Biofouling - What is it and why should we care?
Do you ever wonder why the rocks near the water's edge are slimy or what the layer of barnacles on the bottom of ships are called? These are examples of biofouling. Biofouling can occur on any solid-water interface but can also be found at water-oil, water-air and solid-air interfaces. When we consider Ocean Food Sourcing we must not only consider where we get our marine products from but also the impacts of maintaining the marine industry. Biofouling causes many issues in the marine industry through environmental and economic effects and is one of the largest challenges facing marine industries.
There are four stage of biofouling:
The first stage is molecular fouling which takes around a minute to form on any solid-water surface including your dishcloths and toothbrushes. This conditioner biofilm is <100𝜇m and changes the physicochemical properties of the surface and affects the bacterial adhesion.
The second stage, microbial biofouling, takes up to 24 hours where bacteria and single celled algae adhere to the surface. These bacteria and diatoms then secrete a sticky extracellular polymer matrix which establishes the functional and structural integrity of the biofilm.
Stage three is the formation of a biofilm which takes around a week to form. This is where the microalgaes and protozoans (single celled animals) attach.
The final stage of the biofouling process is the formation of a macro-community in 2-3 weeks. When more macroalgaes adhere to the biofilm and invertebrates such as molluscs barnacles and sponges also attach, completing the process of biofouling.
The economic effects of biofouling in the marine environment is huge. The US Navy spends 1 billion USD/year on antifouling. In 2005 it was estimated to cost $56 million a year in increased fuel consumption in one US destroyer alone. Barnacles are responsible for 80-90% of all biofouling. Barnacles increase fuel consumption up to 40% and increase corrosion as the larvae fracture the paints on ships. It is also very expensive to take a boat out of the water to clean the biofouling film both in terms of time and money. The hull is also not entirely cleaned due to where the boat sits on its rack. This creates very niche environments for the biofouling organisms.
There are around 1200 known species of barnacles which can be seen in biofouling. Barnacles are exclusively found in marine ecosystems and tend to live in shallow and tidal waters. They are sessile (nonmobile) suspension feeders which have two swimming larval stages. Cyprid and nauplius. As larvae the barnacles swim around in the prevailing currents until they find a solid substrate on which to make their home. There are two things the barnacles look for before attaching themselves to the substrate. Firstly is it a good place to feed and secondly are there any other barnacles around. Once the barnacles decide it is a good place to live they start secreting “cement” which holds them firmly in place. Barnacle cement is a fascinating substance. It is durable and tough and resistant to chemical breakdown. We don’t know a huge amount about this cement but research is being done into its use in dentistry and in medicine to repair blood vessels and nerves.
Marine biofouling prevention is a worldwide industry worth $5 billion per year and growing (2014). In 2006 a Antifoulant Wishlist was created and desired characteristics for antifoulants were decided. However, since the formation of the 2006 Antifoulant Wishlist, very few methods were tested before being painted on ships. They worked in theory but had devastating consequences.
Antifoulant Wishlist (2006):
Antifoulants must be:
Anticorrosive
Antifouling
Environmentally acceptable
Economically viable
Long life
Resistant to abrasion/biodegradation/erosion
Smooth
Antifoulants must not be:
Toxic
Persistent in the environment
Expensive
Chemically unstable
A target for non-specific species
The need for antifoulants began when Aristotle noticed small “fish” (barnacles) able to slow down shops in the 4th century BC. Pre 1850 pitch, lead, copper, sulfur, arsenic mixed with oil, wax, tar or asphalt were used as antifoulants. In the 18th century lead sheathing was replaced with copper sheathing with iron or zinc nails (in the Royal Navy from 1761 and in the US Navy from 1781). However it was not until 1824 that Humphrey Davy demonstrated that copper dissolution in seawater prevented fouling. The issue with copper used as an antifoulant is that steel and iron hulls react with copper and so the use of paints containing toxinants began between 1850-1950. ‘Hot plastic paints’ made of metallic soap of copper sulphate were used from 1854 but they were expensive and difficult to apply. ‘Cold plastic paints’ from 1926, last up to 18 months. And in the 20th century we started using organometallic paints and DDT.
Dichlorodiphenyltrichloroethane (DDT) is a colorless, tasteless, and almost odorless crystalline chemical compound originally developed as an insecticide, and ultimately becoming infamous for its environmental impacts. Contact poison exerts its effect by disorganising the nervous system. This results in hyperactivity, paralysis and death. Although the use of DDT in Western countries has been banned since the 1970's, they are still detected in the marine environment due to its extreme stability (half-life of 15 years). It also has a very high tendency towards biomagnification. DDT is still used in agriculture and occasionally leaches into the marine environment.
Tributyltin (TBT) is an equally toxic antifoulant which has had devastating effects on oyster populations. As little as 20ng/l can affect larval growth and 1µg/l affects the formation of embryos. In the Arcachon Bay (West coast of France) the effects of TBT have been studied and shown to cost US$ 147 million a year to the oyster farmers of the region. All TBT coatings have been banned since 2008 however it is unknown what the lasting effects are.
There are many suggestions of biocide free antifoulants. Silicones work when the boat is travelling between 15-30 knots but the vessel must not be idle for long otherwise they will be susceptible to fouling. There is also concern of silicone contamination problems as many of the curing agents may contain TBT.
We could also take inspiration from the natural environment and ask ourselves how marine organisms remain free of biofouling. Shark placodes (scales) are amazing antifoulants and if we could design materials to emulate their structure we could have an entirely chemical free antifoulant. Shark placodes designs need to be 2 μm to inhibit algal settlement and 40 μm to inhibit cyprid settlement.
The use of microfibres is already on the market lasting 3-5 years. By incorporating fibres perpendicular to the surface when vessels are in motion it can prevent biofouling. However microfibres increase hull roughness and so increase drag and fuel consumption.
Biofouling and the use of antifoulants is a hugely important aspect of the marine industry. Without antifoulants the cost of fuel increases massively. However it is also essential that we consider what is in the antifoulants and what their effect is on the marine environment.
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