Structural colour in the brown algal genus Sporochnus (Sporochnales, Phaeophyceae)

ABSTRACT

The colour produced by an object’s microstructure rather than by pigments is generally called structural colour. Various macroalgae have some mechanisms of structural colouration. The presence of structural colour in the brown algae is well known in the Dictyotales and Fucales but has recently been reported in the Sporochnales where Sporochnus dotyi has structures in the terminal assimilatory filaments (trichothallic hairs) that produce a pronounced green iridescence. Based on observations by light microscopy, the structural colour of this species was caused by iridescent bodies, 10–15 µm in diameter, within the cells of assimilatory filaments. In contrast, another Sporochnus species known as S. radiciformis in Japan (S. radiciformis auct. japon.) also contains iridescent bodies of 6–10 µm diameter in the assimilatory filaments, but no significant structural colour was observed. In epi-illumination observations using red, green and blue LED light sources, iridescent bodies of S. dotyi showed stronger reflection of green compared with red and blue, whereas the reflections were similar among those colours in S. radiciformis. Transmission electron microscopy (TEM) using a rapid freezing method revealed that homogeneous globules 130–160 nm in diameter were tightly packed inside the iridescent bodies of S. dotyi, whereas the sizes of the globules were markedly irregular in iridescent bodies of S. radiciformis. Therefore, we conclude that the structural colouration of S. dotyi is caused by the interference and diffraction of incident light caused by the crystal lattice-like structure of tightly packed and regularly arranged globules of homogeneous size in the iridescent bodies, which is similar to the mechanism causing iridescence in opal. A similar opal-like structure of iridescent bodies causing structural colouration was recently reported in Ericaria selaginoides, but that of S. dotyi lacked the dynamic shift of the globules shown in E. selaginoides that resulted in a change of the wavelength of reflected light.

Highlights

• Structural colouration, previously known only in Dictyotales and Fucales in Phaeophyceae, is newly reported in Sporochnales.

• Green structural colour of Sporochnus dotyi is caused by the iridescent bodies within trichothallic hairs.

• Iridescent bodies of S. dotyi includes densely packed homogeneous globules, whereas they are markedly irregular in size in S. radiciformis auct. japon. which lacks structural colour.

KEYWORDS:

• Iridescence
• iridescent body
• opal
• Phaeophyceae
• Sporochnus
• structural colouration

Acknowledgements

We are grateful to Akihiro Nagata for collecting the fresh specimens of Sporochnus radiciformis, Chikako Nagasato for the use of TEM, and Eric Henry for his helpful comments in revising the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at https://doi.org/10.1080/09670262.2024.2340020.

Supplementary fig. S1. Spectra of LED clip light used for epi-illumination using light microscopy.

Supplementary fig. S2. (a) Structure of opal, (b) mechanism causing structural colour in opal.

Author contributions

Hiroshi Kawai: Collection of specimens, examination by light microscopy, drafting and editing manuscript; Taizo Motomura: examination by TEM and manuscript editing.