Structural colour in the brown algal genus <i>Sporochnus</i> (Sporochnales, Phaeophyceae)
Creators: name: Kawai, Hiroshi
name: Motomura, Taizo
Publisher: name: Taylor & Francis
Year: 2024
Subjects: subject: Biophysics
subject: Biochemistry
subject: Physical Sciences not elsewhere classified
subject: Microbiology
subject: FOS: Biological sciences
subject: Molecular Biology
subject: Biotechnology
subject: Environmental Sciences not elsewhere classified
subject: Ecology
subject: Science Policy
subject: Inorganic Chemistry
subject: FOS: Chemical sciences
subject: Plant Biology

description: 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 <i>Sporochnus dotyi</i> 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 <i>Sporochnus</i> species known as <i>S. radiciformis</i> in Japan (<i>S. radiciformis auct. japon</i>.) 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 <i>S. dotyi</i> showed stronger reflection of green compared with red and blue, whereas the reflections were similar among those colours in <i>S. radiciformis</i>. 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 <i>S. dotyi</i>, whereas the sizes of the globules were markedly irregular in iridescent bodies of <i>S. radiciformis</i>. Therefore, we conclude that the structural colouration of <i>S. dotyi</i> 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 <i>Ericaria selaginoides</i>, but that of <i>S. dotyi</i> lacked the dynamic shift of the globules shown in <i>E. selaginoides</i> that resulted in a change of the wavelength of reflected light. Structural colouration, previously known only in Dictyotales and Fucales in Phaeophyceae, is newly reported in Sporochnales.Green structural colour of <i>Sporochnus dotyi</i> is caused by the iridescent bodies within trichothallic hairs.Iridescent bodies of <i>S. dotyi</i> includes densely packed homogeneous globules, whereas they are markedly irregular in size in <i>S. radiciformis auct. japon</i>. which lacks structural colour. Structural colouration, previously known only in Dictyotales and Fucales in Phaeophyceae, is newly reported in Sporochnales. Green structural colour of <i>Sporochnus dotyi</i> is caused by the iridescent bodies within trichothallic hairs. Iridescent bodies of <i>S. dotyi</i> includes densely packed homogeneous globules, whereas they are markedly irregular in size in <i>S. radiciformis auct. japon</i>. which lacks structural colour.