Biofouling is a significant problem for the shipping industry as it leads to many functional and financial setbacks. During idle periods, a ship can accumulate biofouling. Within minutes of immersion, a surface, such as a ship hull, accumulates a conditioning film, followed by bacteria and other unicellular organisms (i.e., diatoms). This accumulation is known as slime or microfouling [1
]. Within hours to days, macrofouling communities such as barnacles, sponges, and tunicates may begin to grow, creating a complex biofouling community. This accumulation causes changes in surface roughness that lead to increased drag, lower fuel efficiency, and increased greenhouse gas emissions [2
]. A ship with only biofilm (microfouling) and no macrofouling can cause up to $
1.2 million in fuel consumption [2
]. Significant growth on a ship hull may also require physical removal (e.g., brushing, power washing, scrapping) of the biofouling organisms [2
]. Biofouling can also damage oceanographic equipment and is a known vector for the transfer of invasive species [3
Currently, the most common approach to biofouling prevention on ship hulls is the application of marine coatings. These fall into two main categories: antifouling (AF) and fouling release (FR) [6
]. Antifouling coatings are biocide-based, such as copper, which deters growth, making it hard for other organisms to attach and develop [1
]. The drawback is these coatings emit copper and other chemicals into the water, causing environmental issues [8
] and potentially resulting in nutrient loading [7
]. Fouling release coatings have been developed as an alternative to biocide coatings. These coatings are often silicone-based and work by reducing the adhesion strength of organisms, allowing for easier removal either via cleaning or hydrodynamic forces imparted on the ship hull as it moves through water [1
]. Other approaches include mechanical methods which either prevent the buildup of biofouling or remove a fully established biofouling community. Grooming, which uses remotely operated vehicles (ROVs) equipped with brushes to frequently and gently wipe the ship hull, can remove fouling before it has time to fully establish [10
]. Grooming at a frequency of once a week has been effective in removing micro- and macrofouling on both AF and FR coatings [10
]. Cleaning is another alternative, which is a reactive approach to fouling control, as it aims to remove growth via power washing either in or out of water. Currently, there are concerns that grooming and cleaning may release coating biocidal materials into the water; thus, there are methods in place to capture the effluent [11
]. While there are many types of antifouling methods and strategies available, there is still a need for solutions which are environmentally friendly [12
Ultraviolet-C (UVC) light is commonly used for the prevention of bacteria in the medical field. For medicinal purposes, it has proven to be 99% effective in eliminating bacterial biofilms growing on catheters. UVC light (254 nm) damages bacterial cells by attacking their DNA [15
]. Recently, it has been applied in the marine field to prevent biofouling formation on multiple surfaces due to its increased affordability, and it is also considered an environmentally friendly method of biofouling removal or prevention [16
]. UV radiation can disrupt the detection and settlement of coral larvae along with decreasing the biofilm formation on various surfaces [16
]. Barnacles have also been shown to undergo periods of blindness when exposed to UVB light, preventing them from detecting surfaces and settling [21
]. Salters and Piola (2017) [17
] used embedded UVC LEDs as a way to prevent biofouling and identified no biofouling within a small vicinity of the light source.
Recent experiments have investigated the use of UVC as an externally applied source for biofouling prevention on ship hulls. Hunsucker et al. (2019) [22
] and Braga et al. (2020) [23
] determined that biofouling was prevented or significantly decreased in abundance when various frequencies of UVC were applied. Both experiments proved UVC can work in synergy with the fouling control coatings (biocidal and fouling release) mentioned above. On the basis of these prior experiments, a three-part study was designed to address how UVC can be used in synergy with different surfaces while considering the importance of exposure frequency. UVC has yet to be applied to surfaces with varying reflective properties or colors that are unattractive to biofouling organisms. The use of inert surfaces removes the use of antifouling or fouling release coatings, in turn removing the adverse effects that these coatings could potentially have on the environment. Additionally, the use of color is known to influence biofouling settlement and recruitment. Swain et al. (2006) [24
] found that white surfaces contained less biofouling than black surfaces. Red-colored surfaces are a highly preferred color for settlement of barnacles including Balanus amphitrite
, tubeworms, and some coral species [25
]. Colors reflect light at different frequencies, and it is thought that color would influence settlement during periods when there is no UVC exposure.
The aim of this study was to investigate the interaction of UVC on surfaces with different reflective properties and color, as well as its effect on biofouling settlement. Additionally, only daily exposure intervals (1 min/day and 1 min/6 h) were tested in previous studies. Thus, study examines various exposure times and intervals (e.g., 5 min/week and 10 min twice a week) in order to prevent biofouling formation.
2. Materials and Methods
A three-part study was designed to address how UVC interacts with surfaces. Specifically, Section 2.1
looked at how surface color and UVC exposure work in synergy to prevent biofouling settlement, while Section 2.2
compared the interaction of UVC light with surfaces of different reflective properties (stainless steel and polycarbonate). In Section 2.1
and Section 2.2
, low doses of UVC known to have a limited effect on fouling [22
], were applied to panels to allow for a comparison of fouling intensity and composition. Section 2.3
investigated the effectiveness of different time intervals to prevent biofouling formation on the surfaces used in Section 2.1
and Section 2.2
2.1. UVC and Color
To test the synergistic effect of UVC with color, red and white surfaces were selected according to prior research on biofouling settlement [25
]. Both colors were printed on weatherproof computer paper. Each colored paper was sandwiched between 10 cm × 20 cm × 0.16 cm polycarbonate plates to create a uniform surface (herein referred to as a panel). The polycarbonate panels were 1.6 mm thick to allow for the color paper to be clearly visible. General Electric (GE) all-purpose silicone was used to seal the edges of panels to prevent water ingression which would alter the color and paper. Trilux-33 (Interlux), an antifouling spray paint, was used as a border on the panels to prevent the edge effect of unwanted biofouling.
Eight polycarbonate panels were housed on two sides of a 3D printed box (referred to as Box 1), which also allowed water and biofouling larvae to flow through [22
] (Figure 1
). A 25 W Aqua UVC (254 nm) lamp was placed in the center, at a distance of 25 mm from the panels [22
]. In fresh water, the light intensity of the UVC lamp 25 mm from a surface was measured at 1.31 ± 0.88 µW/
. Four replicates of red polycarbonate panels and four white polycarbonate panels were placed in Box 1 (Table 1
). A PRIME-digital lighting timer was used to set to a time frequency of 1 min per day within the box [22
]. Control panels were constructed in the same fashion and were hung on a PVC frame with no UVC exposure.
2.2. UVC and Reflectance
An experiment was designed to determine how fouling interacted with surfaces that reflect UVC or absorb it. Substrates of stainless steel and polycarbonate were used because they are both inert and have known specular reflectance at 254 nm. Stainless steel reflects 55% [30
] and polycarbonate reflects 0% [31
] of UVC light at near-normal angles of incidence. The reflectivity of the two substrates was measured in the lab using the same lamp as seen in Figure 1
and a Solar Light (model no. PMA2122-WP) UVC sensor. Laboratory testing confirmed that stainless steel panels reflected significantly more UVC light than the polycarbonate panels.
In order to test the interaction of UVC with surfaces of different reflectance, three different treatments were established (Table 1
): all polycarbonate panels (Box 1), all stainless steel panels (Box 2), and a combination of polycarbonate panels and stainless steel panels (Box 3). Note that the box setup used to investigate the synergistic effect of UVC and color was also Box 1. Eight replicates of 316 stainless steel, polished to an ASTM 480 # 8 mirror finish [32
], were cut into 10 cm × 20 cm panels and placed in Box 2. Box 3 contained four stainless steel panels on one side of the box and four red polycarbonate panels on the opposite side of the box, to test for the indirect effect of the reflective stainless steel on polycarbonate. Each box was set to a time frequency of 1 min per day using a PRIME-digital lighting timer based on prior experiments [22
]. A set of control panels was constructed for each surface material, immersed during the same time period, and not exposed to UVC.
2.3. UVC and Exposure Intervals
According to the above results from Section 2.1
, two separate time–frequency experiments were conducted to investigate the duration in which UVC would be the most effective on the inert surfaces. Color was not considered as a factor following the results from Section 2.1
. The previous two experiments, Section 2.1
and Section 2.2
, tested UVC at daily intervals (Table 1
), which was able to provide reduced macrofouling settlement. In order to determine if a weekly dose could also be effective, three weekly frequencies were tested for 1 month immersion (Table 1
): 1 min a week, 5 min a week, and 10 min a week. Following these results, a second experiment was conducted using 10 min UVC intervals at a more frequent rate: 10 min twice a week (every Monday and Friday), 10 min three times a week (every Monday, Wednesday, and Friday), and 10 min every day.
The same box setup described above and shown in Figure 1
was used during these experiments, housing four stainless steel panels and four polycarbonate panels. Each box contained a 25 W Aqua UVC (254 nm) lamp placed in the center at a distance of 25 mm from the panels on each side of the box and connected to a PRIME-digital lighting timer to control the frequency. A set of control panels was constructed for each surface material, immersed, and not exposed to UVC.
Testing was conducted at the Center for Corrosion and Biofouling Control’s static immersion facility located at Port Canaveral, Florida (28°24′31.01″ N, 80°37′39.54″ W). The average salinity at this location is 34 ± 2 ppt, and the average water temperature is 27 ± 2 °C. Biofouling is high year-round, with seasonality observed with different fouling organisms. For example, in the warmer months (June, July, August), encrusting bryozoans, calcareous tubeworms, and barnacles dominate, while, in the cooler months (December, January, February), arborescent bryozoans and biofilms dominate. All panels were immersed 0.5 m below the surface water at Port Canaveral for a 1 month period [33
Fouling coverage was visually assessed monthly. Only organisms that were directly attached to the surface were recorded [33
]. For experiments that continued past 1 month of immersion, all panels were cleaned back and wiped down with vinegar, which is a mild acid that helps neutralize the surface and remove settlement cues. After cleaning, the timer was then switched to the next set of predetermined time frequencies.
2.5. Statistical Analysis
A permutational multivariate analysis of variance (PERMANOVA) was performed on data collected for each month, to compare the total biofouling communities based on UVC treatment. A PERMANOVA analysis was run on both UVC-exposed and nonexposed panels to compare color (red and white), surface material (polycarbonate and stainless steel), the indirect effect of UVC, and the exposure intervals. A nonmetric multidimensional scaling (MDS) plot was also conducted, when PERMANOVA results proved to be significant, to display differences in fouling communities based on surface material. A SIMPER analysis was used to indicate differences among fouling communities for the MDS plots. In addition, a multivariate analysis of variance (ANOVA) followed by a Tukey test was used to compare individual biofouling groups (i.e., barnacles, tubeworms) for different surface materials and between exposed samples and controls. All analyses were done using R statistical software (2019).
UVC was applied to surfaces of different colors and two different materials, polycarbonate and stainless steel, to study the synergistic effect on biofouling prevention. This research demonstrated that UVC may be applied intermittently to various surfaces submerged in seawater as a means to control or to prevent fouling. It showed that the effectiveness is moderated by the type of surface that is being treated, and by the frequency and duration of the treatment. No significant differences in total biofouling recruitment were observed for the UVC-treated colors evaluated by these experiments. However, it was found that, in certain cases, calcareous tubeworm recruitment to the red surfaces was greater than that to the white surfaces. These results were similar to the results of Satheesh and Wesley (2010) [27
], who looked at substrate color effects on the settlement of tubeworms, and Swain et al. (2006) [24
], who looked at the effects of black and white surfaces on in situ fouling communities. Both studies found that fouling organisms settle on darker shades of color. Swain et al. (2006) [24
] had increased settlement on black surfaces and Satheesh and Wesley (2010) [27
] had greater settlement on red surfaces. Color preference is a contributing factor in biofouling recruitment and settlement for specific organisms, but it appears to be hindered with UVC exposure due to no statistical difference with respect to fouling. The UVC treatment significantly reduced the tubeworm abundance on treated panels versus the controls. As another possibility, UVC exposure to the surrounding water column could have potentially limited the settlement of tubeworms compared to the controls. However, it is unclear exactly what factor resulted in similar fouling communities.
Comparisons were made of direct UVC exposure on polycarbonate and stainless steel panels to those subjected to indirect influence. A lower abundance of biofouling was seen on both polycarbonate and stainless steel panels that were exposed indirectly. The low abundance of fouling may be due to the coupling of both surfaces reflecting and absorbing the UVC light. For example, polycarbonate reflects 0% of UVC [31
], which indicates that it is absorbing the light. The absorption of light could potentially be heating the surfaces, preventing fouling from settling. The same statement could be made about stainless steel because it only reflects 50% of light [30
]. The differences may also be the result of spatial variation within the test site, which could be teased out with additional replication and testing. Overall, throughout the different experiments, stainless steel had more macrofouling than the polycarbonate, which was seen specifically for tubeworms, tunicates, and encrusting bryozoan. Kim and Kang (2020), [34
] saw similar results for stainless steel and polyvinyl chloride (PVC) when exposing UVC to foodborne pathogens. Bacterial colonies were less abundant on PVC (another commonly used inert surface) compared to stainless steel, indicating that reflectance may not play a role in biofouling prevention as much as the actual surface material.
While differences in color or surface material may play a small role, the major factor influencing settlement on stainless steel and polycarbonate was the duration and frequency of UVC exposure. Panels exposed for intervals of two (MF) and three times (MWF) a week resulted in biofouling communities that were similar. However, a significant decrease in fouling was seen with daily exposures of UVC. This indicated that, with a high enough dose of UVC, most if not all macrofouling can be prevented on both substrates. Hunsucker et al. (2019) [22
] and Braga et al. (2020) [23
] also found that longer exposure times to antifouling and fouling release coatings were effective at preventing both biofilms and macrofouling. While more work is needed to understand the impacts of UVC exposure at the different life stages of biofouling organisms, previous studies have also found veliger [35
] and nematode larvae [36
] to have higher rates of mortality after long exposures to UV irradiance. Both organisms were exposed to continuous UV light for longer than 24 h; however, high mortality was observed within the first 24 h. These studies correlate with what was observed when UVC was exposed to panels for 10 min a day herein. The panels exposed to UVC for 10 min a day had minimal fouling, indicating that a slight increase in time may be equivalent to continuous exposure and may display complete biofouling prevention or removal. For the current study, testing was conducted in warmer months of Florida which are typically the more aggressive fouling season, thus requiring a longer UVC exposure. During lower fouling months or environments, a lower time and frequency of UVC exposure would be required to prevent fouling [22
]. Studies comparing fouling response in different seasons will need to be further investigated since the current study was conducted in the summer months where animal larvae are the dominate fouling organisms compared to the winter months, which are more algal-dominated. Along with exposure time and intensity, distance can play a role in UVC effectiveness. Braga et al. (2020) [23
] applied UVC light to pre-existing adult barnacles attached to an inert surface and saw greater mortality on panels closer to the lamp (25 mm) than farther away (275 mm).
On both polycarbonate and stainless steel, biofouling was minimal when UVC was applied for long and frequent intervals, which was also observed by Hunsucker et al. (2019) [22
] and Braga et al. (2020) [23
]. According to the results herein, UVC can be considered a more effective environmentally friendly use for biofouling prevention compared to fouling control coatings. Applying UVC to inert coatings or substrates could reduce the use of antifouling coatings such as copper, which can leach, and other chemicals from antifouling coatings into water bodies. UVC exposure to copper-based coatings has demonstrated that UVC may accelerate coating release [22
]. Further testing is still needed to understand how long-term UVC impacts both fouling control coatings and inert surfaces. Furthermore, material selection should be considered due to the possibility of surface degradation by the UVC exposure [37