The effects of model polysiloxane and fouling-release coatings on embryonic development of a sea urchin (Arbacia punctulata) and a fish (Oryzias latipes)

Abstract

In recent decades attention has focused on the development of non-toxic fouling-release coatings based on silicone polymers as an alternative to toxic antifouling coatings. As fouling-release coatings gain market share, they will contribute to environmental contamination by silicones. We report effects of eight model polysiloxane and three commercial foul-release coatings on embryonic development of sea urchins and fish, Japanese medaka. We used model coatings because they have known composition and commercially available components and molecules leaching from these coatings have been partially characterized. The commercial fouling-release coatings are purported to be non-toxic and components are proprietary. Our goal was to expose embryos of well studied model animals to the coatings to determine if the complex mixtures leaching from the coatings impact development. Urchins were chosen because development is rapid and embryos can enter the non-slip layer over surfaces. Medaka was chosen because the female deposits the sticky eggs onto the anal fin and then scrapes them off onto surfaces. Embryos were confined in water over coatings in 24 well plates. Fresh model coatings had no effect on urchin development while commercial fouling-release coatings inhibited development. Fish embryos had delayed hatching, increased mortality of hatchlings and dramatically decreased ability of hatchlings to inflate the swim bladder and reduced hatching success on all coatings. After one-month immersion of coatings in running seawater to simulate initial application in the marine environment, sea urchin embryos died when placed over model silicones. Effects of the commercial coatings were reduced but included retarded development. Effects on fish embryos over leached coating were reduced compared to those of fresh coating and included decreased hatching success, decreased hatchling survival and inability to inflate the swim bladder for commercial coatings. These findings suggest, similar to medical conclusions, compounds leaching from silicone coatings can impact development and the topic deserves study.

Introduction

Negative environmental impacts of biocides such as tributyltin and copper have led to regulations and bans on their use in marine biofouling control coatings (Rittschof, 2001, van Wezel and van Wlaardingen, 2004, Yebra et al., 2004). A result is research and development efforts focused on non-toxic measures such as fouling-release coatings. These coatings present a surface which reduces the adhesion strength of settling organisms (Berglin et al., 2001, Clare, 1998) enabling cleaning by pressurized water jets or in some cases by hydrodynamic forces generated as the ship moves (Berglin et al., 2003). Surface properties of the coatings including surface free energy, elastic modulus and thickness are important to the fouling-release performance (Brady and Singer, 2000, Webster and Chisholm, 2010).

The concept of fouling-release coatings has been in development for over three decades (Rittschof, 2009). A great deal of research has been conducted on fouling-release coating systems based upon silicones and fluoropolymers. Silicone polymers, based on poly-dimethyl-siloxane (PDMS), appear to out-perform fluoropolymers and be a viable fouling-release coating system (Rittschof, 2009, Stein et al., 2003). Incorporation of low molecular weight silicone oils into silicone coatings improves their fouling-release properties (Meyer et al., 2006, Milne, 1977, Rittschof et al., 2008, Stein et al., 2003, Truby et al., 2000). The composition of commercial coatings is proprietary.

Fouling-release coatings, particularly those based on silicones, have been developed and sporadically available on the market since 1990s (Finnie and Williams, 2010, Townsin and Anderson, 2009). The sales of these coatings rose significantly when manufacturers voluntarily withdrew TBT coatings from the market in 2003 and the International Maritime Organization (IMO) convention banning TBT antifouling paints was adopted in 2008 (Townsin and Anderson, 2009). Now commercial fouling-release coatings are used in the protection of many man-made structures. Since the smooth surface properties of fouling-release coatings help reduce hydrodynamic drag and improve fuel savings, their sales have increased (Dafforn et al., 2011, Finnie and Williams, 2010, Townsin and Anderson, 2009).

Fouling-release coatings are presumed environmentally friendly and are not subject to government regulations that apply to coatings containing biocides. However, their biological effects are poorly understood. Watermann et al. (1997) found none of the silicone coatings they tested caused mortality of barnacle Balanus amphitrite cyprids even though eluates contained organotin catalysts in silicone polymerization and organotin is highly toxic to marine organisms. The leachate of two silicone fouling-release coatings obtained from GE Silicones, i.e., RTV11^® and RTV11^® amended with polydimethyldiphenylsilicone (PDMSPS) oil, produced lethal response in mysid shrimp Mysidopsis bahia, but not in silverside fish Menidia beryllina (Truby et al., 2000). Rittschof and Holm (1997) discovered that fouling-release coatings leached compounds that killed barnacle nauplii and at least one of the silicone oil detergents described in a patent on fouling-release coatings was very toxic to barnacle larvae. Silicone oils, which are often incorporated in fouling-release coatings, trap and suffocate marine organisms (Nendza, 2007). Finally, components of silicones at the surface of silicones interact with enzymes involved in curing of biological glues (Rittschof et al., 2011).

The increasing introduction of fouling-release coatings, particularly silicone coatings, into the marine environment makes hazard assessments necessary, especially with the lack of proven non-toxic commercial alternatives and the fact that there is currently no regulation. The aim of this study was to evaluate the effects of a series of model silicone coatings of known composition and commercial fouling-release coatings on embryo development of sea urchins and fish. Surface associated compounds from the model silicones have been partially characterized and the mixtures and individual components are known to alter the activity of enzymes (Rittschof et al., 2011). Nothing is known about compounds associated with the commercial fouling-release coatings. We confined the embryos in the presence of the non-toxic surfaces to provide a worst case exposure scenario. Our goal was to expose embryos of common species to the coatings to determine if the complex mixtures leaching from the coatings impact development. Embryos were used as test organisms since embryonic development is vital to the rest of the life-cycle and is a sensitive stage. Urchin development was chosen because it is rapid and extremely well studied (Angerer and Angerer, 2003, Giudice, 1986, Kobayashi and Okamura, 2002). Urchin embryos are small and fit in the nonslip layer of a ship at the dock or of other surfaces. Medaka development has been studied for decades (González-Doncel et al., 2003, Iwamatsu, 2004, Shi and Faustman, 1989). Medaka adults lay transparent eggs daily and rub them off their anal fin onto substrates. Thus the fish eggs could be in contact with the surface during development. Since the egg membranes are clear and the embryo muscles are transparent, one can watch development of all major organs. The larvae do not develop a calcified backbone until three days after hatching enabling easy observation of swim bladder inflation, behavior and deformities.