See seaweeds in a new light: secret behind iridescent ‘Irish Moss’ unlocked

The shifting colours of iridescence are more often associated with soap bubbles or oil slicks than with edible plants. But this new study reveals the origin of this effect in a seaweed.

Irish moss, or Chondrus crispus, is a North Atlantic species of red seaweed that is eaten in various forms around the world.

Researchers found that at the tips of the seaweed fronds, the protective outer layer known as cuticle consists of several transparent layers. When these layers are submerged in water, they bend light in such a way that blue light is reflected. As a result, the seaweed becomes iridescent.

The scientists fired beams of electrons through cross-sections of C. crispus 1.000 times thinner than a human hair, using a technique called Transmission Electron Microscopy (TEM).

Professor Juliet Brodie, a research leader at the Natural History Museum and co-author of the paper, said, “We have conclusively shown the mechanism of colour production in Irish Moss for the first time. Our findings could be useful in research on the impact of climate change and for new applications of material properties such as UV protection.”

She continues: “If you live near the sea, you can see Irish Moss iridescence for yourself. On a bright, sunny day, their iridescence is best observed along 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.”

The purpose of iridescence in seaweed is not yet known. It may protect the plants from harmful ultraviolet radiation. The tips of the fronds, where iridescence is strongest, are the most vulnerable part– this is where growth takes place, so if the tips are removed then the plant’s chances of survival could be reduced. This suggests that iridescence may play a role in confusing predators, affording extra protection to the seaweed.

The images revealed that The tips of the fronds, where iridescence is strongest, are the most vulnerable part of C. crispus – this is where growth takes place, so if the tips are removed then the plant’s chances of survival could be reduced. This suggests that iridescence may play a role in confusing predators, affording extra protection to the seaweed.

Dr Silvia Vignolini, a co-author 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."

Irish Moss is not just pleasing to look at, however – it is also useful. It is the basis for a thickening agent that is used in a variety of dairy and meat products under the name E407. In Scotland and Ireland, the seaweed is also used to make a sweet similar to pannacotta. In fact, studies in World War II assessed the stocks of C. crispus around the UK coast because it was a valuable source of food at a time of strict rationing.

Irish Moss was also important in the production of an agar substitute, which was needed to culture microbes such as penicillin, an antibiotic that has saved millions of lives around the world.

Notes for editors

Images: Please download and credit: © The Trustees of the Natural History Museum, London (Not for publication)

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