Colonisation of Antifouling Paints by Marine Bacteria

Antifouling paints, containing either tri phenyl tin fluoride or cuprous oxide as the toxin, and their non-toxic, empty-matrix analogues were used in this study. Panels were exposed in tanks of running seawater and their bacterial load estimated from viable counts of surface swabbings. The viable counts indicated broadly similar colonisation patterns for the organotin and the two empty-matrix paints. Initial colonisation was rapid: bacteria were detected within four hours of immersion; reaching 2 -2 cnT after 8 hours and 2 cnv after 4 days. Numbers then oscillated between 2 and 2 cnv until the end of the experiment on day 23. In contrast, bacteria were not detected on the copper paint until day 3, although by day 10, and for the remainder of the experiment, this paint supported the highest number of bacteria. Interestingly, the initial rate of colonisation was essentially the same as for the other three paints (2 cm~^ by day 4; 2 cm" by day 8). In addition, samples of exposed panels were taken for direct observation of the developing microfouling communities in the scanning electron microscope. This has shown that the two empty matrix paints acquire a relatively thick fouling layer composed of diverse microorganisms, mostly bacteria and diatoms, bound up with silt. Fouling of the organotin paint was less extensive: bacteria were predominant, forming discrete colonies with the cells often anchored by thin adhesive threads; some patches of silt were also present. In contrast, the copper paint became colonised by an almost continuous layer of bacteria, embedded in a sheet-like matrix of adhesive polymer. The results are discussed with reference to the toxicity of conventional antifouling compounds to bacteria and the probable effect of bacterial fouling on the leaching of toxins from the paints. Introduction non-living material is followed by the attachment and T , . , . f . growth of bacteria, diatoms and other microorganisms In the manne environmenthe growth of organisms on ^ ^,)'̂ ^ durj this Succession) man-made^s ructures constitutes the problem of manne $ md ̂ ^ ^ ̂ £ ^ fouling and leads to increased operating costs, especially f Λ' f ,. -x TM · · * r i_. ·' / V "" · " «~ **·ι j o to a mature fouling community. There is evidence to for shipping (see Townsin, Byrne, Milne and Svensen ' . ., A ,.'. f . J . . 1 1 /ΪΛΟΛ\Τ · r ur ι\ τ * A suggest that this fouling sequence is a true ecological (1980) for a »mew of smp-fouhng economics). To retard succession Zo ell (1938 1939) K^J^ (1951) the rate of fouling, a range of special paints has been and ^ ̂ ̂ ^ bearm „ J developed These release compounds into the layer of ^ ̂ ̂ ^ ̂ ietuement rate of marine seawater adjacent to the hull in sufficient concentration to ̂ ̂ ^ ^ found ̂ kill the settlement stages of fouling organisms. Practical rr " . ;. , , . , . , , X, " . , . ,. , f . ,. , , effect with diatom and other microalgal films, considerations limit the amount of paint which can be applied at any one time and consequently the amount of On toxic substrates, such as antifouling paints, the later toxin available for release is limited. Once the toxin . stages of fouling are delayed until the surface has lost release rate (leaching rate) falls below the level at which some or all of its toxicity. In 1935, ZoBell and Allen fouling is prevented the vessel must be dry-docked and stated that bacterial fouling films probably form a repainted. The current generation of antifouling paints protective barrier on toxic surfaces. Both Floodgate are effective for only 1 to 3 years, depending upon the (1971) and Corpe (1973) state that bacterial adhesive type of paint used and the operating conditions of the polysaccharides may chelate heavy metal antifouling vessel " compounds. Whilst Rorem (1955), Friedman and Dugan (1968) and Lasik and Gordiyenko (1977) have shown There is a well=documented succession in the developthat polysaccharides similar to those produced by marine ment of a marine fouling community. The adsorption of fouling bacteria can chelate metal ions. 0006-8055/81/0024-0185$02.00 " © by Walter de Gruyter & Co. · Berlin · New York 186 Dempsey: Colonisation of antifouling paints by marine bacteria Although the importance of bacterial fouling has been recognised for many years, the majority of studies have relied upon the direct observation of glass slides to monitor its development. For opaque antifouling paints this approach is not possible, also those bacteria susceptible to the antifouling toxin may not be killed until after they have settled. In an attempt to overcome these problems, viable counts of detached bacteria together with direct observation of paints in the scanning electron microscope (S.E.M.) have been used in this study. Materials and Methods Samples of commercial antifouling paint based on the two most common antifouling agents, copper and organotin, were used. The cuprous oxide paint had a vinyl matrix whilst the tri phenyl tin fluoride (TPTF) paint had a chlorinated rubber matrix. In addition, non-toxic paints based on each matrix-type were used as controls. These contained sodium chloride in place of the anti-