Assessing the antifouling properties of cold-spray metal embedment using loading density gradients of metal particles.

Particles of copper, bronze and zinc were embedded into a polymer using cold-spray technology to produce loading density gradients of metal particles. The gradients were used to identify the species with the highest tolerance to the release of copper and zinc ions. The gradients also established the minimum effective release rates (MERRs) of copper and zinc ions needed to prevent the recruitment of fouling under field conditions. Watersipora sp. and Simplaria pseudomilitaris had the highest tolerances to the release of metal ions. Copper and bronze gradient tubes were similar in their MERRs of copper ions against Watersipora sp. (0.058 g m(-2) h(-1) and 0.054 g m(-2) h(-1), respectively) and against S. pseudomilitaris (0.030 g m(-2) h(-1) and 0.025 g m(-2) h(-1), respectively). Zinc was not an effective antifoulant, with failure within two weeks. In conclusion, cold-spray gradients were effective in determining MERRs and these outcomes provide the basis for the development of cold-spray surfaces with pre-determined life-spans using controlled MERRs.

Biocrude yield and productivity from the hydrothermal liquefaction of marine and freshwater green macroalgae.

Six species of marine and freshwater green macroalgae were cultivated in outdoor tanks and subsequently converted to biocrude through hydrothermal liquefaction (HTL) in a batch reactor. The influence of the biochemical composition of biomass on biocrude yield and composition was assessed. The freshwater macroalgae Oedogonium afforded the highest biocrude yield of all six species at 26.2%, dry weight (dw). Derbesia (19.7%dw) produced the highest biocrude yield for the marine species followed by Ulva (18.7%dw). In contrast to significantly different yields across species, the biocrudes elemental profiles were remarkably similar with higher heating values of 33-34MJkg(-1). Biocrude productivity was highest for marine Derbesia (2.4gm(-2)d(-1)) and Ulva (2.1gm(-2)d(-1)), and for freshwater Oedogonium (1.3gm(-2)d(-1)). These species were therefore identified as suitable feedstocks for scale-up and further HTL studies based on biocrude productivity, as a function of biomass productivity and the yield of biomass conversion to biocrude.

Using textured PDMS to prevent settlement and enhance release of marine fouling organisms.

The antifouling efficacy of a series of 18 textured (0.2-1000 µm) and non-textured (0 µm) polydimethylsiloxane surfaces with the profiles of round- and square-wave linear grating was tested by recording the settlement of fouling organisms in the laboratory and in the field by monitoring the recruitment of a multi-species fouling community. In laboratory assays, the diatoms Nitzschia closterium and Amphora sp. were deterred by all surface topographies regardless of texture type. Settlement of propagules of Ulva sp. was lower on texture sizes less than the propagule size, and settlement of larvae of Saccostrea glomerata and Bugula neritina was lower on texture sizes closest to, but less than, the sizes of larvae. After a six month field trial, all textured surfaces lost their deterrent effect; however, the foul-release capabilities of textures were still present. High initial attachment was correlated with most fouling remaining after removal trials, indicating that fouling organisms recruited in higher numbers to surfaces upon which they attached most strongly.

Combining a photocatalyst with microtopography to develop effective antifouling materials.

Polydimethylsiloxane surfaces textured with a square-wave linear grating profile (0, 20, 200, 300 and 600 µm), and embedded with a range of photocatalytic titanium dioxide (TiO2) nanoparticle loadings (3.75, 7.5, 11.25 and 15 wt.%), were used to test the combined efficacy of these technologies as antifouling materials. Settlement of the fouling bryozoan species Bugula neritina was quantified in the laboratory under two intensities of UV light. The lowest settlement rates were observed on 20 µm surfaces. However, texture effects were not as critical to larval settlement as the presence of TiO2. In conjunction with UV light, TiO2 completely inhibited larval metamorphosis even at the lowest loading (3.75 wt.%) and the lowest intensity of UV light (24 W m(-2)). Recruitment of B. neritina was also quantified in field trials and showed similar results to laboratory assays. The lowest recruitment was observed on 20 and 200 µm surfaces, with recruitment being significantly lower on all surfaces containing TiO2. Therefore for B. neritina, although all TiO2 loadings were effective, 3.75 wt.% can be used as a minimum inhibitory concentration to deter larval settlement and the addition of a 20 µm texture further increases the deterrent effect.

Polyurethane seismic streamer skins: an application of cold spray metal embedment.

Cold spray metal embedment is an innovative antifouling (AF) technology that delivers metal particles with AF properties into many thermoplastic polymers. AF efficacy was quantified for low (22.1 ± 4.8 g m(-2)) and high (101.1 ± 10.8 g m(-2)) densities of copper particles embedded into polyurethane (PU) seismic streamer skins, which are used in geophysical exploration. Failure of each Cu-embedded treatment was defined as settlement of hard foulers. Low-density streamers failed after 42 days while high-density streamers failed after 210 days. Most importantly, the high-density streamers were completely free of hard foulers including the barnacle Amphibalanus reticulatus during this time period. In conclusion, cold-spray metal embedment is an effective AF technology for PU seismic streamer skins, under intense fouling conditions. Higher copper particle densities enhance AF longevity and the effect of density provides a tool to extend efficacy and enhance AF performance for specific polymers.

Enhancing the efficacy of fouling-release coatings against fouling by Mytilus galloprovincialis using nanofillers.

Fouling-release (FR) coatings minimise the adhesion strength of fouling organisms. This study describes improved technologies to control the settlement and adhesion of the important fouling organism Mytilus galloprovincialis by incorporating the nanofillers titanium dioxide (TiO(2)) and carbon nanotubes (CNTs) in polydimethylsiloxane (PDMS) matrices. The incorporation of TiO(2) prevented larval settlement when photoactivated with UV light, even at the lowest concentration of the nanofiller (3.75 wt%). Notably, there was 100% mortality of pediveligers exposed to photoactivated TiO(2). However, plantigrades initially settled to photoactivated TiO(2), but their adhesion strength was significantly reduced on these surfaces in comparison to blank PDMS. In addition, plantigrades had high mortality after 6 h. In contrast to the enhanced antifouling and FR properties of PDMS incorporating TiO(2), the incorporation of CNTs had no effect on the settlement and adhesion of M. galloprovincialis.

Cold spray metal embedment: an innovative antifouling technology.

The study demonstrates that embedment of copper particles into thermoplastic polymers (polymers) using cold spray technology is an effective deterrent against fouling organisms. Two polymers, high-density polyethylene (HDPE) and nylon were metallised with copper powder using cold spray technology. After 250 days in the field, Cu-embedded HDPE and copper plate controls were completely free of hard foulers compared to Cu-embedded nylon and polymer controls which were heavily fouled with both soft and hard fouling. Antifouling (AF) success is related to the interaction between the properties of the polymers (elastic modulus and hardness) and the cold spray process which affect particle embedment depth, and subsequently, the release of copper ions as determined by analytical techniques. Embedding metal using cold spray equipment is shown to be an effective AF technology for polymers, in particular those that are difficult to treat with standard AF coatings, with efficacy being a function of the interaction between the cold spray metal and the polymer recipient.

Enhancing the settlement and attachment strength of pediveligers of Mytilus galloprovincialis by changing surface wettability and microtopography.

Surface wettability and microtopography can either enhance or deter larval settlement of many sessile marine organisms. This study quantifies the effect of these surface properties on the settlement of pediveligers of Mytilus galloprovincialis, using polymers spanning a range of wettability and microtextured polydimethylsiloxane (PDMS). Furthermore, the adhesion strength of settled pediveligers on microtextured PDMS surfaces was quantified using a flow chamber. Settlement was enhanced at the hydrophilic end of the wettability spectrum, where mean settlement on nylon reached 33.5 ± 13.1%. In contrast, mean settlement on the most hydrophobic polymer (PDMS) was 4.2 ± 3.2%. Microtopography had a much stronger effect compared to wettability, where 400 µm textured PDMS enhanced settlement above 90%. Settlement preferences were also positively correlated to adhesion strength at flow rates of 4 knots, with all initially settled pediveligers on smooth PDMS detaching, while 79.9 ± 5.7% of pediveligers remained on the 400 µm texture.

Optimising settlement assays of pediveligers and plantigrades of Mytilus galloprovincialis.

The mussel Mytilus galloprovincialis is a common aquaculture species, and also a major fouling organism that has negative economic impacts. There are no standard assay conditions for this important species and therefore, this study quantified the effect of key factors on the settlement of pediveligers and plantigrades. Density dependent settlement did not occur for either pediveligers or plantigrades. Settlement increased in drop assays in a 12 h light:12 h dark cycle, while bottom shade had no effect of any magnitude. In addition, settlement was significantly enhanced by storing pediveligers for between 4 and 24 days at 4 °C. Overall, these data provide the template to optimise and standardise static laboratory settlement assays for mussels in order to develop materials that either enhance settlement for the aquaculture industry, or deter settlement for antifouling applications. Furthermore, simple mechanisms such as storage at 4 °C can enhance settlement beyond current methods used in aquaculture hatcheries.

Biomimetic characterisation of key surface parameters for the development of fouling resistant materials.

Material science provides a direct route to developing a new generation of non-toxic, surface effect-based antifouling technologies with applications ranging from biomedical science to marine transport. The surface topography of materials directly affects fouling resistance and fouling removal, the two key mechanisms for antifouling technologies. However, the field is hindered by the lack of quantified surface characteristics to guide the development of new antifouling materials. Using a biomimetic approach, key surface parameters are defined and quantified and correlated with fouling resistance and fouling removal from the shells of marine molluscs. Laser scanning confocal microscopy was used to acquire images for quantitative surface characterisation using three-dimensional surface parameters, and field assays correlated these with fouling resistance and fouling release. Principle component analysis produced a major component (explaining 54% of total variation between shell surfaces) that correlated with fouling resistance. The five surface parameters positively correlated to increased fouling resistance were, in order of importance, low fractal dimension, high skewness of both the roughness and waviness profiles, higher values of isotropy and lower values of mean surface roughness. The second component (accounting for 20% of variation between shells) positively correlated to fouling release, for which higher values of mean waviness almost exclusively dictated this relationship. This study provides quantified surface parameters to guide the development of new materials with surface properties that confer fouling resistance and release.

The role of nano-roughness in antifouling.

Nano-engineered superhydrophobic surfaces have been investigated for potential fouling resistance properties. Integrating hydrophobic materials with nanoscale roughness generates surfaces with superhydrophobicity that have water contact angles (theta) >150 degrees and concomitant low hysteresis (<10 degrees ). Three superhydrophobic coatings (SHCs) differing in their chemical composition and architecture were tested against major fouling species (Amphora sp., Ulva rigida, Polysiphonia sphaerocarpa, Bugula neritina, Amphibalanus amphitrite) in settlement assays. The SHC which had nanoscale roughness alone (SHC 3) deterred the settlement of all the tested fouling organisms, compared to selective settlement on the SHCs with nano- and micro-scale architectures. The presence of air incursions or nanobubbles at the interface of the SHCs when immersed was characterized using small angle X-ray scattering, a technique sensitive to local changes in electron density contrast resulting from partial or complete wetting of a rough interface. The coating with broad spectrum antifouling properties (SHC 3) had a noticeably larger amount of unwetted interface when immersed, likely due to the comparatively high work of adhesion (60.77 mJ m(-2) for SHC 3 compared to 5.78 mJ m(-2) for the other two SHCs) required for creating solid/liquid interface from the solid/vapour interface. This is the first example of a non-toxic, fouling resistant surface against a broad spectrum of fouling organisms ranging from plant cells and non-motile spores, to complex invertebrate larvae with highly selective sensory mechanisms. The only physical property differentiating the immersed surfaces is the nano-architectured roughness which supports longer standing air incursions providing a novel non-toxic broad spectrum mechanism for the prevention of biofouling.

Attachment point theory revisited: the fouling response to a microtextured matrix.

This paper examines attachment point theory in detail by testing the fouling attachment of several fouling groups to a microtextured matrix. Static bioassays were conducted on polycarbonate plates with nine equal regions, comprising eight scales of microtexture (4-512 microm) and one untextured region. The microtextures examined were continuous sinusoidal ridges and troughs of defined height and width. Attachment over the microtextured plates was examined for the diatom Amphora sp., the green alga Ulva rigida, the red alga Centroceras clavulatum, the serpulid tube worm Hydroides elegans and the bryozoan Bugula neritina. It was found that the size of the microtexture in relation to the size of the settling propagules/larvae was important in the selection of attachment sites. Attachment was generally lower when the microtexture wavelength was slightly smaller than the width of the settling propagules/larvae and increased when the wavelength was wider than their width. The effect of attachment points was weak for small motile microfoulers (Amphora sp. and U. rigida) (7 microm), strong for large macrofouling larvae (H. elegans and B. neritina) (129-321 microm) and non-existent for the non-motile algal spores (C. clavulatum) (37 microm). This study reinforces the potential of using attachment points to develop surfaces with increased fouling resistance or, alternatively, surfaces which promote the attachment of selected target sizes of motile propagules or larvae.

Furanones.

The red alga Delisea pulchra has been a model organism for understanding the ecological role of secondary metabolites as natural antifoulants. Furanones are produced by the plant and delivered to the surface at a concentration where they regulate bacterial colonisation and the settlement of epibiota. This biological understanding has led to the application of furanones as inhibitors of bacterial- and macro-fouling. Furanones inhibit bacterial colonisation and biofilm development through interference with a key bacterial quorum-sensing pathway, the acylated homoserine lactone regulatory system in Gram-negative bacteria. They also interfere with the alternative AI-2 signalling system in Gram-negative and Gram-positive bacteria. Synthetic programs have developed a library of more than 200 furanone and furanone-analogues including surface attached-furanones. These furanone analogues are potent anti-infectives and inhibit pathogenic phenotypes in Gram-negative and Gram-positive bacteria as demonstrated in-vitro using gene microarrays, and in-vivo using mouse models. Additionally, furanones inhibit the expression of bacterial exo-enzymes that actively degrade components of the immune system thereby enhancing the immune response. Surface-attached furanones immobilised on catheters also inhibit bacterial attachment and retain activity for extended periods. Furanones are strong deterrents of the settlement and growth of macrofouling organisms and as such have potential application as a marine antifouling technology. Laboratory antifouling assays have been used to identify effective and safe furanone-analogues while field trials of furanones incorporated into coatings and polymers demonstrate efficacies similar to commercial biocides. Further development is required to control the release of compounds from suitable carriers to extend coating/polymer lifespans. This review summarises the extensive work on furanones focusing on their natural and applied antifouling activities.

Testing attachment point theory: diatom attachment on microtextured polyimide biomimics.

This paper explores diatom attachment to a range of laser etched polyimide surfaces to directly test 'attachment point theory'. Static bioassays were conducted on microtextured polyimide surfaces using four diatom species, Fallacia carpentariae, Nitzschia cf. paleacea, Amphora sp. and Navicula jeffreyi with cell sizes ranging from 1-14 microm. The microtextured polyimides were modelled from natural fouling resistant bivalve surfaces and had wavelengths above, below and at the same scale as the diatom cell sizes. Diatoms attached in significantly higher numbers to treatments where the numbers of attachment points was highest. The lowest diatom attachment occurred where cells were slightly larger than the microtexture wavelength, resulting in only two theoretical points of attachment. The results support attachment point theory and highlight the need to address larval/cell size in relation to the number of attachment points on a surface. Further studies examining a range of microtexture scales are needed to apply attachment point theory to a suite of fouling organisms and to develop structured surfaces to control the attachment and development of fouling communities.

Differential community development of fouling species on the pearl oysters Pinctada fucata, Pteria penguin and Pteria chinensis (Bivalvia, Pteriidae).

A field experiment documented the development of fouling communities on two shell regions, the lip and hinge, of the pearl oyster species Pinctada fucata, Pteria penguin and Pteria chinensis. Fouling communities on the three species were not distinct throughout the experiment. However, when each species was analysed separately, fouling communities on the lip and hinge of P. penguin and P. chinensis were significantly different during the whole sampling period and after 12 weeks, respectively, whereas no significant differences could be detected for P. fucata. There was no significant difference in total fouling cover between shell regions of P. fucata and P. chinensis after 16 weeks; however, the hinge of P. penguin was significantly more fouled than the lip. The most common fouling species (the hydroid Obelia bidentata, the bryozoan Parasmittina parsevalii, the bivalve Saccostrea glomerata and the ascidian Didemnum sp.) showed species-specific fouling patterns with differential fouling between shell regions for each species. The role of the periostracum in determining the community development of fouling species was investigated by measuring the presence and structure of the periostracum at the lip and hinge of the three pearl oyster species. The periostracum was mainly present at the lip of the pearl oysters, while the periostracum at the hinge was absent and the underlying prismatic layer eroded. The periostracum of P. fucata lacked regular features, whereas the periostracum of P. penguin and P. chinensis consisted of a regular strand-like structure with mean amplitudes of 0.84 microm and 0.65 microm, respectively. Although the nature and distribution of fouling species on the pearl oysters was related to the presence of the periostracum, the periostracum does not offer a fouling-resistant surface for these pearl oyster species.

A new metabolite from the marine bacterium Vibrio angustum S14.

The new metabolite [1-(2'-methylpropoxy)-2-hydroxy-2-methylpropoxy]butane was isolated from the cell-free culture supernatant of the marine bacterium Vibrio angustum S14 as part of studies investigating the role of chemical signals in prokaryote--prokaryote and prokaryote--eukaryote interactions. The structure was elucidated by interpretation of its high-field NMR and mass spectrometric data. [1-(2'-Methylpropoxy)-2-hydroxy-2-methylpropoxy]butane induced the acylated homoserine lactone (AHL) reporter system in Agrobacterium tumefaciens and bioluminescence in Vibrio harveyi.

Induction of metamorphosis in the sea urchin Holopneustes purpurascens by a metabolite complex from the algal host Delisea pulchra.

Most benthic invertebrates have complex life cycles with planktonic larvae that return to the substratum to settle and metamorphose into a benthic stage. Although naturally produced chemical cues have long been thought to be important for the settlement or metamorphosis of invertebrate larvae, few ecologically relevant chemical cues have been clearly identified. The marine echinoid Holopneustes purpurascens has a complex life cycle, with a planktonic, nonfeeding dispersive larva that metamorphoses into a benthic stage that lives in the canopy of subtidal benthic algae such as the red alga Delisea pulchra and the kelp Ecklonia radiata. Recently recruited juveniles are found primarily on D. pulchra, and we hypothesized that this was in response to a chemical cue produced by this alga. Competent larvae metamorphosed in the presence of D. pulchra, or seawater surrounding this alga, but not in response to the presence of E. radiata or its extracts. A cue for metamorphosis was isolated and characterized from D. pulchra and found to be a water-soluble complex of the sugar floridoside and isethionic acid in a 1:1 molar ratio. The floridoside-isethionic acid complex also triggered settlement in H. purpurascens; however, this response was less specific than metamorphosis and was reversible. Larvae of H. purpurascens also metamorphosed in the presence of several other species of red, but not brown or green, algae from their habitat. Floridoside is found only in red algae, suggesting that the floridoside-isethionic acid complex may be acting as a cue for metamorphosis in other red algae as well as in D. pulchra.

Inhibition of luminescence and virulence in the black tiger prawn (Penaeus monodon) pathogen Vibrio harveyi by intercellular signal antagonists.

Expression of luminescence in the Penaeus monodon pathogen Vibrio harveyi is regulated by an intercellular quorum sensing mechanism involving the synthesis and detection of two signaling molecules, one of which is N-hydroxy butanoyl-L-homoserine lactone and the other of which is uncharacterized. Indirect evidence has suggested that virulence, associated with a toxic extracellular protein, and luminescence in V. harveyi are coregulated. In this study the effects of an acylated homoserine lactone antagonist produced by the marine alga Delisea pulchra on luminescence and toxin production in a virulent strain of V. harveyi were analyzed. Luminescence and toxin production were both inhibited by the signal antagonist at concentrations that had no impact on growth. Toxin production was found to be prematurely induced in V. harveyi cultures incubated in a 10% conditioned medium. Additionally, a significant reduction in the toxicity of concentrated supernatant extracts from V. harveyi cultures incubated in the presence of the signal antagonist, as measured by in vivo toxicity assays in mice and prawns, was observed. These results suggest that intercellular signaling antagonists have potential utility in the control of V. harveyi prawn infections.

A novel and sensitive method for the quantification of N-3-oxoacyl homoserine lactones using gas chromatography-mass spectrometry: application to a model bacterial biofilm.

A method is reported for the quantification of 3-oxoacyl homoserine lactones (3-oxo AHLs), a major class of quorum-sensing signals found in Gram-negative bacteria. It is based on the conversion of 3-oxo AHLs to their pentafluorobenzyloxime derivatives followed by gas chromatography-mass spectrometry (electron capture-negative ion). The method used [13C16]-N-3-oxo-dodecanoyl homoserine lactone ([13C16]-OdDHL) as the internal standard, and its validity was tested by spiking the supernatant and cell fractions with three levels of 3-oxo AHLs, i.e. 1, 10 and 100 ng per sample. These showed the method to be both sensitive (S/N ratio >10:1 for 1 ng) and accurate. The assay was applied to the biofilm and effluent of a green fluorescent protein (GFP)-expressing strain of Pseudomonas aeruginosa (6294) culture grown in flow cells. Biofilm volume was determined for three replicate flow cells by confocal scanning laser microscopy. OdDHL was detected in the biofilm at 632 +/- 381 microM and the effluent at 14 +/- 3 nM. The biofilm concentration is the highest level so far reported for an AHL in a wild-type bacterial system. The next most abundant 3-oxo AHL in the biofilm and effluent was N-3-oxo-tetradecanoyl homoserine lactone (OtDHL) at 40 +/- 15 microM and 1.5 +/- 0.7 nM respectively. OtDHL is unreported for P. aeruginosa and has an activity equivalent to OdDHL in a lasR bioassay. Two other 3-oxo AHLs were detected at lower concentrations: N3-oxo-decanoyl homoserine lactone (ODHL) in the biofilm (3 +/- 2 microM) and effluent (1 +/- 0.1 nM); and N-3-oxo-octanoyl homoserine lactone (OOHL) in the effluent (0.1 +/- 0.1 nM).

Quorum-sensing cross talk: isolation and chemical characterization of cyclic dipeptides from Pseudomonas aeruginosa and other gram-negative bacteria.

In cell-free Pseudomonas aeruginosa culture supernatants, we identified two compounds capable of activating an N-acylhomoserine lactone (AHL) biosensor. Mass spectrometry and NMR spectroscopy revealed that these compounds were not AHLs but the diketopiperazines (DKPs), cyclo(DeltaAla-L-Val) and cyclo(L-Pro-L-Tyr) respectively. These compounds were also found in cell-free supernatants from Proteus mirabilis, Citrobacter freundii and Enterobacter agglomerans [cyclo(DeltaAla-L-Val) only]. Although both DKPs were absent from Pseudomonas fluorescens and Pseudomonas alcaligenes, we isolated, from both pseudomonads, a third DKP, which was chemically characterized as cyclo(L-Phe-L-Pro). Dose-response curves using a LuxR-based AHL biosensor indicated that cyclo(DeltaAla-L-Val), cyclo(L-Pro-L-Tyr) and cyclo(L-Phe-L-Pro) activate the biosensor in a concentration-dependent manner, albeit at much higher concentrations than the natural activator N-(3-oxohexanoyl)-L-homoserine lactone (3-oxo-C6-HSL). Competition studies showed that cyclo(DeltaAla-L-Val), cyclo(L-Pro-L-Tyr) and cyclo(L-Phe-L-Pro) antagonize the 3-oxo-C6-HSL-mediated induction of bioluminescence, suggesting that these DKPs may compete for the same LuxR-binding site. Similarly, DKPs were found to be capable of activating or antagonizing other LuxR-based quorum-sensing systems, such as the N-butanoylhomoserine lactone-dependent swarming motility of Serratia liquefaciens. Although the physiological role of these DKPs has yet to be established, their activity suggests the existence of cross talk among bacterial signalling systems.