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Shining seaweeds reveal the secret to their iridescence

The common British seaweed Irish moss displays a blue shine using unique structures that bend light, according to a new study.

In bright sunlight, the tips of the seaweed Chondrus crispus, known as Irish moss, display a shifting bluish colour much like that seen on oil slicks or soap bubbles. The seaweed tips bend sunlight to reflect blue and ultraviolet light, scientists have shown.   

The seaweed has a protective coating known as a cuticle. At the growing tip of the seaweed’s fronds, this cuticle has multiple layers that reflect and bend the light to create an iridescent effect. The investigation into the structure of this cuticle is published today in the journal Nature Scientific Reports by a team that includes Museum Research Chair Prof Juliet Brodie.

The type of colour production discovered is known as structural colour. It is created by structures in the seaweed, rather than by a chemical pigment such as the melanin that colours human skin. It’s not just the structure of the cuticle layers that is important for the seaweed - the layers also need to be wet to reflect the light.

Beauty and substance

The function of iridescence in C. crispus is unknown, but the fact that it strongly reflects ultraviolet light could be a clue to its purpose. Prof Brodie said: ‘C. crispus lives in intertidal habitats -  shallow areas such as rock pools where the seaweed tips in particular are exposed to the Sun. Reflecting UV light may offer them some protection from potentially damaging radiation.’

Only the tips of the seaweed, where growth takes place, are iridescent. This makes the tips the most vulnerable sections, suggesting another possible function - confusing prey and preventing the tips from being eaten.   

Climate change and fabulous fabrics

C. crispus could be among the first species to be affected by rising seawater temperatures caused by climate change. They are already stressed because they live close to the ocean’s surface, making them particularly vulnerable to warming waters. ‘This species could be an indicator species - a sort of canary in the coal mines,’ said Prof Brodie.

The reflective properties of seaweeds could be of benefit to us when looking for new ways to create colours through biomimetics. Biomimetics is the imitation of nature’s structures and materials to solve human problems, and the mechanisms used by seaweeds could point to new ways of producing colour fabrics and paints.

Dr Silvia Vignolini, one of the study’s co-authors, from the Department of Chemistry at the University of Cambridge, said: ‘Understanding the materials and the structures that are responsible for the production of colour without using toxic pigment inspires us to produce novel materials with brighter-coloured appearances.’

See for yourself

C. crispus already has a range of uses around the world, from a food source to a thickening agent for jellies and ice creams. It was also investigated as a substitute for agar - a material used for growing useful bacteria such as penicillin - during World War II.

Now is a good time to spot iridescent seaweeds, according to Prof Brodie. ‘If you live near the sea, you can see Irish moss iridescence for yourself. On a bright, sunny day their iridescence is best observed in rock pools on rocky shores. Late spring and early summer are particularly good times to see it, so it really is prime time for seaweed watching right now.’