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Biofouling: It's Not Just Barnacles Anymore
(Released March 2004)

 
  by Marianne Stanczak  

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Every spring when the snow melts, there it is - rust. Unsightly, almost evil, rust plagues us. That small reddish-brown speck is suddenly a massive crater, slowly sucking the mechanical life out of your vehicle. Did a chunk of fender just fall off?

We know this rust is usually caused by road salt or elements in the air reacting with a vehicle's metal surface, but where do other examples of rust originate? We also know that we could prevent or at least stall an automobile's corrosion by rust proofing our cars, washing off excess salt and dirt, and so on. Easy enough, right? But what would you do with a Navy warship or an offshore drilling rig? Obviously more than bringing out the pail of soapy water on a Sunday afternoon. In tropical waters, street salt is certainly not causing corrosion and deterioration of ships' hulls, water-cooling systems or offshore structures. So what is? And how are these and other industries, the livelihood of which depends on being corrosion-free, solving the problem? Before a problem can be solved, it must first be defined.

Biofouling is simply the attachment of an organism or organisms to a surface in contact with water for a period of time. That explanation sounds fairly straightforward, but there are several organisms that cause biofouling, many different types of surfaces affected by it, and, due to the work of scientists, engineers and others, scores of solutions to the problem. Even this definition greatly simplifies what really occurs. This article examines the causes of biofouling and the numerous, innovative solutions being derived from our evolving knowledge of these causes.

Biofouling occurs worldwide in various industries, from offshore oil and gas industries in China and the Indian Ocean, to fishing equipment in the Caspian Sea, to cooling systems in the Chesapeake Bay. One of the most common biofouling sites is on the hulls of ships, where barnacles are often found. The most obvious problem of growth on a ship is the eventual corrosion of the hull, leading to the ship's deterioration. Even before corrosion occurs, if left unattended, organic growth can increase the roughness of the hull, thereby decreasing its maneuverability and increasing drag. 1 This domino effect continues when the ship's fuel consumption increases, in some cases by 30%.2 This in turn has economic and environmental consequences, as increased fuel consumption leads to increased output of greenhouse gases.3 Economic losses are tremendous, as fuel accounts for up to 50% of marine transportation costs.4

Biofouling is everywhere. Parts of a ship other than the hull are affected as well: heat exchangers, water-cooling pipes, propellers, even the ballast water.5 Heating and cooling systems biofouling might also be found in power stations or factories. Just like a clogged drain in your kitchen or bathroom, buildup of matter inside cooling system pipes decreases performance.6 Again, fouling causes a domino effect. Equipment must be cleaned frequently, at times with harsh chemicals, and the obstruction of piping can lead to a shutdown of plants and economic losses.7

Fishing and fish farming are also affected, with mesh cages and trawls harboring fouling organisms. In Australia, biofouling accounts for about 80% of the pearling industry's costs.8 Gold- and silicon-based components of microelectrochemical drug delivery devices are susceptible to biofouling, as are machines in the papermaking and pulping industries and underwater instrumentation.9 Yet another place biofouling organisms lurk is piping and sprinkler system nozzles of fire protection systems.10

The problem is more serious in tropical waters. Cold waters have a low prevalence of biofouling, perhaps because of the physiology of the organisms responsible.11 It is not only barnacles that create difficulties, and as we've seen, it's not just on ships' hulls; moreover, the old method of scraping barnacles is not the only solution.

Biofouling is not as simple a process as it sounds. Organisms do not usually simply suck onto a substrate like a suction cup. The complex process often begins with the production of a biofilm.

A biofilm is a film made of bacteria, such as Thiobacilli or other microorganisms, that forms on a material when conditions are right.12 Nutrient availability is an important factor; bacteria require dissolved organic carbon, humic substances and uronic acid for optimum biofilm growth.13 Biofilms do not have to contain living material; they may instead contain such once living material as dead bacteria and/or secretions.14 Bacteria are not the only organisms that can create this initial site of attachment (sometimes called the slime layer); diatoms, seaweed, and their secretions are also culprits. Coral reef diatoms' attachment depends on pH, and as in the Achnanthes and Stauronesis diatoms, the molecular structure of the organism.15 The study of the biology of the Achnanthes longipes (Bacillariophyceae) diatom can determine which temperatures produce maximum growth.16

The growth of a biofilm can progress to a point where it provides a foundation for the growth of seaweed, barnacles, and other organisms. In other words, microorganisms such as bacteria, diatoms, and algae form the primary slime film to which the macroorganisms such as mollusks, seasquirts, sponges, sea anemones, bryozoans, tube worms, polychaetes and barnacles attach.17

This initial process does not occur in a random fashion. Conditions must be favorable, including proper pH, humidity and nutrient availability.18 Organisms appear to be particular; for example, bacteria creating biofilms on stainless and carbon steels and recirculating cooling systems are similar physiologically and often the same species.19 Biochemistry may determine if and where biofilms attach, as in the case of Vibrio alginolyticus, a bacteria which produces organic compounds sensitive to changes in temperatures and pH.20 Chemistry and biology also determine which organisms attach to the biofilm. A barnacles microbiology prevents or assists it in settling on substrates.21

Barnacles (a type of marine crustacean), encrusting bryozoans, mollusks, tube worms, and zebra mussels create a type of fouling known as calcareous (hard) fouling, while organisms such as algae, slimes and hydroids make up non-calcareous (soft) fouling.22 As mentioned earlier, studying an organism's biology and chemistry may determine where it settles, what prevents it from settling, and therefore, which technique or techniques to apply.

Many factors contribute to an organism's settlement on a substrate. Blue mussel attachment has been studied with relation to different chemical elements, and the green mussel, Perna viridis, and Great Lakes zebra mussels have been studied in relation to chlorine.23 The effects of water turbulence on spores of the bacterium Bacillus thuringiensis have been studied.24 Composition of metal surfaces and even color of an organism have been examined. The settlement of the biofilm created by Pseudomonas fluorescens, another type of bacterium, can be determined by flow of water; maximum development occurs around 1 m/sec.25 Zebra mussels (Dreissena polymorpha) and water flow velocity have been studied - settlement of the mussels does not occur at velocities greater than 2m/sec.26 Barnacles will actually detach (from a silicone surface, most often an antifouling coating) at 10-15 knots (~5.1 - 7.7 m/sec) but most effectively detach at 30 knots (~15 m/sec).27 Turbulence and speed also help to detach seaweed and kelp from substrates.28 Where organisms reside and thrive - quiet waters, flowing waters, or the tidal zone - is another important consideration.

One of the primary ways to prevent biofouling is to select the appropriate material out of which to make a structure. This may be accomplished in coordination with the biological knowledge of biofouling organisms. For example, zebra mussels find aluminum-bronze distasteful, so they tend to avoid such structures.29 Cupronickels (copper-nickel alloys) have good biofouling and corrosion resistance, and therefore are often used for surfaces or surface coatings.30 Two of the most popular materials used are 90/10 and 70/30 copper-nickel alloys (90%Cu-10%Ni and 70%Cu-30%Ni, respectively).31 This method may not be effective in every situation, especially with ships that travel great distances through waters of different temperatures and salinity, rendering a change in materials' resistances to biofouling.

One of the earliest methods of solving the problem was simply to scrape the hulls of ships. This solution, although simple and relatively effective, poses one not so obvious major problem spread of invasive species. This is illustrated best with the population explosion of zebra mussels (Dreissena polymorpha) in the Great Lakes region. The mussels are picked up by fishing equipment, ships and other vessels and transported to non-native waters where they wreak havoc on native environments. To counter the spread of invasive species, many areas have established hull-cleaning laws that state that any material removed from a hull must be collected and disposed of properly.32

When cleaning (or scraping) becomes time consuming or ineffective, industries turn to perhaps the most widely accepted method of controlling and preventing biofouling - antifouling coatings. One of the most popular of these is tin-based coatings, specifically triorganotin- or organostannic- or simply, TBT-coatings. These are also considered self-polishing, as there is a controlled hydrolysis (decomposition) of the surface, which releases the TBT in a slow, steady fashion from the substrate. When a substrate (e.g. a ship's hull) is in motion, the water wears the compound away, leaving behind particles. TBT-coatings are highly effective in reducing/controlling biofouling; however, they are also highly toxic to marine organisms. You may ask: isn't that what is desired? TBT-coatings are toxic to biofouling organisms, but also to non-target organisms. TBT interferes with major biological processes such as growth, reproduction and immunity, on a cellular level.33 Some antifouling paints have a leaching rate of more than 4 micrograms of TBT per day. That may sound insignificant, but damage to an organism can occur in low concentrations - as little as less than 1 ppb (1 part per billion) - and the life of a TBT-coating can be as great as five years.34

Due to the toxicity of TBT, the United States Navy discontinued use of such paints at the beginning of the decade, and others are following suit.35 Plans are underway worldwide to phase out TBT by late 2007 or 2008.36 An application ban begins with boats of less than 25m, then all lengths, and by 2007/2008, even the presence of the chemical on hulls will be prohibited, even on coatings applied before the ban.

With the solution of one problem, another arises: how to replace TBT coatings. Since copper is a successful deterrent of biofouling for instance pipe made of copper kills E. coli in drinking water it has been used as a coating.37 Many, however, worry about its effects on the environment. In fact, along with the phase-out of TBT, some want to eliminate copper-based coatings, claiming they are responsible for the same negative effects as TBT.38 A unique example is the copper-based antifouling paint used to prevent the amoebic gill disease-causing protozoan Neoparamoeba pemaquidensis from attaching to salmon nets; Paradoxically, in one experiment, the use of copper paint increased the presence of N. pemaquidensis.39 Because this is an isolated study, more research is needed on the relationship between copper and the protozoan. However, in the process, more biofouling prevention techniques may be discovered.

Some in the metals industry claim that there is no evidence proving adverse effects on organisms by copper-based antifouling paints.40 To resolve these contradictory viewpoints that could harm the copper industry and/or the natural environment, much more research needs to be done and is being done. For example, it has been discovered that a copper-nickel alloy will release copper ions much more slowly than pure copper.41 This information can be used to develop new techniques, as well as reduce copper use.

In the event that copper-based coatings are banned, what are other methods of preventing biofouling? There is currently a wealth of options, and more are surely on the horizon. We may first reexamine the idea that prevention starts with selecting the right material. Titanium alloys, such as UNSR50400, UNSR52400, and UNSR53400 (200206-35-1369), have been shown to exhibit little or no toxicity in marine animals.42 Titanium surfaces may be further protected by maintaining a more than 2m/sec water velocity to which organisms cannot attach. Titanium is also immune to microbiologically influenced corrosion (MIC), indicating that only larger organisms such as mussels or barnacles are responsible for inducing corrosion.43 For those titanium surfaces that cannot maintain the 2m/sec velocity, or for stationary structures, chlorination is an option. Chlorination has been used in the Great Lakes to inhibit biofouling of the zebra mussel Dreissena polymorpha3 and, in other parts of the world to inhibit the green mussel Perna viridis.44 However, some studies show that chlorination is unlikely to work against mussels if they are the predominant foulers once again raising concerns that chlorine is toxic.

Due to concerns about polluting waters and poisoning organisms, many have pursued research to create non-toxic coatings. One of these is known as a foul-release coating.45 Usually made of polymers (plastics), these coatings are non-toxic and are thought to have a natural resistance to biofouling by creating a low surface tension and having a low glass transition temperature.46 Polymers utilized in these coatings are silicones and fluoropolymers and ethyl vinyl acetates. 47 Cornell University in Ithaca, NY, this past year created two types of antifouling coatings - one hydrophilic (water-loving) and one hydrophobic (water-hating). These coatings in essence make a ship self-cleaning, creating a slippery substrate resistant to organisms - from bacteria to barnacles - as it moves through the water.48

Other man-made solutions are abundant. A technique commonly used against diatoms is called pulse laser irradiation - the longer the duration of each pulse, the greater the mortality of organisms.49 Unfortunately, this radiation is not species-specific and can harm non-target organisms. Plasma pulse technology does not use chemicals or heat; it transmits energy directly into the water, which may cause harmful shockwaves or steam bubbles.50 Pulsed electric fields, frequently used in pipes, create acoustic waves. Unlike plasma pulse technology, this process does not create shockwaves that could affect cooling/heating systems. In addition, electric fields do not kill, but stun organisms, clearly lowering mortality of non-target organisms. 51

Another unique method for decreasing biofouling starts not with coatings, not with materials selection, but with where the materials (in this case, boats) are housed when not in use. Enclosed marinas are much more likely to contain biofouling organisms than unenclosed marinas. Tides and currents assist in the flushing and renewal of water in a marina. Harbors can be designed to ensure maximum flushing capacity, because marinas with breakwaters retain more water than marinas without, leaving a greater build-up of fouling species. 52

There may be no greater way to fight nature than with nature itself; it is important to study fouling organisms' biology in order to help prevent them from becoming a nuisance. Nemertine pyridyl alkaloids (chemical compounds from worms) may be used for the inhibition of Balanus amphitrite (barnacle) larvae.53 Studies in Caribbean sponges have shown that purified, extracted compounds from them deter bacterial attachment. Since bacterial attachment is often the initial step of the biofouling process, these sponges and their chemistry may help to prevent the succession to larger organisms.54

A member of the ascidian group, Distaplia nathensis, is also showing promise - used in an extract, it inhibits byssal production in the mussel Perna indica. A byssus is a mass of filaments which the mussel uses to attach itself to a surface.55 Dark brown bacteria provide the best attachment for oyster larvae, but there are other bacteria which produce polysaccharides that are toxic to oysters; in other words, they may be used to prevent biofouling by oysters. 56 Immunoglobulins provide a natural biocide for planktonic (floating) and sessile (attached) bacteria.57 Some species of bacteria can be used against other biofouling organisms. Pseudoalteromonas spp., marine bacteria, produces bioactive compounds with an inhibitory effect and can be used to prevent biofouling by algae like Ulva lactuca and by barnacles like Hydroides elegans and Balanus amphitrite.58 Research is still needed to determine the exact method of applying this knowledge.

These solutions are, of course, the tip of the iceberg. Many organisms are responsible for fouling, and much information is still to be learned so that more environmentally friendly prevention methods may be used. Not all organisms that cause biofouling and its subsequent damage, are evil - Dreissena bugensis and the infamous zebra mussel, Dreissena polymorpha are known as biofoulers, but their productivity and filtration capacity are important for maintaining water quality.59

Using natural methods may be more cost effective than specialized coatings, materials, or techniques. It is obvious that something needs to be done because of the economic impacts that biofouling has on so many industries. These industries' research might serve to overcome the still-common misconception that businesses cannot remain profitable without harming the environment.

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