially bivalves, which can completely block pipes over the course of time. It is important to carry out these thermal treatments regularly in order to prevent the colonisation of larvae right from the start.
Perhaps other substances will be available in the future to combat fouling. Biotoxins, for example, some of which are more effective than synthetic compounds. Research is focusing on bufalin, a cardiotonic toxin that was originally isolated from the poison of Chinese toads and forms part of many traditional Chinese remedies. Bufalin is said to be more than 100 times stronger than TBT and also effective against barnacles.