M.J. Cowling

The early stages of marine biofouling and its effect on two types of optical sensors

This paper records the results of an investigation into the effects of biofouling on optical marine sensors and the organisms responsible for the deterioration in sensor accuracy. Two kinds of commercial sub-surface optical marine sensor, commonly used to measure water quality, were operated in a natural marine environment and allowed to foul while measurements of the actual conditions were made daily using clean instruments. A number of glass and acrylic coupons were placed in the same environment and were removed at intervals throughout the trial. These coupons were examined and the biological population quantified. Deterioration in the optical properties of the coupons was measured using image analysis and UV-visible spectroscopy. The results from the coupons were compared with the results from the commercial instruments. It was observed that the major deterioration in instrumental accuracy occurred when a bacterial population exceeding 105/mm2 was found on the coupons. The algal population had little effect on the instruments over this time period. The acrylic coupons supported a lower fouling population, apparently due to the increased solubility of acrylic in seawater. The two optical techniques returned similar patterns of results for the surface area fouled, although the numerical values returned by each technique were different. Neither of these two techniques returned values directly comparable with the deterioration in accuracy of the commercial instruments. The trial took place on the Isle of Cumbrae in the Firth of Clyde on the west coast of Scotland, U.K.

An alternative approach to antifouling based on analogues of natural processes

A number of marine organisms are able to resist fouling pressure and remain essentially free of fouling. Some organisms are totally devoid of even the first stages of biofilm formation involving bacteria and microalgae. A key feature in recent research has been the realisation that previous low adherence technology is an insufficient technical solution and that natural models, based on marine and other organisms, incorporate other passive techniques for fouling resistance. These characteristics may be incorporated into physical analogues of the natural processes. This paper describes ways of producing physical analogues of some such characteristics, the application of such techniques to surfaces in the marine environment and the environmental impact. The paper includes some results of recent trials and a cost comparison.

The effect of benzalkonium chloride concentration on nine species of marine diatom

The effect of varying concentrations of benzalkonium chloride (BAK) on nine diatom species is measured, and related to the variation in tolerance levels of different species seen elsewhere in the literature. Different species showed different effective levels; however, all species were non-viable at 1×10−3% BAK. The technique being laboratory based and, therefore, immune from seasonal influences, is quick to perform and is easily adapted for bioassay work.

A novel technique to prevent bacterial fouling, using imposed surface potential

A. KERR, T. HODGKIESS, M.J. COWLING, C.M. BEVERIDGE, M.J. SMITH AND A.C.S. PARR. 1998. The effect of modest imposed surface potentials on the adhesion of marine bacteria to an electrically conducting layer deposited on silica glass is recorded. A positive shift increased bacterial settlement. However, a negative shift in potential was extremely beneficial in reducing numbers of adhered bacteria. An applied surface potential of − 66 mV SCE resulted in the bacterial population decreasing to approximately 12% of that on the uncharged reference sample. There was no further significant decrease in the adhered bacterial population when the magnitude of the negative potential was increased. The potential was maintained with very little current flow (less than 0·25 nA mm−2). The results were not due to any effect of the material used and therefore the technique could be useful for reducing bacterial fouling in many situations, including medical applications.

Method for the measurement of the diffusion coefficient of benzalkonium chloride

Biofilm formation on the optical ports of cameras and underwater sensors is the primary cause of their reduced useful deployment time. The use of a transparent hydrogel coating containing the cationic surfactant benzalkonium chloride has been shown to extend the deployment times for up to 12 weeks for these instruments. In order to predict the effective lifetime of these coatings it was necessary to obtain the diffusion coefficient of the benzalkonium chloride used in the coatings. Benzalkonium chloride can have different alkyl chain lengths ranging from C8H17 to C18H37 with chain length greatly affecting its chemical properties. The benzalkonium chloride materials investigated here were mixtures of C12H25 and C14H29 as well as C14H29 on its own. These materials were selected for their proven biofilm resistant qualities. The diaphragm diffusion cell technique was investigated for its applicability to the measurement of diffusion coefficients of molecules with surfactant properties and the ability to form micelles. The method was found to be satisfactory for the cationic surfactant benzalkonium chloride. The average value of the membrane cell integral diffusion coefficient D was 7.78 x 10(-6) cm2 s(-1) at 25 degrees C and there was no significant effect of alkyl chain length on the measured value of D.

The non-toxic effects of a novel antifouling material on oyster culture

The production traits (size, weight, mortality and condition) of Crassostrea gigas oysters were examined when cultured in cages coated with a novel antifouling material consisting of a polymer film based on a hydrogel which had been loaded with the active material (benzalkonium chloride—BCl), a mixture of alkyldimethylbenzylammonium chlorides. Production traits of adult oysters and growth rate of larvae obtained from BCl-exposed adults were equal to the controls. Toxicity of BCl, as assessed by the oyster embryo bioassay, was two orders of magnitude lower than TBT and one order of magnitude lower than copper, the active compounds presently used in antifouling paints.

Optimising optical port size on underwater marine instruments to maximise biofouling resistance

The limiting effects of biofouling have restrained the widespread deployment of optical marine sensors. This has necessitated the development of biofilm resistant coatings that are not detrimental to the quality of any measurement recorded. A comprehensive study has been carried out into the effect of changes in the diameter of optical ports. It was found that increasing the diameter of an optical port in the range 10-46 mm reduced the biofouling per unit area. Changes in the diameter did not affect the release of an antimicrobial agent from a hydrogel-based coating. The results show that the operational lifetime of optical ports is significantly improved for diameters over 30 mm and it is suggested that this should be considered a preferred design minimum for optical ports, regardless of the size of the underlying sensor.

The effects of salinity and temperature on the behaviour of polyacrylamide gels

Polymer gels are capable of undergoing large volume changes under the influence of solvent composition and temperature. Studies have been conducted on the effects of salinity and temperature on hydrolysed polyacrylamide gels. Three salinities were investigated from 5 parts per thousand (ppt) to 35 ppt, which is approximately the salinity of natural seawater in temperate waters. For each of the salinities, the effect of temperature from 5°C to 40°C with 5°C increments was investigated. It was found that hydrolysed polyacrylamide gels shrank in all the solutions, this effect being most pronounced at the high salinity (35 ppt), with a smaller volume decrease noted in 20 ppt and 5 ppt salinities, respectively. The effect of temperature was minimal, with all solutions promoting a decreasing volume change as the temperature increased. The polyacrylamide gels remained whole in the experiments with no visible signs of degradation. The cyclical volumetric strain behaviour of the gels was also investigated by alternate exposure to saline solutions and distilled water. Cyclical swelling and deswelling of the gels was observed which, in some cases, was fully reversible.

The biofouling resistant properties of six transparent polymers with and without pre-treatment by two antimicrobial solutions

Six bulk polymers potentially suitable for use as optical ports of underwater instruments were exposed to a solution of marine bacteria after soaking in distilled water or surfactant solutions. The effect of the surfactant solutions was to reduce fouling build-up on four of the six polymers. The presence of the surfactant altered the surface energy of the polymers. The surfactant reduced the importance of physical characteristics, such as surface roughness, on fouling build-up. It was found that untreated polyethylene terephthalate out-performed polymethyl methacrylate, over short time periods. This result was repeated when these polymers were tested on optical underwater instruments exposed to a marine environment.

Some physical factors affecting the accumulation of biofouling

The effects of surface roughness and microsolubility on fouling levels are examined using glass and acrylic samples. It is found that both of these, often overlooked, physical characteristics have a noticeable effect on the rate of fouling. The microsolubility of acrylic results in lower fouling than found on glass despite the higher hydrophobicity of acrylic and the resultant increase in initial attraction for fouling organisms. Fouling levels were found to increase with increasing surface roughness and therefore studies on the fouling susceptibility of different materials should report the roughness values of the samples examined.