Promoting Intertidal Sciences blog | Dr Mike Kent | Nov 26th 2018

The sight of a rainbow arching across an otherwise grey sky at Polzeath in late November quickened my pulse and filled me with a sense of awe and wonder. Its sheer beauty was overwhelming. It might surprise you that a slimy seaweed can evoke similar sensations for similar reasons.

Cystoseira tamariscofolia is a type of brown alga found in the Mediterranean Sea and off the Atlantic coast as far north as Cornwall (and common in the pools at Baby Bay). From its description as “… a bushy seaweed, up to 60 cm in length but usually 30-45 cm, with a cylindrical frond and irregular branches”, you would not think it anything special. And take it out of water, it appears very ordinary. But view it on a sunny day, fully submerged in the clear and crisp waters of a Cornish rock pool, and you will realise why the common name of this species is the Rainbow Wrack. Under water, the Rainbow Wrack glimmers and glows to produce a blue-green sheen and a shimmering array of other colours.

Unlike bioluminescent fish and plankton, the Rainbow Wrack does not produce its own colourful display. Instead, the wrack has nano-sized oil droplets which form opal-like structures in the fronds. These droplets refract, reflect and interfere with light in much the same way as opal-like hard structures on the body surface of iridescent beetles and butterflies. But unlike the structures of insects, those of the Rainbow Wrack are inside the seaweed and are made of liquid. What’s even more interesting is that researchers led by Drs Heather Whitney and Ruth Oulton at Bristol University have found that Rainbow Wrack varies its iridescence according to the conditions. In dim light, it self-assembles its oil droplets into a tightly packed lattice to produce a strong iridescence; in bright sunlight, it loosens the lattice to dim its iridescent glow. When the seaweed dies, the lattice of oil-droplets break down and the seaweed loses its ability to glow.

The opal-like oil-droplets are surrounded by chloroplasts (the cellular structures responsible for photosynthesis) suggesting to the researches that changes in the packing of the droplets might regulate the amount of light reaching the chloroplasts. The Bristol researchers claim that if they can fully understand and control the mechanism, they could produce biodegradable, switchable display technology which could be used in labeling and packaging, or to make very efficient, low cost solar cells.

You can find out more about this fascinating topic in the recent scientific paper written by the Bristol team (M. Lopez-Garcia et al., 2018 Light-induced dynamic structural color by intracellular 3D photonic crystals in brown algae. Sci. Adv. 4, eaan8917). But if you just want to enjoy the spectacular optical delights of the Rainbow Weeds, go down to Baby Bay, search in the rock pools and you are almost guaranteed to find this most beautiful of seaweeds and, as the Psalmist instructs

Take a good look at God’s wonders—

they’ll take your breath away. (Psalm 66,5)