A novel geometry for a laboratory-based larval settlement assay

Effect of Multilayer Termination on Nonspecific Protein Adsorption and Antifouling Activity of Alginate-Based Layer-by-Layer Coatings

Layer-by-layer (LbL) assembly is a versatile platform for applying coatings and studying the properties of promising compounds for antifouling applications. Here, alginate-based LbL coatings were fabricated by alternating the deposition of alginic acid and chitosan or polyethylenimine to form multilayer coatings. Films were prepared with either odd or even bilayer numbers to investigate if the termination of the LbL coatings affects the physicochemical properties, resistance against the nonspecific adsorption (NSA) of proteins, and antifouling efficacy. The hydrophilic films, which were characterized using spectroscopic ellipsometry, water contact angle goniometry, ATR-FTIR spectroscopy, AFM, XPS, and SPR spectroscopy, revealed high swelling in water and strongly reduced the NSA of proteins compared to the hydrophobic reference. While the choice of the polycation was important for the protein resistance of the LbL coatings, the termination mattered less. The attachment of diatoms and settling of barnacle cypris larvae revealed good antifouling properties that were controlled by the termination and the charge density of the LbL films.

Sol-Gel-Based Hybrid Materials as Antifouling and Fouling-Release Coatings for Marine Applications

Hybrid materials (HMs) offer unique properties as they combine inorganic and organic components into a single material. Here, we developed HM coatings for marine antifouling applications using sol-gel chemistry and naturally occurring polysaccharides. The coatings were characterized by spectroscopic ellipsometry, contact angle goniometry, AFM, and ATR-FTIR, and their stability was tested in saline media. Marine antifouling and fouling-release properties were tested in laboratory assays against the settlement of larvae of the barnacle Balanus improvisus and against the settlement and removal of the diatom Navicula incerta. Furthermore, laboratory data were confirmed in short-term dynamic field assays in Florida, USA. All hybrid coatings revealed a superior performance in the assays compared to a hydrophobic reference. Within the hybrids, those with the highest degree of hydrophilicity and negative net charge across the surface performed best. Alginate and heparin showed good performance, making these hybrid materials promising building blocks for fouling-resistant coatings.

Effects of crosslink density in zwitterionic hydrogel coatings on their antifouling performance and susceptibility to silt uptake

Hydrogel coatings effectively reduce the attachment of proteins and organisms in laboratory assays, in particular when made from zwitterionic monomers. In field experiments with multiple species and non-living material, such coatings suffer from adsorption of particulate matter. In this study, the zwitterionic monomer 3-[N-(2-methacryloyloxyethyl)-N,N-dimethylammonio] propanesulfonate (SPE) was copolymerized with increasing amounts of the photo-crosslinker benzophenon-4-yloxyethyl methacrylate (BPEMA) to systematically alter the density of crosslinks between the polymer chains. The effect of increasing crosslink density on the antifouling (AF) performance of the coatings was investigated in laboratory assays and fields tests. In both cases, the AF performance was improved by increasing the crosslinker content. The coatings reduced protein, diatom, and barnacle accumulation, and showed better resistance to biomass accumulation. The findings underline that the marine AF performance of hydrogel coatings does not only depend on the specific chemical structure of the polymers, but also on their physico-chemical properties such as rigidity and swelling.

Antifouling Properties of Dendritic Polyglycerols against Marine Macrofouling Organisms

Dendritic polyglycerols (PGs) were synthesized and postmodified by grafting of poly(ethylene glycol) (PEG) and polypropylene glycol (PPG) diglycidyl ether groups, and their antifouling and fouling-release properties were tested. Coating characterization by spectroscopic ellipsometry, contact angle goniometry, attenuated total internal reflection-Fourier transform infrared spectroscopy (ATR-FTIR), and atomic force microscopy showed brushlike morphologies with a high degree of microscale roughness and the ability to absorb large amounts of water within seconds. PGs with three different thicknesses were tested in laboratory assays against settlement of larvae of the barnacle Balanus improvisus and against the settlement and removal of zoospores of the alga Ulva linza. Very low coating thicknesses, e.g., 11 nm, reduced the settlement of barnacles, under static conditions, to 2% compared with 55% for an octadecyltrichlorosilane reference surface. In contrast, zoospores of U. linza settled readily but the vast majority were removed by exposure to a shear force of 52 Pa. Both PEG and PPG modification increased the antifouling properties of the PG films, providing a direct comparison of the ultralow fouling properties of all three polymers. Both, the modified and the nonmodified PGs are promising components for incorporation into amphiphilic fouling-resistant coatings.

Chemistry-specific surface adsorption of the barnacle settlement-inducing protein complex

Gregarious settlement in barnacle larvae (cyprids) is induced by a contact pheromone, the settlement-inducing protein complex (SIPC). The SIPC has been identified both in the cuticle of adult barnacles and in the temporary adhesive secretion (footprint) of cyprids. Besides acting as a settlement inducer, the presence of the SIPC in footprints points to its additional involvement in the adhesion process. SIPC adsorption behaviour was therefore investigated on a series of self-assembled monolayers (SAMs) by surface plasmon resonance at the pH of seawater (8.3). Fibrinogen and α2-macroglobulin (A2M) (blood complement protease inhibitors with which the SIPC shares 29% sequence homology) were used in the adsorption experiments as positive and negative standards, respectively. The mass uptake of the SIPC was comparable to that of fibrinogen, with adsorption observed even on the protein-resistant oligo(ethylene glycol) surface. Notably, on the positively charged SAM the SIPC showed a kinetic overshoot, indicating a metastable configuration causing the amount of adsorbed protein to temporarily exceed its equilibrium value. A2M adsorption was low or negligible on all SAMs tested, except for the positively charged surface, indicating that A2M adsorption is mainly driven by electrostatics. Evaluation of SIPC non-specific adsorption kinetics revealed that it adsorbed irreversibly and non-cooperatively on all surfaces tested.

Worldwide genetic differentiation in the common fouling barnacle, Amphibalanus amphitrite

Amphibalanus amphitrite is a common fouling barnacle distributed globally in tropical and subtropical waters. In the present study, the genetic (mitochondrial cytochrome oxidase subunit I) and morphological differentiation in A. amphitrite from 25 localities around the world were investigated. The results revealed three clades within A. amphitrite with a genetic divergence of ~ 4% among clades, whereas there were no diagnostic morphological differences among clades. Clade 1 is widely distributed in both temperate and tropical waters, whereas Clade 3 is currently restricted to the tropical region. The deep divergence among clades suggests historical isolation within A. amphitrite; thus, the present geographical overlaps are possibly a result of the combined effects of rising sea level and human-mediated dispersals. This study highlights the genetic differentiation that exists in a common, widely distributed fouling organism with great dispersal potential; future antifouling research should take into account the choice of lineages.

Barnacle cyprid motility and distribution in the water column as an indicator of the settlement-inhibiting potential of nontoxic antifouling chemistries

Testing of new coatings to control fouling frequently involves single-species laboratory bioassays. Barnacle cyprids are among the most widely used model organisms in marine biofouling research, and surfaces that inhibit their settlement are considered to be promising candidates for new coating concepts. An analysis of motility parameters (mean velocity and swimming area coefficient) and distribution of cyprids of Balanus amphitrite in different swimming regions in the vicinity of model surfaces (self-assembled monolayers) is presented. The data are correlated with the settlement preferences of cyprids on these surfaces. Cyprids were predominantly found in interfacial regions and the transition frequencies between swimming regions of different depths were determined.

Correlation between surface chemistry and settlement behaviour in barnacle cyprids (Balanus improvisus)

In laboratory-based biofouling assays, the influence of physico-chemical surface characteristics on barnacle settlement has been tested most frequently using the model organism Balanus amphitrite (= Amphibalanus amphitrite). Very few studies have addressed the settlement preferences of other barnacle species, such as Balanus improvisus (= Amphibalanus improvisus). This study aimed to unravel the effects of surface physico-chemical cues, in particular surface-free energy (SFE) and surface charge, on the settlement of cyprids of B. improvisus. The use of well-defined surfaces under controlled conditions further facilitates comparison of the results with recent similar data for B. amphitrite. Zero-day-old cyprids of B. improvisus were exposed to a series of model surfaces, namely self-assembled monolayers (SAMs) of alkanethiols with varying end-groups, homogenously applied to gold-coated polystyrene (PS) Petri dishes. As with B. amphitrite, settlement of cyprids of B. improvisus was influenced by both SFE and charge, with higher settlement on low-energy (hydrophobic) surfaces and negatively charged SAMs. Positively charged SAMs resulted in low settlement, with intermediate settlement on neutral SAMs of similar SFE. In conclusion, it is demonstrated that despite previous suggestions to the contrary, these two species of barnacle show similar preferences in response to SFE; they also respond similarly to charge. These findings have positive implications for the development of novel antifouling (AF) coatings and support the importance of consistency in substratum choice for assays designed to compare surface preferences of fouling organisms.

Molecular surface chemistry in marine bioadhesion

This review covers the in situ molecular physicochemical characterisation of bioadhesives at solid/liquid interfaces, with the aim of elucidating the adhesion strategies that lie at the root of marine biofouling. It focuses on three major foulers: mussels, algae and barnacles. The dispersal of these organisms, their colonisation of surfaces, and ultimately their survival rely critically on the ability of the organisms' larvae or spores to locate a favourable settlement site and undergo metamorphosis, thus initiating their sessile existence. Differences in the composition of adhesive secretions and the strategies employed for their temporary or permanent implementation exists between the larval and adult life stages. To date, only a few adhesive secretions from marine fouling organisms have been adequately described in terms of their chemical composition, and a survey revealed the presence of certain recurrent functional groups, specifically catechol, carboxylate, monoester-sulphate and -phosphate. This review will describe the binding modes of such functionalities to wet mineral/metal oxides surfaces. Such functionalities will be ranked based on their ability to bind to hydrophilic surfaces replacing surface-bound water (Langmuir adsorption constant) as well as other adsorbates (competitive adsorption). A plausible explanation for the propensity of the reviewed adhesive functionalities to bind to hydrous metal oxide surfaces will be given on the basis of the Hard and Soft Acids and Bases principle, Hofmeister effects and entropic considerations. From the in situ analysis of marine organism bioadhesives and adsorption studies of functionalities relevant to the bioadhesion process, insights can be gleaned for a knowledge-based innovation of antifouling strategies and the synthesis of strong, durable adhesive materials, which are suitable for implementation in wet environments.