Margot M. Arnould-Pétré1,2, Bruno Jesus2, Christopher Williamson3, Juliet Brodie1
1Science, The Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom
2Institut des Substances et Organismes de la Mer, ISOMer, UR 2160, Nantes Université, 44000 Nantes, France
3Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol, BS8 1QU, United Kingdom
Seaweeds are colourful organisms, and pigmentation is not the sole mechanism behind the colours of some species: structural colour occurs due to nano-structures causing refraction and interference in the incident light and reflecting particular wavelengths. It results in intense, sharp and often iridescent shades where the tissue appears to shift colours with the angle of view. In red algae, two known types of mechanisms create structural colour: multi-layers and iridescent bodies. In this study, we focus on two species of the red algal genus Chondria (Rhodomelaceae) where structural colour is due to intra-cellular iridescent bodies. In Britain, Chondria coerulescens is a summer annual species growing on very sheltered, silty shores. Structural colour gives it strikingly turquoise to purple hues throughout the branches. Chondria scintillans is found on slightly more exposed rocky shores all year around and more significantly in late winter. Its structural colour is less conspicuous and shows as light turquoise spots on the branches. Both species grow on the lower shore and shallow subtidal waters along the Atlantic coast of Europe. Chondria scintillans appears to be expanding its range northwards and has only recently been reported for the first time for Britain from the South Devon. We hypothesise that structural colour provides a protective mechanism against damaging light while also enhancing photosynthetic efficiency under low light conditions. Using PAM fluorometry, we raise questions on the impact of the iridescent bodies on the photosynthetic activity, seasonal variations, and possible differences in the photophysiology of these two closely related species.