How do sea squirts stay attached to rocks amid crashing waves and strong currents? Recent research has revealed that sea squirts do not simply secrete adhesive substances. Instead, they possess a unique system where they package these materials into nano-sized (nm) condensates, deliver them to the destination, and then unpack them for use on-site.
Professor Dong Soo Hwang of the Division of Environmental Science and Engineering, the Division of interdisciplinary Bioscience & Bioengineering, and the Graduate School of convergence Science and Technology at POSTECH (Pohang University of Science and Technology) has elucidated a previously unknown internal delivery mechanism for underwater adhesive materials in sea squirts by studying their rhizoids. This study was published in the online edition of the Proceedings of the National Academy of Sciences (PNAS).
Around the world, "ocean desertification" — the rapid disappearance of seaweed due to rising water temperatures and pollution—is accelerating. As the seafloor becomes barren, efforts are being made to artificially cultivate seaweed and transplant it back into the ocean. However, a recurring problem has been the seaweed's inability to properly attach to rocks or the seabed during its early growth stages. Although researchers investigated how marine organisms use root-like rhizoids to anchor themselves stably, the complex mechanism remained a mystery.
The research team found a crucial clue while analyzing the underwater adhesive proteins secreted from sea squirt rhizoids. They discovered that sea squirts do not secrete adhesive proteins in a simple liquid state. Instead, they combine the proteins with metal ions to create solid nanocondensates, which are tightly packaged within cells for transport. These nanocondensates, coordinated with ions such as iron (Fe), chromium (Cr), and vanadium (V), were found to act as a "protective case," shielding the proteins from the external environment as they move through the body.
Once the nanocondensates are secreted outside the cell and reach the cuticle layer of the rhizoid, the particle structure rearranges, activating the adhesive proteins within. Notably, the researchers observed a transition in the role of the metal ions: while they contribute to structural stabilization during transport, they detach and are released during the actual adhesion phase.
This mechanism is distinct from the adhesion strategy of mussels, another marine organism. In mussels, the amino acid "DOPA" within adhesive proteins binds directly with metal ions to create strong adhesion. In other words, while the binding with metal ions is the core principle of adhesion in mussels, for sea squirts, metal ions play a key role in safely "delivering" the adhesive materials. Sea squirts have uniquely solved the problem of how to deliver the adhesive rather than just the composition of the adhesive itself.
This study provides important clues for understanding why seaweed often fails to attach stably to rocks in its early stages. Based on these findings, the research is expected to contribute to the development of bio-adhesives that can assist in the early attachment of seaweed or function reliably in underwater environments.
Professor Dong Soo Hwang stated, "The biggest challenge in creating sea forests and cultivating seaweed has been the issue of early attachment, but through this research, we have come to understand the principles of the rhizoid adhesion system. This could become a significant turning point that contributes to addressing climate change, marine ecosystem restoration, and even solving food security issues."
This research was supported by Korea Institute of Marine Science & Technology Promotion(KIMST) funded by the Ministry of Oceans and Fisheries.
