Barnacles — a major problem for shipping The first phase of Marine Paint focused on the barnacles that are one of the major problems for shipping. The lifecycle of the barnacle consists of six sequential larval stages, in the last of which it develops into a cyprid larva. During this non-feeding larval stage, its overriding objective is to find a surface to adhere to. Inside the larva are two cement glands in which the adhesive is stored. When the larva has found a surface, it stands on its antennae and secretes adhesive from the glands via cement ducts in the antennae, and adheres to the surface. It then metamorphosizes from a free-swimming larva into an immobile adult animal that starts building the wellknown, volcano-like, hard structure, strongly attached to the surface. The barnacle´s adhesive consists of a unique set of proteins. These proteins are liquid while they are inside the barnacle, and then harden to form a very strong and durable adhesive. 1. Efficacy of medetomidine
Working with medetomidine concentrations as low as 0.025 % wt. in the paint prevent barnacle fouling. Moreover, field tests have also verified that the efficacy remains for multi-season tests. Thus far, panels have been tested for 24 months with remaining efficacy. Field tests have also demonstrated that medetomidine is effective against three species of barnacles and one type of tube-forming polychaete.
2. Mode of action
It was found that the barnacle responded to the presence of medetomidine by increased leg movement leading to difficulties in adhering to the surface. All cells in an organism are connected by different communication systems, either neuronal or by circulating hormones. The signal molecules, neurotransmitters or hormones, act through binding to a specific protein on the cell surface, the receptor. The receptor then transports the signal from the outside of the cell to the inside, inducing a biological response. One class of receptors on the cell surface is the G-protein coupled receptors (GPCRs) and we have found it to be a so-called octopamine receptor that binds the antifouling substance medetomidine.
The first hypothesis regarding mode of action was that medetomidine inhibited cement secretion. The reasoning behind this was that if no cement (adhesive proteins) was secreted, it would be impossible for the barnacle cyprid larva to settle. This hypothesis was disproved and instead, an increase in motility was seen while adding medetomidine. A barnacle cyprid larva can swim as fast as 1 mm/s or twice its length each second, which is better than corresponding swimming records for humans. With such a swimming speed, the exploratory surface behaviour necessary for settlement of the barnacle cyprid larva is effectively blocked. Thereby a deterrent behaviour is promoted and the attachment and permanent settlement is inhibited.
3. The genome of the barnacle larva
From the barnacle biology part of the programme a cDNA library of the barnacle cyprid larva is now available and the sequencing and identification of proteins significant in barnacle biology is running. The emerging knowledge of the genome can be used for the development of fast screening methods in the search for future antifouling substances.
4. The formulation of a paint
A marine paint formulation contains many different types of chemicals,
the most important of which are solvent, binder, filler, pigment and stabilizer. The selection of each chemical and their respective ratio in the coating is based on economy and performance. Most often, paint development concentrates on optimizing economy and mechanical properties, rather than on biocides or a specific slow release system. The state-of-the-art in this respect is the self-polishing coatings, which in contact with water, break down by hydrolysis to form an erosive outer layer in the coating. The biocidal leakage occurs only in the erosive zone and this per se, makes the coating a slow release system. Medetomidine has an antifouling effect even without a slow release system specialized for medetomidine. However, in order to optimize the amounts needed in the coating we have refined the release mechanism by using a small amount of transition metal oxide particles that effectively binds medetomidine to prevent leakage through the part of the coating not in the erosive zone.
The reason for using transition metal oxide particles is that imidazole groups (such as in medetomidine) bind very strongly to transitional metal oxides in non-polar solvents, such as xylene. By adding, for example, a small amount of ZnO particles with a high surface area it is possible to retain medetomidine in dry coating. When the seawater hydrolyzes the outer layer in the coating, medetomidine desorbs from the particles and is free to act as an antifouling agent. Tests have validated the hypothesis showing both better efficacy and leakage rates when transition metal oxide nanoparticles were used together with medetomidine.
5. Ecotoxicology
The ecotoxicology and hazard assessment of medetomidine is based on tests of a large number of marine non-target organisms and have identified several important responses e.g. metabolic down-regulation, interference with detoxification processes, pigmentation disturbances and inactivation of burrowing activity in sediment fauna. However these effects occur in concentrations much higher than can be expected in worst-case marine scenarios. The emission of medetomidine used in the evaluations is in the ng/cm2/day range which is far lower than demonstrated for other biocides. Medetomidine has been evaluated as a promising anti-fouling candidate and with a recommendation to I-Tech to proceed with the registration of medetomidine according to the BPD.
