Ocean Allies: How Sponges, Corals, and Sea Cucumbers Inspire Cleaner, Greener Anti-Fouling Coatings

The Ocean's Secret Weapon: Natural Chemical Anti-Foulants from Sessile Marine Life

Written by: Monica Medappa .PhD

For centuries, keeping the hulls of ships free from biological attachments has been a costly and destructive process of biofouling has been a major challenge ^{1, 2, 3}. Biofouling, which starts with an invisible film of organic particles and bacteria, progresses to a tenacious layer of algae, mussels, and barnacles, causing billions of dollars in damage and disruptions annually ⁴. It increases hydrodynamic drag and decreases maneuverability of naval and merchant ships, which in turn increases fuel consumption and voyage costs ⁵.

The Problem with Traditional Anti-Fouling

The most effective traditional solution was the coating tributyltin (TBT). However, TBT was found to be highly toxic and soluble in seawater⁶, interfering with the endocrine system of marine mollusks and causing severe environmental issues, such as sexual abnormalities⁷. This critical environmental impact led to a global ban on TBT antifouling coatings around 2008⁸.

Synthetic antifouling agents like 4,5-dichloro-2-octylisothiazolone (DCOIT) ⁹and the biocide Econea (4-bromo-2-(4-chlorophenyl)-5-(trifluoromethyl)-1H-pyrrole-3-carbonitrile)¹⁰ effectively combat fouling, yet their potential toxicity to non-target marine species demands thorough environmental evaluation¹¹.

Since the TBT ban, an efficient, environmentally acceptable replacement coating with a long half-life has not yet been fully developed. This has driven urgent research towards biomimetic (nature-inspired)¹² and bio-inspired solutions. The ocean's own sessile (non-moving) organisms, which have evolved sophisticated defenses to keep their surfaces clean¹³, are providing the blueprint.

How Biofouling Works (and How Nature Stops It)

What is biofouling and how can we stop it?⁴ provides a simple outline of the steps in biofouling. Biofouling is a sequential, multi-step process:

1. Conditioning Film: The rapid formation of a conditioning film (adsorption of organic particles) on a submerged surface.

2. Biofilm Formation: This is quickly followed by the colonization of bacteria forming a sticky, protective biofilm.

3. Macro-Fouling: Finally, the settlement of larger organisms like barnacles and algae (macro-foulers) occurs on the surface.

The process of Biofouling

Sessile marine invertebrates, such as sponges and soft corals, have developed specialized chemical warfare to disrupt this process. Their key strategies include:

Quorum Sensing Interference ¹⁴: Bacteria in a biofilm communicate via quorum sensing (QS) to coordinate their growth. A key strategy in some organisms is the production of compounds that directly interfere with the anti-quorum sensing/bacterial communication network to disrupt biofilm formation and coordination¹⁵.

Direct Toxicity¹⁶: Producing direct-acting secondary metabolites or toxic peptides to poison or impair microorganisms and inhibit the growth of macro-fouling animals¹⁶.

Surface Defense¹⁷: Some sponges utilize a natural coating of bio-silica which is suggested to inhibit the initial conditioning film, preventing the first step in the biofouling sequence¹⁷.

Key Anti-Fouling Organisms and Compounds

1. Marine sponges

These can be likened to 'little factories for bioactive secondary metabolites'¹⁶. They rely on chemical defenses for survival, including fighting off competition, deterring predators, and controlling surface fouling¹⁶.

Venus’ flower basket (Euplectella aspergillum) found in deep waters of the Pacific

Terpenoids: One of the most potent and diverse groups of molecules utilized by sponges¹⁸. The sponge terpenoids ageloxime-D and manoalide are examples of compounds reported to interfere with bacterial biofilm formation without disrupting cellular growth¹⁸. Manoalide, from Luffariella variabilis, is effective as a Quorum Sensing Inhibitor (QSI) against both Gram-positive and Gram-negative species¹⁴.

Pyrrole-Imidazole Alkaloids (PIAs)¹⁹: A class of potent molecules found exclusively in marine sponges.

Dihydrooroidin (DHO), an easily synthesized variant of the PIA oroidin, showed strong potential as an antifouling agent. When combined with marine-based paint and tested in tank trials, the DHO paint had 125% less biomass than the paint-only controls after three weeks. Importantly, DHO was found to be non-toxic in mammalian cytotoxicity assays up to six times the dosage used in the paint trials²⁰.

Derivatives of the 2-aminoimidazole (2-AI) moiety, found in many PIAs, have been studied for their ability to act as non-microbicidal biofilm modulators. The lead 2-AIT, SPAR, has shown promise as an adjuvant for conventional antibiotic therapy, able to resensitize drug-resistant bacteria like MRSA to antibiotics²¹.

2. Sea Cucumbers (Class Holothuroidea)

These echinoderms have developed chemical defenses, primarily triterpene glycosides, to protect themselves from predators²².

A common, large sea cucumber (Bohadschia argus)

Saponins (Triterpene Glycosides): These compounds are naturally potent antimicrobial, antifungal, and cytotoxic agents²².

Studies on various sea cucumber species, including Holothuria atra, H. nobilis, and H. glaberrima, have shown that saponins were responsible for the observed anti-fouling activities²³.

Crude extracts from the genera Bohadschia and Actinopyga showed stronger anti-fouling activities than Holothuria species, which is thought to be due to significantly higher concentrations of total saponins²³.

3. Soft Corals (Class Anthozoa)

Soft corals belonging to the clade Anthozoa show the highest number of promising bioactive compounds among cnidarians. About 7% of compounds described in cnidarians show antifouling activity²⁴.

Soft coral

Cembranoids and Sesquiterpenes: Several cembranoid compounds from the Caribbean gorgonian Pseudoplexaura flagellosa can disrupt quorum sensing and inhibiting biofilm maturation²⁵.

A sesquiterpene compound from the gorgonian Echinogorgia pseudossapo showed significant anti-settlement activity towards barnacle larvae²⁶.

Pukalide Derivatives and Steroids: Pukalide derivatives²⁷ and 2,11-cyclized diterpenoids have displayed pronounced antifouling activity²⁸.

Two steroids isolated from the gorgonian coral Subergorgia suberosa inhibited the settlement of bryozoan larvae²⁶.

Conclusion: Harnessing Nature for Sustainable Energy Infrastructure

The global movement away from toxic TBT to environmentally sound solutions is essential for the future of renewable marine energy.

Wavegen is aiming to develop innovative wave energy converters that rely on large, permanently submerged structures in the harsh marine environment. Biofouling on these structures can drastically reduce efficiency, impede necessary maintenance, and lead to corrosive damage - all factors that undermine the commercial viability of wave energy.

The research into natural chemical anti-foulants offers a pathway to sustainable operations for Wavegen and the entire ocean energy sector. Biomimetic compounds like Dihydrooroidin (DHO), from sponges, or effective quorum-sensing inhibitors from soft corals, provide the key to creating next-generation coatings that are highly effective yet non-toxic to the surrounding ecosystem.

Wavegen is committed to not only utilizing these natural solutions but exploring the most ethical and sustainable ways to synthesize and apply them.

How can Wavegen leverage these nature-inspired antifoulants? Through research into scalable, eco-friendly synthesis, the company envisions coatings that boost wave energy performance without harming marine life.  This commitment includes investing in responsible, large-scale synthesis of these naturally inspired, environmentally friendly antifoulants, ensuring the supply chain is sound and the ecological impact is minimal. By strategically integrating these non-toxic, biomimetic compounds, Wavegen can ensure its marine infrastructure maintains peak efficiency and longevity.

By investing in the development and large-scale synthesis of these naturally inspired, environmentally friendly antifoulants. The wisdom of 800-million-year-old sessile organisms is now poised to power the future.

What are your thoughts on biomimetic anti-foulants? Comment below or email us to join the conversation on sustainable ocean tech.