Plant Cuttings

This image, of “L’algue rouge Asparagopsis taxiformis à la Réunion (lagon de Saint-Leu)” by Jean-Pascal Quod, is used under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Colour is a most attractive and arresting property of many living things. It is achieved in three ways (Atrouli Chatterjee, 2022), pigments (Qian Tang et al., 2025) and dyes (JB Stothers & Edward Noah Abrahart), bioluminescence (Katie Pavid), and structural means (Yosola Olorunshola).

Colour is traditionally the way that seaweeds are categorised, into ‘greens’ (e.g., sea lettuce), ‘browns’ (e.g., kelp and wrack), and ‘reds’ (e.g., laver). The different colours in those macroalgae are achieved by combinations of pigments, most of which play a role in capturing sunlight energy for photosynthesis. For example, red seaweeds contain the green pigment chlorophyll (Paul May; Robert A Andersen & Ralph A Lewin), the blue pigment R-phycocyanin (Zhengxin Chen et al., 2023), and the red pigment R-phycoerythrin (Chen et al., 2023), all of which have roles in photosynthesis (Aline P Martins et al., 2023). But, colouration in seaweeds isn’t necessarily uniform throughout the plant, nor confined to pigments, or relevant to photosynthesis. Which is where the concept of structural colour* comes into the story.

Although categorised as a red seaweed, the fronds of Asparagopsis taxiformis**^, *** have distinctive blue tips, which are sometimes white in colour. Not unsurprisingly, these colourful structures caught the eye of Hiroshi Kawai & Taizo Motomura (2025). Using transmission electron microscopy (TEM) to look at the very-small-sized, fine-structural details of cells in those blue/white tips, they examined the gland cells in this seaweed. TEM revealed that these specialised cells contained a ‘large refractile body’ which hosts numerous ‘nanospheres’ (which have the capacity to manipulate light to produce either the blue or the white structural colour).

Blue tips are found where the nanospheres are uniformly-sized (“140–240 nm” in diameter (page 249) [but which they appear to correct to “200–240 nm” on page 251 of the article]), and reflect a single wavelength of light. However, as those gland cells mature the variably-sized spheres – 180–1000 nm in diameter – found there reflect a much-broader spectrum of light that appears white. Additionally, Kawai & Motomura (2025) remind us that the refractile bodies of Asparagopsis species contain compounds such as bromides that “are considered to act as defence compounds against epiphytic organisms such as bacteria and herbivorous animals”.

Whilst they don’t completely rule out a potential phytoprotective role of the nanosphere-containing refractile bodies, Kawai & Motomura (2025) suggest that this blue colouration – that’s intimately related to the presence of anti-herbivory compounds – may act as a warning signal (i.e., an example of aposematism (Bibiana Rojas et al., 2015; Tim Caro & Graeme Ruxton, 2019; Sarah Lee)) to would-be herbivores.

Additionally, they speculate that the white-coloured tips – which mask the more usual red colouration of the seaweed – might act as camouflage (Diane Boudreau et al.) to deter herbivorous animals reliant on visual cues. By comparison with animals that exhibit this phenomenon, Kawai & Motomura (2025) categorise this putative seaweed camouflage as an example of ‘masquerading’, explaining that “A masquerading species is one whose appearance causes its predators or prey to misclassify it as a different object found in the environment, causing the observer to change its behaviour in a way that enhances the survival of the masquerader (Niu et al., 2018)”. In their paper, Kawai & Motomura (2025) also consider several other macroalgae in which structural colours may have similar roles as those postulated for Asparagopsis.

Consequently, they conclude that “Based on these observations, we consider that structural colouration is probably widely used for communicative functions in macroalgae, as in animals and land plants” (Kawai & Motomura (2025). Whilst it’s always satisfying to be able to relate a structure to a function, these suggestions have yet to be examined. Mr P Cuttings looks forward to being able to share news that this hypothesis has been tested and supported. Happy sciencing, Hiroshi Kawai & Taizo Motomura!

* For more about structural colour (mainly, but not exclusively, botanical), see Beverley J Glover & Heather M Whitney, 2010; Anne Osterrieder; Jiyu Sun et al., 2013; Chris J Chandler et al., 2015; Edwige Moyroud et al., 2017; Paula J Rudall; Angeli Mehta; Chiara A Airoldi et al., 2019; George Degen; Andrew Richard Parker, 2000; Nathan Masters; Juliet Brodie et al., 2021; Atrouli Chatterjee, 2022; Cédric Finet, 2023; Alistair Daynes; Miranda A Sinnott-Armstrong et al., 2023; R Middleton & M Sinnott-Armstrong, 2024; here; here; and here.

** In terms of the etymology (Cristina Gusano) of the seaweed’s scientific name, its generic name (Shashank Goswami) Asparagopsis means ‘asparagus-like’ – presumably because the overall form of the plant reminded the seaweed’s namer of the plant asparagus. Its specific epithet – taxiformis, means resembling the genus Taxus, the generic name for the tree known as yew, whose leafy branches (Lizzie Harper) do look a little like the seaweed’s fronds.

*** If you think you’ve heard of this alga before, it may be in connection with burping or farting cows (!), as indicated by this statement: “Cows burp tons of the greenhouse gas methane that comes from their foregut fermentation. However, if only a small percentage of their diet is A. taxiformis, this is greatly reduced” (quoted from here). For more on that story, see Michael Battaglia; Lorenna Machado et al., 2014; Christopher RK Glasson et al., 2022; Emma Nyløy et al., 2023; Maggie Harrison Dupré; here; and here.

REFERENCES

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Juliet Brodie et al., 2021. Does structural color exist in true fungi? J. Fungi 7: 141; https://doi.org/10.3390/jof7020141

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