Calcite-specific coupling protein in barnacle underwater cement

Adaptive Adhesions of Barnacle-Inspired Adhesive Peptides

The strategy of robust adhesion employed by barnacles renders them fascinating biomimetic candidates for developing novel wet adhesives. Particularly, barnacle cement protein 19k (cp19k) has been speculated to be the key adhesive protein establishing the priming layer in the initial barnacle cement construction. In this work, we systematically studied the sequence design rationale of cp19k by designing adhesive peptides inspired by the low-complexity STGA-rich and the charged segments of cp19k. Combining structure analysis and the adhesion performance test, we found that cp19k-inspired adhesive peptides possess excellent disparate adhesion strategies for both hydrophilic mica and hydrophobic self-assembled monolayer surfaces. Specifically, the low-complexity STGA-rich segment offers great structure flexibility for surface adhesion, while the hydrophobic and charged residues can contribute to the adhesion of the peptides on hydrophobic and charged surfaces. The adaptive adhesion strategy identified in this work broadens our understanding of barnacle adhesion mechanisms and offers valuable insights for designing advanced wet adhesives with exceptional performance on various types of surfaces.

Biomineralization in Barnacle Base Plate in Association with Adhesive Cement Protein

Barnacles strongly attach to various underwater substrates by depositing and curing a proteinaceous cement that forms a permanent adhesive layer. The protein MrCP20 present within the calcareous base plate of the acorn barnacle Megabalanus rosa (M. rosa) was investigated for its role in regulating biomineralization and growth of the barnacle base plate, as well as the influence of the mineral on the protein structure and corresponding functional role. Calcium carbonate (CaCO3) growth on gold surfaces modified by 11-mercaptoundecanoic acid (MUA/Au) with or without the protein was followed using quartz crystal microbalance with dissipation monitoring (QCM-D), and the grown crystal polymorph was identified by Raman spectroscopy. It is found that MrCP20 either in solution or on the surface affects the kinetics of nucleation and growth of crystals and stabilizes the metastable vaterite polymorph of CaCO3. A comparative study of mass uptake calculated by applying the Sauerbrey equation to the QCM-D data and quantitative X-ray photoelectron spectroscopy determined that the final surface density of the crystals as well as the crystallization kinetics are influenced by MrCP20. In addition, polarization modulation infrared reflection-absorption spectroscopy of MrCP20 established that, during crystal growth, the content of β-sheet structures in MrCP20 increases, in line with the formation of amyloid-like fibrils. The results provide insights into the molecular mechanisms by which MrCP20 regulates the biomineralization of the barnacle base plate, while favoring fibril formation, which is advantageous for other functional roles such as adhesion and cohesion.

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

Self-assembling Bioadhesive Inspired by the Fourth Repetitive Sequence of Balanus albicostatus Cement Protein 20 kDa (Balcp-20 k)

Barnacle cement proteins are multi-protein complexes composed of a series of functionally related synergistic proteins that enable barnacles to adhere strongly and consistently to various underwater substrates. There is no post-translational modification of barnacle cement proteins, which provides a possibility for the synthesis of similar adhesive materials. Balcp-20 k has four repetitive sequences with multiple conserved cysteine groups. Whether these repeats are separate functional units and the role of cysteine in adhesion is not clear. In order to investigate the adhesion properties of Balcp-20 k, we amplified and expressed R4 (DHLACNAKHPCWHKHCDCFC)₄, which is a quadruple repeat of Balcp-20 k's fourth repetitive sequence, and S0R4 (DHLASNAKHPSWHKHSDSFS)₄, all cysteine of R4 replaced by serine. Analysis showed that R4 had a similar structure to Balcp-20 k, and the amyloid fibrils structure formed by self-assembly of R4 played an important role in improving the adhesion strength. The absence of disulfide bonds in S0R4 prevents self-assembly, and the failure of self-assembly after the reduction of disulfide bonds of R4 by DTT indicates that disulfide bonds play an important role in self-assembly. With adhesion and coating analysis, it was found that R4 has good adhesion on different materials surfaces, which is better than Balcp-20 k, while S0R4 has weak adhesion, which is only better than BSA.

An essential role of disulfide bonds for the hierarchical self-assembly and underwater affinity of CP20-derived peptides

Barnacles are typical fouling organisms strongly adhere to immersed solid substrates by secreting proteinaceous adhesives called cement proteins (CPs). The self-assembly of the CPs forms a permanently bonded layer that binds barnacles to foreign surfaces. However, it is difficult to determine their natural structure and describe their self-assembly properties due to the abundance of cysteines in whole-length CP20. A putative functional motif of Balanus albicostatus CP20 (BalCP20) was identified to present distinctive self-assembly and wet-binding characteristics. Atomic-force microscopy (AFM) and transmission electron microscope (TEM) investigations showed that wildtype BalCP20-P3 formed grain-like spindles, which assembled into fractal-like structures like ears of wheat. SDS-PAGE, AFM, and LSCM showed that DTT treatment opened up disulfide bonds between cysteines and disrupted fractal-like structures. Additionally, these morphologies were abolished when one of the BalCP20-P3 four cysteines was mutated by alanine. Circular dichroism (CD) results suggested that the morphological diversity among BalCP20-P3 and its mutations was related to the proportion of α-helices. Finally, quartz crystal microbalance with dissipation (QCM-D) detected that BalCP20-P3 and its mutations with diverse self-assemblies occupied different affinities. The above results demonstrated that cysteines and disulfide bonds played a crucial role in the self-assembly and wet binding of BalCP20-P3. The work provides new ideas for the underwater bonding of BalCP20 and developing new bionic underwater adhesives.

Convergent evolution of barnacles and molluscs sheds lights in origin and diversification of calcareous shell and sessile lifestyle

The calcareous shell and sessile lifestyle are the representative phenotypes of many molluscs, which happen to be present in barnacles, a group of unique crustaceans. The origin of these phenotypes is unclear, but it may be embodied in the convergent genetics of such distant groups (interphylum). Herein, we perform comprehensive comparative genomics analysis in barnacles and molluscs, and reveal a genome-wide strong convergent molecular evolution between them, including coexpansion of biomineralization and organic matrix genes for shell formation, and origination of lineage-specific orphan genes for settlement. Notably, the expanded biomineralization gene encoding alkaline phosphatase evolves a novel, highly conserved motif that may trigger the origin of barnacle shell formation. Unlike molluscs, barnacles adopt novel organic matrices and cement proteins for shell formation and settlement, respectively, and their calcareous shells have potentially originated from the cuticle system of crustaceans. Therefore, our study corroborates the idea that selection pressures driving convergent evolution may strongly act in organisms inhabiting similar environments regardless of phylogenetic distance. The convergence signatures shed light on the origin of the shell and sessile lifestyle of barnacles and molluscs. In addition, notable non-convergence signatures are also present and may contribute to morphological and functional specificities.

Adhesive Materials Inspired by Barnacle Underwater Adhesion: Biological Principles and Biomimetic Designs

Wet adhesion technology has potential applications in various fields, especially in the biomedical field, yet it has not been completely mastered by humans. Many aquatic organisms (e.g., mussels, sandcastle worms, and barnacles) have evolved into wet adhesion specialists with excellent underwater adhesion abilities, and mimicking their adhesion principles to engineer artificial adhesive materials offers an important avenue to address the wet adhesion issue. The crustacean barnacle secretes a proteinaceous adhesive called barnacle cement, with which they firmly attach their bodies to almost any substrate underwater. Owing to the unique chemical composition, structural property, and adhesion mechanism, barnacle cement has attracted widespread research interest as a novel model for designing biomimetic adhesive materials, with significant progress being made. To further boost the development of barnacle cement-inspired adhesive materials (BCIAMs), it is necessary to systematically summarize their design strategies and research advances. However, no relevant reviews have been published yet. In this context, we presented a systematic review for the first time. First, we introduced the underwater adhesion principles of natural barnacle cement, which lay the basis for the design of BCIAMs. Subsequently, we classified the BCIAMs into three major categories according to the different design strategies and summarized their research advances in great detail. Finally, we discussed the research challenge and future trends of this field. We believe that this review can not only improve our understanding of the molecular mechanism of barnacle underwater adhesion but also accelerate the development of barnacle-inspired wet adhesion technology.

A novel recombinant chimeric bio-adhesive protein consisting of mussel foot protein 3, 5, gas vesicle protein A, and CsgA curli protein expressed in Pichia pastoris

Despite various efforts to produce potent recombinant bio-adhesive proteins for medical purposes, efficient production of a safe and feasible bio-glue is not yet a commercial reality due to the weak properties or low expression levels. Here, a feasible expression system has been developed to produce strong recombinant fusion bioinspired protein using mussel foot protein 3 and 5 (Mfps) along with gas vesicle protein A (GvpA) of Anabaena flos-aquae, and a curli protein CsgA from E. coli, expressed under the control of alcohol oxidase (AOX1) promoter for high-level production in yeast P. pastoris using pPICZα vector. Purified chimeric proteins were first evaluated using western blotting, and their remaining dihydroxyphenylalanine (DOPA) was measured in the modified proteins by NBT assay. We further elucidated the mechanistic properties of obtained adhesive protein assembly in various pH levels based on its different subunits using atomic force microscopy (AFM) when adsorbed onto the mica surface. We found that both combinational structural features of subunits and post-translational changes during expression in yeast host have led to potent adherence due to higher DOPA residues specially in acidic condition and tetrad complex which is higher than that of earlier reports in prokaryotic systems. We believe that our obtained chimeric protein resulted from the fusion of GvpA and CsgA proteins with DOPA-containing Mfp proteins, expressed in the methylotrophic yeast, P. pastoris, not only presents a candidate for future biomedical applications but also provides novel biological clues used for high-performance bioinspired biomaterial designation.

High-Throughput Screening of Heterologous Functional Amyloids Using Escherichia coli

Escherichia coli remains one of the most widely used workhorse microorganisms for the expression of heterologous proteins. The large number of cloning vectors and mutant host strains available for E. coli yields an impressively wide array of folded globular proteins in the laboratory. However, applying modern functional screening approaches to interrogate insoluble protein aggregates such as amyloids requires the use of nonstandard expression pathways. In this chapter, we detail the use of the curli export pathway in E. coli to express a library of gene fragments and variants of a functional amyloid protein to screen sequence traits responsible for aggregation and the formation of nanoscale materials.

Protein-mediated bioadhesion in marine organisms: A review

Protein-mediated bioadhesion is one of the crucial physiological processes in marine organisms, by which they can firmly adhere to underwater substrates. Most marine adhesive organisms are biofoulers, causing negative effects on marine ecosystems and huge economic losses to aquaculture and maritime industries. Furthermore, adhesive proteins in these organisms are promising bionic candidates for high-performance artificial materials with great application value. In-depth understanding of the bioadhesion in marine ecosystems is of dual significance for resolving biofouling issue and developing marine bionic products. Here, we review the research progress of protein-mediated bioadhesion in marine organisms. The adhesion processes such as protein biosynthesis and secretion are similar among organisms, but the detailed features such as compositions, structures, and molecular functions of adhesive proteins are distinct. Hydroxylation, glycosylation, and phosphorylation are important post-translational modifications during the processes of adhesion. The contents of some amino acids such as glycine, tyrosine and cysteine involved in underwater adhesion are significantly higher, which is a sequence feature of barnacle cement and mussel foot proteins. The amyloid structures and conserved domains/motifs such as EGF and vWFA distributed in adhesive proteins are involved in the underwater adhesion. In addition, the oxidative cross-linking also plays an important role in marine bioadhesion. Overall, the unique and common features identified for the protein-mediated bioadhesion in diverse marine organisms here provide background information and essential reference for characterizing marine adhesive proteins and associated functional domains, formulating antifouling strategies, and developing novel biomimetic adhesives.

Transthyretin: From Structural Stability to Osteoarticular and Cardiovascular Diseases

Transthyretin (TTR) is a tetrameric protein transporting hormones in the plasma and brain, which has many other activities that have not been fully acknowledged. TTR is a positive indicator of nutrition status and is negatively correlated with inflammation. TTR is a neuroprotective and oxidative-stress-suppressing factor. The TTR structure is destabilized by mutations, oxidative modifications, aging, proteolysis, and metal cations, including Ca2+. Destabilized TTR molecules form amyloid deposits, resulting in senile and familial amyloidopathies. This review links structural stability of TTR with the environmental factors, particularly oxidative stress and Ca2+, and the processes involved in the pathogenesis of TTR-related diseases. The roles of TTR in biomineralization, calcification, and osteoarticular and cardiovascular diseases are broadly discussed. The association of TTR-related diseases and vascular and ligament tissue calcification with TTR levels and TTR structure is presented. It is indicated that unaggregated TTR and TTR amyloid are bound by vicious cycles, and that TTR may have an as yet undetermined role(s) at the crossroads of calcification, blood coagulation, and immune response.

Mussel-inspired hybrid network hydrogel for continuous adhesion in water

The mussel-inspired catechol-based strategy has been widely used in the development of adhesives. However, the properties of the obtained adhesives were still severely limited in a humid environment, particularly in water. In this study, a facile and versatile approach was proposed to prepare an underwater adhesion hydrogel. First, dopamine (DA) was grafted on oxidized carboxymethylcellulose (OCMC) to obtain dopamine-grafted oxidized carboxymethylcellulose (OCMC-DA). Second, the acrylamide (AM) monomer was conjugated with OCMC-DA by a Schiff base reaction, and then polymerized to form an OCMC-DA/PAM hydrogel. The properties of the resulting hydrogel have been fully characterized. The underwater adhesion strength of the hydrogel can reach as high as 86.3 ± 7.2 kPa and reduced to 43 ± 3.4 kPa after being immersed in water for 9 days. More remarkably, we found that the maximal adhesion strength was shown when the G' and G'' of the hydrogel were very close. Moreover, we demonstrated the mechanical properties of our fabricated hydrogel by compressive tests and rheological analysis. The adhesive hydrogel also exhibits excellent biocompatibility.

Proteogenomic Characterization of the Cement and Adhesive Gland of the Pelagic Gooseneck Barnacle Lepas anatifera

We focus on the stalked goose barnacle L. anatifera adhesive system, an opportunistic less selective species for the substrate, found attached to a variety of floating objects at seas. Adhesion is an adaptative character in barnacles, ensuring adequate positioning in the habitat for feeding and reproduction. The protein composition of the cement multicomplex and adhesive gland was quantitatively studied using shotgun proteomic analysis. Overall, 11,795 peptide sequences were identified in the gland and 2206 in the cement, clustered in 1689 and 217 proteinGroups, respectively. Cement specific adhesive proteins (CPs), proteases, protease inhibitors, cuticular and structural proteins, chemical cues, and many unannotated proteins were found, among others. In the cement, CPs were the most abundant (80.5%), being the bulk proteins CP100k and -52k the most expressed of all, and CP43k-like the most expressed interfacial protein. Unannotated proteins comprised 4.7% of the cement proteome, ranking several of them among the most highly expressed. Eight of these proteins showed similar physicochemical properties and amino acid composition to known CPs and classified through Principal Components Analysis (PCA) as new CPs. The importance of PCA on the identification of unannotated non-conserved adhesive proteins, whose selective pressure is on their relative amino acid abundance, was demonstrated.

The Quantitative Proteome of the Cement and Adhesive Gland of the Pedunculate Barnacle, Pollicipes pollicipes

Adhesive secretion has a fundamental role in barnacles' survival, keeping them in an adequate position on the substrate under a variety of hydrologic regimes. It arouses special interest for industrial applications, such as antifouling strategies, underwater industrial and surgical glues, and dental composites. This study was focused on the goose barnacle Pollicipes pollicipes adhesion system, a species that lives in the Eastern Atlantic strongly exposed intertidal rocky shores and cliffs. The protein composition of P. pollicipes cement multicomplex and cement gland was quantitatively studied using a label-free LC-MS high-throughput proteomic analysis, searched against a custom transcriptome-derived database. Overall, 11,755 peptide sequences were identified in the gland while 2880 peptide sequences were detected in the cement, clustered in 1616 and 1568 protein groups, respectively. The gland proteome was dominated by proteins of the muscle, cytoskeleton, and some uncharacterized proteins, while the cement was, for the first time, reported to be composed by nearly 50% of proteins that are not canonical cement proteins, mainly unannotated proteins, chemical cues, and protease inhibitors, among others. Bulk adhesive proteins accounted for one-third of the cement proteome, with CP52k being the most abundant. Some unannotated proteins highly expressed in the proteomes, as well as at the transcriptomic level, showed similar physicochemical properties to the known surface-coupling barnacle adhesive proteins while the function of the others remains to be discovered. New quantitative and qualitative clues are provided to understand the diversity and function of proteins in the cement of stalked barnacles, contributing to the whole adhesion model in Cirripedia.

Insights into the Synthesis, Secretion and Curing of Barnacle Cyprid Adhesive via Transcriptomic and Proteomic Analyses of the Cement Gland

Barnacles represent one of the model organisms used for antifouling research, however, knowledge regarding the molecular mechanisms underlying barnacle cyprid cementation is relatively scarce. Here, RNA-seq was used to obtain the transcriptomes of the cement glands where adhesive is generated and the remaining carcasses of Megabalanus volcano cyprids. Comparative transcriptomic analysis identified 9060 differentially expressed genes, with 4383 upregulated in the cement glands. Four cement proteins, named Mvcp113k, Mvcp130k, Mvcp52k and Mvlcp1-122k, were detected in the cement glands. The salivary secretion pathway was significantly enriched in the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of the differentially expressed genes, implying that the secretion of cyprid adhesive might be analogous to that of saliva. Lysyl oxidase had a higher expression level in the cement glands and was speculated to function in the curing of cyprid adhesive. Furthermore, the KEGG enrichment analysis of the 352 proteins identified in the cement gland proteome partially confirmed the comparative transcriptomic results. These results present insights into the molecular mechanisms underlying the synthesis, secretion and curing of barnacle cyprid adhesive and provide potential molecular targets for the development of environmentally friendly antifouling compounds.

Adhesion of acorn barnacles on surface-active borate glasses

Concerns about the bioaccumulation of toxic antifouling compounds have necessitated the search for alternative strategies to combat marine biofouling. Because many biologically essential minerals have deleterious effects on organisms at high concentration, one approach to preventing the settlement of marine foulers is increasing the local concentration of ions that are naturally present in seawater. Here, we used surface-active borate glasses as a platform to directly deliver ions (Na+, Mg2+ and BO43-) to the adhesive interface under acorn barnacles (Amphibalanus (=Balanus) amphitrite). Additionally, surface-active glasses formed reaction layers at the glass-water interface, presenting another challenge to fouling organisms. Proteomics analysis showed that cement deposited on the gelatinous reaction layers is more soluble than cement deposited on insoluble glasses, indicating the reaction layer and/or released ions disrupted adhesion processes. Laboratory experiments showed that the majority (greater than 79%) of adult barnacles re-attached to silica-free borate glasses for 14 days could be released and, more importantly, barnacle larvae did not settle on the glasses. The formation of microbial biofilms in field tests diminished the performance of the materials. While periodic water jetting (120 psi) did not prevent the formation of biofilms, weekly cleaning did dramatically reduce macrofouling on magnesium aluminoborate glass to levels below a commercial foul-release coating. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Three-dimensional structure of Megabalanus rosa Cement Protein 20 revealed by multi-dimensional NMR and molecular dynamics simulations

Barnacles employ a protein-based cement to firmly attach to immersed substrates. The cement proteins (CPs) have previously been identified and sequenced. However, the molecular mechanisms of adhesion are not well understood, in particular, because the three-dimensional molecular structure of CPs remained unknown to date. Here, we conducted multi-dimensional nuclear magnetic resonance (NMR) studies and molecular dynamics (MD) simulations of recombinant Megabalanus rosa Cement Protein 20 (rMrCP20). Our NMR results show that rMrCP20 contains three main folded domain regions intervened by two dynamic loops, resulting in multiple protein conformations that exist in equilibrium. We found that 12 out of 32 Cys in the sequence engage in disulfide bonds that stabilize the β-sheet domains owing to their placement at the extremities of β-strands. Another feature unveiled by NMR is the location of basic residues in turn regions that are exposed to the solvent, playing an important role for intermolecular contact with negatively charged surfaces. MD simulations highlight a highly stable and conserved β-motif (β7-β8), which may function as nuclei for amyloid-like nanofibrils previously observed in the cured adhesive cement. To the best of our knowledge, this is the first report describing the tertiary structure of an extracellular biological adhesive protein at the molecular level. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

The expression and characterization of recombinant cp19k barnacle cement protein from Pollicipes pollicipes

Adhesive proteins of barnacle cement have potential as environmentally friendly adhesives owing to their ability to adhere to various substrates in aqueous environments. By understanding the taxonomic breath of barnacles with different lifestyles, we may uncover commonalities in adhesives produced by these specialized organisms. The 19 kDa cement protein (cp19k) of the stalked barnacle Pollicipes pollicipes was expressed in Escherichia coli BL21 to investigate its adhesive properties. Initial expression of hexahistidine-tagged protein (rPpolcp19k-his) yielded low levels of insoluble protein. Co-overproduction of E. coli molecular chaperones GroEL-GroES and trigger factor (TF) increased soluble protein yields, although TF co-purified with the target protein (TF-rPpolcp19k-his). Surface coat analysis revealed high levels of adsorption of the TF-rPpolcp19k-his complex and of purified E. coli TF on both hydrophobic and hydrophilic surfaces, while low levels of adsorption were observed for rPpolcp19k-his. Tag-free rPpolcp19k protein also exhibited low adsorption compared to fibrinogen and Cell-Tak controls on hydrophobic, neutral hydrophilic and charged self-assembled monolayers under surface plasmon resonance assay conditions designed to mimic the barnacle cement gland or seawater. Because rPpolcp19k protein displays low adhesive capability, this protein is suggested to confer the ability to self-assemble into a plaque within the barnacle cement complex. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Comparative Analysis of the Adhesive Proteins of the Adult Stalked Goose Barnacle Pollicipes pollicipes (Cirripedia: Pedunculata)

Adhesion in barnacles is still poorly understood. The cement gland secretes an insoluble multi-protein complex, which adheres very strongly to a variety of substrates in the presence of water. This adhesion mechanism is bioinspiring for the engineering of new adhesive materials, but to replicate this adhesive system, the genes coding for the cement constitutive proteins must be identified and elucidated, and their products characterised. Here, the complete sequences of three cement protein (CP) genes (CP-100K, CP-52K, and CP-19K) isolated from the cement gland of the stalked barnacle Pollicipes pollicipes (order Scalpelliformes) were obtained using RACE PCR. The three genes were compared to the 23 other acorn barnacle CP genes so far sequenced (order Sessilia) to determine common and differential patterns and molecular properties, since the adhesives of both orders have visibly different characteristics. A shotgun proteomic analysis was performed on the cement, excreted at the membranous base of specimens, where the products of the three genes sequenced in the gland were identified, validating their function as CPs. A principal component analysis (PCA) was performed, to cluster CPs into groups with similar amino acid composition. This analysis uncovered three CP groups, each characterised by similar residue composition, features in secondary structure, and some biochemical properties, including isoelectric point and residue accessibility to solvents. The similarity among proteins in each defined group was low despite comparable amino acid composition. PCA can identify putative adhesive proteins from NGS transcriptomic data regardless of their low homology. This analysis did not highlight significant differences in residue composition between homologous acorn and stalked barnacle CPs. The characteristics responsible for the structural differences between the cement of stalked and acorn barnacles are described, and the presence of nanostructures, such as repetitive homologous domains and low complexity regions, and repetitive β-sheets are discussed relatively to self-assembly and adhesion.

Self-Assembled Nanofibers for Strong Underwater Adhesion: The Trick of Barnacles

Developing adhesives that can function underwater remains a major challenge for bioengineering, yet many marine creatures, exemplified as mussels and barnacles, have evolved their unique proteinaceous adhesives for strong wet adhesion. The mechanisms underlying the strong adhesion of these natural adhesive proteins provide rich information for biomimetic efforts. Here, combining atomic force microscopy (AFM) imaging and force spectroscopy, we examine the effects of pH on the self-assembly and adhesive properties of cp19k, a key barnacle underwater adhesive protein. For the first time, we confirm that the bacterial recombinant Balanus albicostatus cp19k (rBalcp19k), which contains no 3,4-dihydroxyphenylalanine (DOPA) or any other amino acids with post-translational modifications, can self-assemble into aggregated nanofibers at acidic pHs. Under moderately acidic conditions, the adhesion strength of unassembled monomeric rBalcp19k on mica is only slightly lower than that of a commercially available mussel adhesive protein mixture, yet the adhesion ability of rBalcp19k monomers decreases significantly at increased pH. In contrast, upon preassembly at acidic and low-salinity conditions, rBalcp19k nanofibers keep stable in basic and high-salinity seawater and display much stronger adhesion and thus show resistance to its adverse impacts. Besides, we find that the adhesion ability of Balcp19k is not impaired when it is combined with an N-terminal Thioredoxin (Trx) tag, yet whether the self-assembly property will be disrupted is not determined. Collectively, the self-assembly-enhanced adhesion presents a previously unexplored mechanism for the strong wet adhesion of barnacle cement proteins and may lead to the design of barnacle-inspired adhesive materials.

Sequence basis of Barnacle Cement Nanostructure is Defined by Proteins with Silk Homology

Barnacles adhere by producing a mixture of cement proteins (CPs) that organize into a permanently bonded layer displayed as nanoscale fibers. These cement proteins share no homology with any other marine adhesives, and a common sequence-basis that defines how nanostructures function as adhesives remains undiscovered. Here we demonstrate that a significant unidentified portion of acorn barnacle cement is comprised of low complexity proteins; they are organized into repetitive sequence blocks and found to maintain homology to silk motifs. Proteomic analysis of aggregate bands from PAGE gels reveal an abundance of Gly/Ala/Ser/Thr repeats exemplified by a prominent, previously unidentified, 43 kDa protein in the solubilized adhesive. Low complexity regions found throughout the cement proteome, as well as multiple lysyl oxidases and peroxidases, establish homology with silk-associated materials such as fibroin, silk gum sericin, and pyriform spidroins from spider silk. Distinct primary structures defined by homologous domains shed light on how barnacles use low complexity in nanofibers to enable adhesion, and serves as a starting point for unraveling the molecular architecture of a robust and unique class of adhesive nanostructures.

Localization of Phosphoproteins within the Barnacle Adhesive Interface

Barnacles permanently adhere to nearly any inert substrate using proteinaceous glue. The glue consists of at least ten major proteins, some of which have been isolated and sequenced. Questions still remain about the chemical mechanisms involved in adhesion and the potential of the glue to serve as a platform for mineralization of the calcified base plate. We tested the hypothesis that barnacle glue contains phosphoproteins, which have the potential to play a role in both adhesion and mineralization. Using a combination of phosphoprotein-specific gel staining and Western blotting with anti-phosphoserine antibody, we identified multiple phosphorylated proteins in uncured glue secretions from the barnacle Amphibalanus amphitrite The protein composition of the glue and the quantity and abundance of phosphoproteins varied distinctly among individual barnacles, possibly due to cyclical changes in the glue secretion over time. We assessed the location of the phosphoproteins within the barnacle glue layer using decalcified barnacle base plates and residual glue deposited by reattached barnacles. Phosphoproteins were found throughout the organic matrix of the base plate and within the residual glue. Staining within the residual glue appeared most intensely in regions where capillary glue ducts, which are involved in cyclical release of glue, had been laid down. Lastly, mineralization studies of glue proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated that proteins identified as phosphorylated possibly induce mineralization of calcium carbonate (CaCO3). These results contribute to our understanding of the protein composition of barnacle glue, and provide new insights into the potential roles of phosphoproteins in underwater bioadhesives.

Barnacle biology before, during and after settlement and metamorphosis: a study of the interface

Mobile barnacle cypris larvae settle and metamorphose, transitioning to sessile juveniles with morphology and growth similar to adults. Because biofilms exist on immersed surfaces on which they attach, barnacles must interact with bacteria during initial attachment and subsequent growth. The objective of this study was to characterize the developing interface of the barnacle and substratum during this key developmental transition to inform potential mechanisms that promote attachment. The interface was characterized using confocal microscopy and fluorescent dyes to identify morphological and chemical changes in the interface and the status of bacteria present as a function of barnacle developmental stage. Staining revealed patchy material containing proteins and nucleic acids, reactive oxygen species amidst developing cuticle, and changes in bacteria viability at the developing interface. We found that as barnacles metamorphose from the cyprid to juvenile stage, proteinaceous materials with the appearance of coagulated liquid were released into and remained at the interface. The patchy material was associated with cuticle expansion and separation during later stages of metamorphosis, and spanned the entire vertical interface in the gap between the juvenile base and the substratum. It stained positive for proteins, including phosphoprotein, as well as nucleic acids. Regions of the developing cuticle and the patchy material itself stained for reactive oxygen species. Bacteria were absent until the cyprid was firmly attached, but populations died as barnacle development progressed. The oxidative environment may contribute to the cytotoxicity observed for bacteria and has potential for oxidative crosslinking of cuticle and proteinaceous materials at the interface.

Integration of Transcriptomic and Proteomic Approaches Provides a Core Set of Genes for Understanding of Scallop Attachment

Attachment is an essential physiological process in life histories of many marine organisms. Using a combination of transcriptomic and proteomic approach, scallop byssal proteins (Sbps) and their associated regulatory network genes were investigated for the first time. We built the first scallop foot transcriptome library, and 75 foot-specific genes were identified. Through integration of transcriptomic-proteomic approach, seven unique Sbps were identified. Of them, three showed significant amino acid sequence homology to known proteins. In contrast, the rest did not show significant protein matches, indicating they are possibly novel proteins. Our transcriptomic and proteomic analyses also suggest that post-translational modification may be one of the significant features for Sbps as well. Taken together, our study provides the first multidimensional collection of a core set of genes that may be potentially involved in scallop byssal attachment.

Protein Aggregation Formed by Recombinant cp19k Homologue of Balanus albicostatus Combined with an 18 kDa N-Terminus Encoded by pET-32a(+) Plasmid Having Adhesion Strength Comparable to Several Commercial Glues

The barnacle is well known for its tenacious and permanent attachment to a wide variety of underwater substrates, which is accomplished by synthesizing, secreting and curing a mixture of adhesive proteins termed “barnacle cement”. In order to evaluate interfacial adhesion abilities of barnacle cement proteins, the cp19k homologous gene in Balanus albicostatus (Balcp19k) was cloned and expressed in Escherichia coli. Here, we report an intriguing discovery of a gel-like super adhesive aggregation produced by Trx-Balcp19k, a recombinant Balcp19k fusion protein. The Trx-Balcp19k consists of an 18 kDa fragment at the N-terminus, which is encoded by pET-32a(+) plasmid and mainly comprised of a thioredoxin (Trx) tag, and Balcp19k at the C-terminus. The sticky aggregation was designated as “Trx-Balcp19k gel”, and the bulk adhesion strength, biochemical composition, as well as formation conditions were all carefully investigated. The Trx-Balcp19k gel exhibited strong adhesion strength of 2.10 ± 0.67 MPa, which was approximately fifty folds higher than that of the disaggregated Trx-Balcp19k (40 ± 8 kPa) and rivaled those of commercial polyvinyl acetate (PVA) craft glue (Mont Marte, Australia) and UHU glue (UHU GmbH & Co. KG, Germany). Lipids were absent from the Trx-Balcp19k gel and only a trace amount of carbohydrates was detected. We postulate that the electrostatic interactions play a key role in the formation of Trx-Balcp19k gel, by mediating self-aggregation of Trx-Balcp19k based on its asymmetric distribution pattern of charged amino acids. Taken together, we believe that our discovery not only presents a promising biological adhesive with potential applications in both biomedical and technical fields, but also provides valuable paradigms for molecular design of bio-inspired peptide- or protein-based materials.

Self-Assembly of Protein Nanofibrils Orchestrates Calcite Step Movement through Selective Nonchiral Interactions

The recognition of atomically distinct surface features by adsorbed biomolecules is central to the formation of surface-templated peptide or protein nanostructures. On mineral surfaces such as calcite, biomolecular recognition of, and self-assembly on, distinct atomic kinks and steps could additionally orchestrate changes to the overall shape and symmetry of a bulk crystal. In this work, we show through in situ atomic force microscopy (AFM) experiments that an acidic 20 kDa cement protein from the barnacle Megabalanus rosa (MRCP20) binds specifically to step edge atoms on {101̅4} calcite surfaces, remains bound and further assembles over time to form one-dimensional nanofibrils. Protein nanofibrils are continuous and organized at the nanoscale, exhibiting striations with a period of ca. 45 nm. These fibrils, templated by surface steps of a preferred geometry, in turn selectively dissolve underlying calcite features displaying the same atomic arrangement. To demonstrate this, we expose the protein solution to bare and fibril-associated rhombohedral etch pits to reveal that nanofibrils accelerate only the movement of fibril-forming steps when compared to undecorated steps exposed to the same solution conditions. Calcite mineralized in the presence of MRCP20 results in asymmetric crystals defined by frustrated faces with shared mirror symmetry, suggesting a similar step-selective behavior by MRCP20 in crystal growth. As shown here, selective surface interactions with step edge atoms lead to a cooperative regime of calcite modification, where templated long-range protein nanostructures shape crystals.

Natural antifouling compounds: Effectiveness in preventing invertebrate settlement and adhesion

Biofouling represents a major economic issue regarding maritime industries and also raise important environmental concern. International legislation is restricting the use of biocidal-based antifouling (AF) coatings, and increasing efforts have been applied in the search for environmentally friendly AF agents. A wide diversity of natural AF compounds has been described for their ability to inhibit the settlement of macrofouling species. However poor information on the specific AF targets was available before the application of different molecular approaches both on invertebrate settlement strategies and bioadhesive characterization and also on the mechanistic effects of natural AF compounds. This review focuses on the relevant information about the main invertebrate macrofouler species settlement and bioadhesive mechanisms, which might help in the understanding of the reported effects, attributed to effective and non-toxic natural AF compounds towards this macrofouling species. It also aims to contribute to the elucidation of promising biotechnological strategies in the development of natural effective environmentally friendly AF paints.

The chemistry of stalked barnacle adhesive (Lepas anatifera)

The results of the first chemical analysis of the adhesive of Lepas anatifera, a stalked barnacle, are presented. A variety of elements were identified in scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) of the adhesive, including Na, Mg, Ca, Cl, S, Al, Si, K and Fe; however, protein-metal interactions were not detected in Raman spectra of the adhesive. Elemental signatures from SEM-EDS of L. anatifera adhesive glands were less varied. Phosphorous was mostly absent in adhesive samples; supporting previous studies showing that phosphoserines do not play a significant role in adult barnacle adhesion. Disulfide bridges arising from Cys dimers were also investigated; Raman analysis showed weak evidence for S-S bonds in L. anatifera. In addition, there was no calcium carbonate signal in the attenuated total reflectance Fourier transform infrared spectra of L. anatifera adhesive, unlike several previous studies in other barnacle species. Significant differences were observed between the Raman spectra of L. anatifera and Balanus crenatus; these and a range of Raman peaks in the L. anatifera adhesive are discussed. Polysaccharide was detected in L. anatifera adhesive but the significance of this awaits further experiments. The results demonstrate some of the diversity within barnacle species in the chemistry of their adhesives.

Experimental strategies for the identification and characterization of adhesive proteins in animals: a review

Adhesive secretions occur in both aquatic and terrestrial animals, in which they perform diverse functions. Biological adhesives can therefore be remarkably complex and involve a large range of components with different functions and interactions. However, being mainly protein based, biological adhesives can be characterized by classical molecular methods. This review compiles experimental strategies that were successfully used to identify, characterize and obtain the full-length sequence of adhesive proteins from nine biological models: echinoderms, barnacles, tubeworms, mussels, sticklebacks, slugs, velvet worms, spiders and ticks. A brief description and practical examples are given for a variety of tools used to study adhesive molecules at different levels from genes to secreted proteins. In most studies, proteins, extracted from secreted materials or from adhesive organs, are analysed for the presence of post-translational modifications and submitted to peptide sequencing. The peptide sequences are then used directly for a BLAST search in genomic or transcriptomic databases, or to design degenerate primers to perform RT-PCR, both allowing the recovery of the sequence of the cDNA coding for the investigated protein. These sequences can then be used for functional validation and recombinant production. In recent years, the dual proteomic and transcriptomic approach has emerged as the best way leading to the identification of novel adhesive proteins and retrieval of their complete sequences.

Ionically cross-linked poly(allylamine) as a stimulus-responsive underwater adhesive: ionic strength and pH effects

Gel-like coacervates that adhere to both hydrophilic and hydrophobic substrates under water have recently been prepared by ionically cross-linking poly(allylamine) (PAH) with pyrophosphate (PPi) and tripolyphosphate (TPP). Among the many advantages of these underwater adhesives (which include their simple preparation and low cost) is their ability to dissolve on demand when exposed to high or low pH. To further analyze their stimulus-responsive properties, we have investigated the pH and ionic strength effects on the formation, rheology and adhesion of PAH/PPi and PAH/TPP complexes. The ionic cross-linker concentrations needed to form these adhesives decreased with increasing pH and ionic strength (although the complexes ceased to form when the parent solution pH exceeded ca. 8.5; i.e., the effective pKa of PAH). Once formed, their ionic cross-links were most stable (as inferred from their relaxation times) at near-neutral or slightly alkaline pH values (of roughly 6.5-9) and at low ionic strengths. The decrease in ionic cross-link stability within complexes prepared at other pH values and at elevated (150-300 mM) NaCl concentrations diminished both the strength and longevity of adhesion (although, under most conditions tested, the short-term tensile adhesion strengths remained above 10(5) Pa). Additionally, the sensitivity of PAH/PPi and PAH/TPP complexes to ionic strength was demonstrated as a potential route to injectable adhesive design (where spontaneous adhesive formation was triggered via injection of low-viscosity, colloidal PAH/TPP dispersions into phosphate buffered saline). Thus, while the sensitivity of ionically cross-linked PAH networks to pH and ionic strength can weaken their adhesion, it can also impart them with additional functionality, such as minimally invasive, injectable delivery, and ability to form and dissolve their bonds on demand.

Adhesive proteins of stalked and acorn barnacles display homology with low sequence similarities

Barnacle adhesion underwater is an important phenomenon to understand for the prevention of biofouling and potential biotechnological innovations, yet so far, identifying what makes barnacle glue proteins ‘sticky’ has proved elusive. Examination of a broad range of species within the barnacles may be instructive to identify conserved adhesive domains. We add to extensive information from the acorn barnacles (order Sessilia) by providing the first protein analysis of a stalked barnacle adhesive, Lepas anatifera (order Lepadiformes). It was possible to separate the L. anatifera adhesive into at least 10 protein bands using SDS-PAGE. Intense bands were present at approximately 30, 70, 90 and 110 kilodaltons (kDa). Mass spectrometry for protein identification was followed by de novo sequencing which detected 52 peptides of 7–16 amino acids in length. None of the peptides matched published or unpublished transcriptome sequences, but some amino acid sequence similarity was apparent between L. anatifera and closely-related Dosima fascicularis. Antibodies against two acorn barnacle proteins (ab-cp-52k and ab-cp-68k) showed cross-reactivity in the adhesive glands of L. anatifera. We also analysed the similarity of adhesive proteins across several barnacle taxa, including Pollicipes pollicipes (a stalked barnacle in the order Scalpelliformes). Sequence alignment of published expressed sequence tags clearly indicated that P. pollicipes possesses homologues for the 19 kDa and 100 kDa proteins in acorn barnacles. Homology aside, sequence similarity in amino acid and gene sequences tended to decline as taxonomic distance increased, with minimum similarities of 18–26%, depending on the gene. The results indicate that some adhesive proteins (e.g. 100 kDa) are more conserved within barnacles than others (20 kDa).

Self-assembly of stiff, adhesive and self-healing gels from common polyelectrolytes

Underwater adhesion has numerous potential medical, household, and industrial applications. It is typically achieved through covalent polymerization and cross-linking reactions and/or the use of highly specialized biological or biomimetic polymers. As a simpler alternative to these covalent and biomimetic strategies, this article shows that stiff, gel-like complexes that adhere to various substrates under water can also be prepared through the ionic cross-linking of common, commercial polyelectrolytes. The gels form spontaneously when synthetic polycations, such as poly(allylamine) (PAH), are mixed with strongly binding multivalent anions, pyrophosphate (PPi) and tripolyphosphate (TPP). The PAH/PPi and PAH/TPP gels exhibit very high storage moduli (G∞′ ≈ 400 kPa), self-heal when torn, and adhere to both hydrophilic and hydrophobic substrates under water (with short-term tensile adhesion strengths of 350–450 kPa). Furthermore, these gels can be dissolved on demand (if adhesion needs to be reversed) by changing the ambient pH, which controls the ionization state of the polyelectrolyte and ionic cross-linker. These properties suggest that synthetic polycations cross-linked with PPi and TPP ions could provide a simple, inexpensive, and scalable platform for underwater adhesion.

First study on gene expression of cement proteins and potential adhesion-related genes of a membranous-based barnacle as revealed from Next-Generation Sequencing technology

This is the first study applying Next-Generation Sequencing (NGS) technology to survey the kinds, expression location, and pattern of adhesion-related genes in a membranous-based barnacle. A total of 77,528,326 and 59,244,468 raw sequence reads of total RNA were generated from the prosoma and the basis of Tetraclita japonica formosana, respectively. In addition, 55,441 and 67,774 genes were further assembled and analyzed. The combined sequence data from both body parts generates a total of 79,833 genes of which 47.7% were shared. Homologues of barnacle cement proteins - CP-19K, -52K, and -100K - were found and all were dominantly expressed at the basis where the cement gland complex is located. This is the main area where transcripts of cement proteins and other potential adhesion-related genes were detected. The absence of another common barnacle cement protein, CP-20K, in the adult transcriptome suggested a possible life-stage restricted gene function and/or a different mechanism in adhesion between membranous-based and calcareous-based barnacles.

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.

Characterization of two 20kDa-cement protein (cp20k) homologues in Amphibalanus amphitrite

The barnacle, Amphibalanus amphitrite, is a common marine fouling organism. Understanding the mechanism of barnacle adhesion will be helpful in resolving the fouling problem. Barnacle cement is thought to play a key role in barnacle attachment. Although several adult barnacle cement proteins have been identified in Megabalanus rosa, little is known about their function in barnacle settlement. In this study, two homologous 20k-cement proteins (cp20k) in Amphibalanus amphitrite, named Bamcp20k-1 and Bamcp20k-2, were characterized. The two homologues share primary sequence structure with proteins from other species including Megabalanus rosa and Fistulobalanus albicostatus. The conserved structure included repeated Cys domains and abundant charged amino acids, such as histidine. In this study we demonstrated that Bamcp20k-1 localized at the α secretory cells in the cyprid cement gland, while Bamcp20k-2 localized to the β secretory cells. The differential localizations suggest differential regulation for secretion from the secretory cells. Both Bamcp20k-1 and Bamcp20k-2 from cyprids dissolved in PBS. However, adult Bamcp20k-2, which was dominant in the basal shell of adult barnacles, was largely insoluble in PBS. Solubility increased in the presence of the reducing reagent Dithiothreitol (DTT), suggesting that the formation of disulfide bonds plays a role in Bamcp20k-2 function. In comparison, Bamcp20k-1, which was enriched in soft tissue, could not be easily detected in the shell and base by Western blot and easily dissolved in PBS. These differential solubilities and localizations indicate that Bamcp20k-1 and Bamcp20k-2 have distinct functions in barnacle cementing.

Mini-review: barnacle adhesives and adhesion

Barnacles are intriguing, not only with respect to their importance as fouling organisms, but also in terms of the mechanism of underwater adhesion, which provides a platform for biomimetic and bioinspired research. These aspects have prompted questions regarding how adult barnacles attach to surfaces under water. The multidisciplinary and interdisciplinary nature of the studies makes an overview covering all aspects challenging. This mini-review, therefore, attempts to bring together aspects of the adhesion of adult barnacles by looking at the achievements of research focused on both fouling and adhesion. Biological and biochemical studies, which have been motivated mainly by understanding the nature of the adhesion, indicate that the molecular characteristics of barnacle adhesive are unique. However, it is apparent from recent advances in molecular techniques that much remains undiscovered regarding the complex event of underwater attachment. Barnacles attached to silicone-based elastomeric coatings have been studied widely, particularly with respect to fouling-release technology. The fact that barnacles fail to attach tenaciously to silicone coatings, combined with the fact that the mode of attachment to these substrata is different to that for most other materials, indicates that knowledge about the natural mechanism of barnacle attachment is still incomplete. Further research on barnacles will enable a more comprehensive understanding of both the process of attachment and the adhesives used. Results from such studies will have a strong impact on technology aimed at fouling prevention as well as adhesion science and engineering.

Barnacle Balanus amphitrite adheres by a stepwise cementing process

Barnacles adhere permanently to surfaces by secreting and curing a thin interfacial adhesive underwater. Here, we show that the acorn barnacle Balanus amphitrite adheres by a two-step fluid secretion process, both contributing to adhesion. We found that, as barnacles grow, the first barnacle cement secretion (BCS1) is released at the periphery of the expanding base plate. Subsequently, a second, autofluorescent fluid (BCS2) is released. We show that secretion of BCS2 into the interface results, on average, in a 2-fold increase in adhesive strength over adhesion by BCS1 alone. The two secretions are distinguishable both spatially and temporally, and differ in morphology, protein conformation, and chemical functionality. The short time window for BCS2 secretion relative to the overall area increase demonstrates that it has a disproportionate, surprisingly powerful, impact on adhesion. The dramatic change in adhesion occurs without measurable changes in interface thickness and total protein content. A fracture mechanics analysis suggests the interfacial material's modulus or work of adhesion, or both, were substantially increased after BCS2 secretion. Addition of BCS2 into the interface generates highly networked amyloid-like fibrils and enhanced phenolic content. Both intertwined fibers and phenolic chemistries may contribute to mechanical stability of the interface through physically or chemically anchoring interface proteins to the substrate and intermolecular interactions. Our experiments point to the need to reexamine the role of phenolic components in barnacle adhesion, long discounted despite their prevalence in structural membranes of arthropods and crustaceans, as they may contribute to chemical processes that strengthen adhesion through intermolecular cross-linking.

Unusual adhesive production system in the barnacle Lepas anatifera: an ultrastructural and histochemical investigation

Adhesives that are naturally produced by marine organisms are potential sources of inspiration in the search for medical adhesives. Investigations of barnacle adhesives are at an early stage but it is becoming obvious that barnacles utilize a unique adhesive system compared to other marine organisms. The current study examined the fine structure and chemistry of the glandular system that produces the adhesive of the barnacle Lepas anatifera. All components for the glue originated from large single-cell glands (70-180 μm). Staining (including immunostaining) showed that L-3,4-dihydroxyphenylalanine and phosphoserine were not present in the glue producing tissues, demonstrating that the molecular adhesion of barnacles differs from all other permanently gluing marine animals studied to date. The glandular tissue and adhesive secretion primarily consisted of slightly acidic proteins but also included some carbohydrate. Adhesive proteins were stored in cytoplasmic granules adjacent to an intracellular drainage canal (ICC); observations implicated both merocrine and apocrine mechanisms in the transport of the secretion from the cell cytoplasm to the ICC. Inside the ICC, the secretion was no longer contained within granules but was a flocculent material which became "clumped" as it traveled through the canal network. Hemocytes were not seen within the adhesive "apparatus" (comprising of the glue producing cells and drainage canals), nor was there any structural mechanism by which additions such as hemocytes could be made to the secretion. The unicellular adhesive gland in barnacles is distinct from multicellular adhesive systems observed in marine animals such as mussels and tubeworms. Because the various components are not physically separated in the apparatus, the barnacle adhesive system appears to utilize completely different and unknown mechanisms for maintaining the liquid state of the glue within the body, as well as unidentified mechanisms for the conversion of extruded glue into hard cement.

Protein-based underwater adhesives and the prospects for their biotechnological production

Biotechnological approaches to practical production of biological protein-based adhesives have had limited success over the last several decades. Broader efforts to produce recombinant adhesive proteins may have been limited by early disappointments. More recent synthetic polymer approaches have successfully replicated some aspects of natural underwater adhesives. For example, synthetic polymers, inspired by mussels, containing the catecholic functional group of 3,4-L-dihydroxyphenylalanine adhere strongly to wet metal oxide surfaces. Synthetic complex coacervates inspired by the Sandcastle worm are water-borne adhesives that can be delivered underwater without dispersing. Synthetic approaches offer several advantages, including versatile chemistries and scalable production. In the future, more sophisticated mimetic adhesives may combine synthetic copolymers with recombinant or agriculture-derived proteins to better replicate the structural and functional organization of natural adhesives.

Atomic force microscopy of the morphology and mechanical behaviour of barnacle cyprid footprint proteins at the nanoscale

Barnacles are a major biofouler of man-made underwater structures. Prior to settlement, cypris larvae explore surfaces by reversible attachment effected by a 'temporary adhesive'. During this exploratory behaviour, cyprids deposit proteinaceous 'footprints' of a putatively adhesive material. In this study, footprints deposited by Balanus amphitrite cyprids were probed by atomic force microscopy (AFM) in artificial sea water (ASW) on silane-modified glass surfaces. AFM images obtained in air yielded better resolution than in ASW and revealed the fibrillar nature of the secretion, suggesting that the deposits were composed of single proteinaceous nanofibrils, or bundles of fibrils. The force curves generated in pull-off force experiments in sea water consisted of regions of gradually increasing force, separated by sharp drops in extension force manifesting a characteristic saw-tooth appearance. Following the relaxation of fibrils stretched to high strains, force-distance curves in reverse stretching experiments could be described by the entropic elasticity model of a polymer chain. When subjected to relaxation exceeding 500 ms, extended footprint proteins refolded, and again showed saw-tooth unfolding peaks in subsequent force cycles. Observed rupture and hysteresis behaviour were explained by the 'sacrificial bond' model. Longer durations of relaxation (>5 s) allowed more sacrificial bond reformation and contributed to enhanced energy dissipation (higher toughness). The persistence length for the protein chains (L(P)) was obtained. At high elongation, following repeated stretching up to increasing upper strain limits, footprint proteins detached at total stretched length of 10 microm.

Underwater adhesive of marine organisms as the vital link between biological science and material science

Marine sessile organisms naturally attach themselves to diverse materials in water by a technique that has so far remained unreproducible. Recent studies on the holdfast of marine sessile organisms have revealed natural concepts that are currently beyond our understanding with respect to the molecular design and macroscopic range. The combination of valuable and practical natural design of biotic adhesives as biomolecular materials, together with continuing efforts towards mimetic design, hold the promise of revolution for future materials. This review focuses on recent advances in the study of barnacle underwater cement, a protein complex whose constituents and the properties of individual components are being uncovered. A comparison is made with the model systems used by the mussel and tubeworm.

The adhesive strategies of cyprids and development of barnacle-resistant marine coatings

Over the last decade, approaches to the development of surfaces that perturb settlement and/or adhesion by barnacles have diversified substantially. Although, previously, coatings research focussed almost exclusively on biocidal technologies and low modulus, low surface-free-energy 'fouling-release' materials, novel strategies to control surface colonisation are now receiving significant attention. It is timely, therefore, to review the current 'state of knowledge' regarding fouling-resistant surface characteristics and their mechanisms of action against settling larvae of barnacles. The role of the barnacle in marine fouling is discussed here in the context of its life cycle and the behavioural ecology of its cypris larva. The temporary and permanent adhesion mechanisms of cyprids are covered in detail and an overview of adult barnacle adhesion is presented. Recent legislation has directed academic research firmly towards environmentally inert marine coatings, so the actions of traditional biocides on barnacles are not described here. Instead, the discussion is restricted to those surface modifications that interfere with settlement-site selection and adhesion of barnacle cypris larvae; specifically, textural engineering of surfaces, development of inert 'non-fouling' surfaces and the use of enzymes in antifouling.