Irish moss changes hue because of plate-like structures – and the finding could lead to waterproof sunblock

[UK] It is used to thicken ice cream, brew beer and is eaten fresh as a tangy salad, but only now have scientists discovered what makes the Irish moss seaweed iridesce.

The red algae Chondrus crispus, which grows abundantly around the coast of Britain, Ireland, the US, Canada and Japan, can take on a blue shine when it is submerged in water.

Researchers found that rather than being caused by pigment, the colour is produced by transparent layers in the plant’s tips.

The image above shows the relfectivity of light at the tips (a) of Irish moss and towards the base (b). The thicker skin of the seaweed at the base prevents the light reaching the transparent plates beneath
The image above shows the relfectivity of light at the tips (a) of Irish moss and towards the base (b). The thicker skin of the seaweed at the base prevents the light reaching the transparent plates beneath

The findings could lead to new materials that can help to protect swimmers from harmful UV light while under the water.

These plate-like layers reflect blue light when wet while closer to the base of the seaweed frond where the skin around the algae is thicker less light is reflected and so they remain red.

The effect only occurs when the seaweed is wet as the water helps give the plates the structure they need to reflect the light.

It is one of a few examples of iridescence that occur living creatures when wet. For example the iridescent features of mourning doves increase by 50 per cent when water is added.

Researchers from the University of Cambridge and the Natural History Museum in London say it is still not completely clear why the seaweed has developed this iridescence, but say it might help protect the plant from damage by ultraviolet light.

Writing in the journal Scientific Reports, Silvia Vignolini, a chemist at the University of Cambridge, and her colleagues said: ‘The reflectance of Chondrus crispus fronds is maximal in the ultraviolet to blue wavelength range, from ~320 to 430 nm.

‘Reflectance in these wavelengths may therefore function to protect against excessive levels of UV-blue light.

‘Although individuals are likely to dehydrate and lose their structural colour within the intertidal zone, many individuals inhabit rock pools and thus remain hydrated with the potential to use structurally-based reflectance to reduce their overall levels of UV absorption.’

Chondrus crispus is harvested around the coast for use by the food industry as a thickener and stabiliser. When dried it turns yellow but when wet it has reddish colour and the tips almost glow blue (as above)
Chondrus crispus is harvested around the coast for use by the food industry as a thickener and stabiliser. When dried it turns yellow but when wet it has reddish colour and the tips almost glow blue (as above)

Chondrus cripus is the industrial source of carrageenan, which is used as a thickener and stabiliser in ice cream and other milk based products.

It is also considered to be a highly nutrious ‘superfood’, rich in omega-3 oils and anti-oxidants.

The researchers used detailed images of Chondrus crispus under a microscope both when hydrated and dehydrated.

They found when hydrated with sea water the tops of the plant had a vivid blue colour that could be seen under the microscope that disappeared as they dried.

They found the carbohydrates in Chondus crispus are rich in chemicals called sulphated polysaccharides which help retain water and cause swelling.

This causes a change in the spaces between the plate-like layers and so changes how it reflects.

The images above show how when wet the structures in the tips reflect blue light (left) but when dried the structures collapse and the intensity of the light reflected decreases dramatically (shown on the right)
The images above show how when wet the structures in the tips reflect blue light (left) but when dried the structures collapse and the intensity of the light reflected decreases dramatically (shown on the right)

Speaking to New Scientist, Professor Juliet Brodie, from the Natural History Museum in London who was part of the research team, said such structures could help to explain why the marine environment is so full of colour.

She said it may also be possible to use the findings to develop new swimwear or even sunscreens that could help to protect the wearer from UV light while in the water.

She said: ‘We don’t entirely know why this iridescence has evolved in the seaweed, but it could be a sunscreen or a way of controlling photosynthesis.

The cuticle layers on the tip of the seaweed (shown in the microscope image above) retain water, causing them to swell and so changing how they reflect light. The scientists believe it may be a natural sunscreen
The cuticle layers on the tip of the seaweed (shown in the microscope image above) retain water, causing them to swell and so changing how they reflect light. The scientists believe it may be a natural sunscreen

‘They live in tidal areas and as the water levels drop they may be exposed to more UV and blue light, so by reflecting it there might be a way of optimising photosynthesis.

‘Thinking laterally, the kind of structures we see could provide the inspiration for making materials that deflect UV light underwater.’

 

 

Photo: The seaweed Irish moss (shown above) has a deep red colour except at its tips where the fronds take on an eerie iridescent blue colour. Now for the first time scientists have unraveled what causes this and found the plate-like layers at the tips swell in sea water to reflect blue light to produce the effect (shown in the inset)

View original article at: Irish moss changes hue because of plate-like structures – and the finding could lead to waterproof sunblock

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