BARNACLES, mussels and algae evolved to thrive underwater on the surfaces of rocks and in crevices. But when they choose to cling instead to ships鈥� hulls, oil platforms and pipelines, the result is havoc. Ships suffer increased drag, leading to lower speeds and higher fuel consumption. Corrosion of pipelines escalates, and fish farm cages become swamped. Worldwide, the cost of keeping these unwanted marine organisms at bay comes to at least $1.4 billion a year.
For ships, the traditional remedy has been a regular visit to the dry dock. There, barnacles and other organisms can be scraped or sandblasted off the hull, which is then covered with a coat of antifouling paint designed to discourage their return. As long as 2000 years ago, hulls were sheathed with lead and smeared with concoctions of oil laced with sulphur and arsenic. And in 1625, a lethal recipe combining arsenic, copper and gunpowder was considered important enough to be worthy of an English patent as an antifouling compound. Modern antifouling coatings containing copper or antibiotics work on similar principles. Currently, the most effective formulations are based on organic compounds containing the metal tin, notably tributyltin (TBT). Applied to naval vessels, these organotins give up to five years鈥� protection. TBT can be incorporated into self-polishing coatings, designed to wear away gradually as water flows by, releasing the barnacle-inhibiting compound at a constant rate.
The downside to TBT started to emerge in the early 1980s, when it became clear that the organisms that foul ships鈥� hulls were not the only victims. In waters contaminated with the antifouling agent, whelks were starting to show sex-changing disorders, and oysters developed abnormally thick shells. As a result, restrictions were imposed on the use of organotins in most of Western Europe, Japan, the US, Canada, Australia and New Zealand. A complete ban on TBT coatings now operates in Japan, while in Britain its use for small boats was outlawed in 1988.
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In the wake of these restrictions, copper compounds have re-emerged as the main active ingredients of antifouling coatings. These mixtures work well in the short term, and the best formulations can last for up to three years. But they can鈥檛 match the five-year lifetime of the self-polishing TBT coating. And like TBT, copper-based antifouling agents work by poisoning the unwanted organisms, so biologists fear that they too could cause environmental problems. The use of copper-based coatings is still allowed, but the US Environmental Protection Agency and the European Union have begun to review their effects.
Awaiting alternatives
The search is now on for different ways of keeping barnacles and other marine life off ships and underwater structures. 鈥淎ll over the world, people agree that there is a need for alternatives,鈥� says Peter Willemsen of the TNO Centre for Coatings Research in the Netherlands. Industry is keen to back the research, fearing that some existing products could be banned. 鈥淭he legislation is not so strict at the moment, because the authorities realise that there are no alternatives,鈥� says Willemsen. 鈥淏ut as soon as there are, then the harmful chemicals will be banned. The industry is aware that there is not much time left.鈥�
One line of attack is to try to create 鈥渘onstick鈥� ships. Make the surface slippery enough and the hapless barnacles will simply fall off. The nonstick coatings developed so far are mostly based on silicone compounds. On fast-moving boats, they can be self-cleaning, says Julian Hunter of International Paint, based in London, which is looking at the success of the coatings on different types of boat. 鈥淒rug-busting boats off the coast of Miami have no problem,鈥� he says. On slower ships, the coatings do not completely prevent fouling, but they do make the surfaces easier to clean. Another problem has been that the coatings are relatively soft. The US Office of Naval Research is spending $4 million per year on its antifouling programme, which is focused on the search for a tougher nonstick coating.
In the meantime, an array of sea creatures 鈥� including corals, sponges and sea squirts 鈥� are pointing the way to natural chemical weapons that might repel potential hangers-on without causing widespread harm. Many aquatic animals, especially those that attach themselves to the bottom of the sea, run the risk of being swamped by the same organisms that foul ships and pipelines, so they have evolved strategies to beat off these enemies. Some corals and fish seem to renew their outer surfaces by exuding mucus that is sloughed off. Others appear to produce chemicals that repel marine organisms.
In laboratories around the world, these creatures are under the spotlight. Their cells and tissues are yielding a rich harvest of compounds which could function as antifouling agents. Extraction techniques are straightforward. Step one is to grind up the tissue and apply a series of solvents to soak up and separate fractions containing different chemical ingredients. Next, these chemicals are teased apart with standard techniques that separate molecules according to their size and chemical structure. At each stage, the fractions are tested to see whether they prevent fouling organisms like barnacle larvae, algal spores and bacteria from settling on the surface of a laboratory dish.
Larva repellant
The fouling creature most often used in these tests is the cypris larva 鈥� the free-living larva of the barnacle, which eventually settles on the bottom of a boat and grows into the more familiar adult form. In the larva鈥檚 exploratory phase it temporarily attaches itself to a surface using an adhesive which it secretes onto attachment discs at the ends of its two antennules. It then 鈥渨alks鈥� over the surface, and if the larva does not like what it finds it can swim off and look for somewhere better. But if the surface is suitable, the larva makes an irreversible commitment to settling there by secreting a cement through ducts in its antennules. The cement permanently fixes the larva in place.
Once the larva has metamorphosed to the adult form, it develops an additional protein-based cement to bind itself to the surface. It is at this stage that the barnacle becomes so difficult to remove from marine structures. So the ideal antifoulant would repel larvae before or during the exploratory phase.
A wealth of potential antifoulants has already been obtained from sea creatures such as corals. In the early 1980s, at Duke University Marine Laboratory in North Carolina, John Costlow and colleagues isolated several substances with antifouling properties from octocorals 鈥� so named because they are covered in sprays of eight tentacles. These natural antifouling agents belong to a class of compounds known as diterpenoid lipids. The researchers obtained two similar lipids, known as pukalide and epoypukalide, from the outer fleshy tissues of the whip coral Leptogorgia virgulata. And from a species of sea pansy, Renilla reniformis, came a previously unknown group of diterpene compounds, now named the renillafoulins. All these natural products turned out to prevent barnacle larvae settling on laboratory dishes, and they did so at concentrations which were four to five orders of magnitude lower than the dose needed to kill them.
The sea grasses have also thrown up some potential antifouling compounds. In the early 1990s, Richard Zimmerman and colleagues at the Hopkins Marine Station found in laboratory tests that a crude extract of Zostera marina, a species of eelgrass, prevented settlement of some marine bacteria, algae, barnacles and tube worms. All the evidence suggested that the vital ingredient was an aromatic compound known as zosteric acid. Zimmerman admits that this compound is not the 鈥渕agic bullet鈥� 鈥� it does not fend off all biofouling organisms. But it has shown some promise in short-term field tests against 鈥渉ard鈥� fouling organisms such as barnacles and tube worms.
The bryozoans that encrust rocks and seaweeds have also yielded antifouling compounds. At the Marine Biotechnology Institute in Tokyo, Wataru Miki isolated a number of potential antifoulants, most notably one called tribromogramine (TBG), from the bryozoan Zoobotryon pellucidum. The results are promising. TBG is only one-tenth as toxic as TBT, but is six to eight times as potent when it comes to inhibiting the settlement of larvae.
Promising sponges
As part of a research programme that is funded by the EU and coordinated by Mike Cowling of the Glasgow Marine Technology Centre, Willemsen has screened 35 species of Caribbean sponges for their ability to repel barnacles. Of the extracts he tested, roughly half discouraged barnacles from settling. Some are toxic to the larvae, but this need not be a problem if the compounds break down readily after they are released, as this would render them harmless to other marine life. It is still a mystery how some compounds repel barnacle larvae without being toxic to them. 鈥淭he barnacles sense something that they don鈥檛 like, and just stay in the water,鈥� says Willemsen.
Even bacteria and algae, usually thought of as fouling organisms themselves, are yielding antifouling compounds. Bacteria are the focus of a study which was begun in 1993 by Staffan Kjelleberg at the University of New South Wales in Sydney. A species of Alteromonas isolated from the surface of a sea squirt has proved especially promising, having yielded three active components. Two of these, a protein and a low molecular weight compound, are stable. The small molecule is toxic, but the protein seems to inhibit the settlement of barnacles, sea squirts and spores of the seaweed Ulva by some nontoxic means which, again, no one understands.
Kjelleberg is reluctant to say more about the nature of the active chemical, and with good reason. He says that several companies have expressed an interest in these substances, and the race to patent them for antifoulant use has begun. These substances could eventually prove highly profitable. Bacteria are generally easy to culture, and extracting the active compounds has been straightforward. Genetically engineering the bacteria to yield more of the active compound, or a more active variant, could also be an option in the future.
Also in Sydney, Peter Steinberg and colleagues have been looking at antifouling agents from a red alga, Delisea pulchra. The alga has produced a family of bromine-containing furanones, which have been found to reduce settlement of barnacles and to curb development of the fertilised eggs of the alga Ulva. The compounds were also found to slow the growth of a particular marine bacterium more effectively than one of the antibiotics used in conventional antifouling coatings. Thus a single compound combines the multiple modes of attack that make commercial antifouling paints so effective, and this makes it a promising natural alternative. Steinberg鈥檚 group recently filed a patent application on the compounds, and intends to formulate coatings for field tests.
Simpler solutions
Another promising feature of the furanones is their versatility. Steinberg found that chemical tinkering with the molecules 鈥� changing a hydroxyl group to an acetate group, for example 鈥� altered their antifouling power by orders of magnitude. This may be the key to finding out exactly how these natural compounds can repel invading organisms without poisoning them.
Zimmerman鈥檚 preliminary work in 1992 on extracts of sea grass points to the same conclusions. A sulphate ester group on the zosteric acid molecule seems to be the key to its antifouling properties. For the pukalide and renillafoulin molecules, again the activity seemed to be attributable to small key areas on the molecules. Dan Rittschof and colleagues at the Duke University Marine Laboratory discovered in the late 1980s that the components likely to be responsible are small oxygen-containing rings known as lactones and furans.
These results are good news. If the key groups can be identified, then they can probably be found in less exotic 鈥渙ff-the-shelf鈥� compounds. This makes research much more likely to yield practical results, and the potential for profit immediately increases. Various lactones and furans are available by the bottleful, so Rittschof thought it worth trying them out on barnacle larvae. Sure enough, some of them inhibited settlement as efficiently as the natural extracts. On barnacle larvae these compounds seem to 鈥減ut the organism to sleep鈥�, and the effect appears to be reversible. Larvae that have been inactivated by the molecules rapidly recover when placed in clean seawater, and go on to attach to clean surfaces where they develop normally.
While the laboratory tests continue, some of the natural antifouling agents are being put to the test in practice. Over the past five years or so, researchers funded by the Office of Naval Research have been applying some of the chemicals, including the lactones, furans and zosteric acid, to rafts at Pearl Harbor in Hawaii and Beaufort, North Carolina. Don Sundberg and colleagues at the University of New Hampshire have prepared miniature capsules which release antifouling compounds over several weeks, at a controlled rate. But Stephen Snyder of the ONR believes that natural compounds are still some way away from giving the five years鈥� protection that the Navy now gets from its antifouling coatings. 鈥淚t will be five to ten years before the copper ablative coatings can be replaced,鈥� he predicts.
Isolating promising compounds is just the first step. Before a compound can replace the toxic concoctions in use today, it will have to be shown to be effective for a wide range of fouling organisms. It will also have to be safe. 鈥淪ome of the natural products may turn out to be too toxic,鈥� says Cowling. TBG, for instance, could not be described as nontoxic, so registration could prove problematic. Approval could be slow to come anyway. In the US, any product that prevents biofouling is classed as a pesticide by the Environmental Protection Agency and the same goes for its counterparts in most other Western countries. For the US alone, registration is likely to take three years or more and cost over $1 million.
Once a suitable compound has been identified and approved, it will have to be engineered to make a coating that stays active for several years. Then it will be up to the chemists or microbiologists to find ways of producing it in commercial quantities. No one would seriously contemplate the alternative, which would be to dredge up tonnes of exotic sea creatures and extract the active agent from them.
Daunting as the quest for natural antifouling compounds is, it is gaining a range of formidable backers, including some of the pharmaceutical companies who have sought out antiviral, antimicrobial and anticancer drugs from the seas. And the sheer number of chemicals produced by organisms that live in the seas makes the natural solution a compelling option. The oceans cover around 70 per cent of our planet, and may harbour 10 million species or more. If only a fraction of them contain useful chemicals, then the 6000 natural substances isolated so far are just a start. Perhaps one day, ship fouling and the toxic paint now needed to prevent it will be an obsolete problem of a bygone age.