Structural colour in Chondrus crispus

Abstract

The marine world is incredibly rich in brilliant and intense colours. Photonic structures are found in many different species and provide extremely complex optical responses that cannot be achieved solely by pigments. In this study we examine the cuticular structure of the red alga Chondrus crispus (Irish Moss) using anatomical and optical approaches. We experimentally measure the optical response of the multilayer structure in the cuticle. Using finite-difference time-domain modelling, we demonstrate conclusively for the first time that the dimensions and organisation of lamellae are responsible for the blue structural colouration on the surface of the fronds. Comparison of material along the apical-basal axis of the frond demonstrates that structural colour is confined to the tips of the thalli and show definitively that a lack of structural colour elsewhere corresponds with a reduction in the number of lamellae and the regularity of their ordering. Moreover, by studying the optical response for different hydration conditions, we demonstrate that the cuticular structure is highly porous and that the presence of water plays a critical role in its ability to act as a structural light reflector.

Structural Color in Marine Algae

“…Figure 2 compares the two main types of architectures responsible for structural color in algae. Firstly, extracellular multilayered structures, [ 29 ] similar to those of land plants. [ 16 ] These structures found in Chondrus crispus or Iridaea sp.…”

“…An example of this system has been shown to produce the blue coloration in the thalli of Chondrus crispus ( Figure 4 ). [ 29 ] Such photonic structures can refl ect, in specular direction, an intense color that overcomes the response from the pigment, and the color of the tissue appears to be strongly metallic (Figure 4 d). …”

“…[ 6 ] In nature, structural color occurs in the marine and terrestrial environments in organisms across the tree of life, including birds, [7][8][9] insects, [10][11][12][13][14] land plants [ 15,16 ] (e.g., in the leaves, [ 17,18 ] fl owers [19][20][21] and fruit [ 22,23 ] ), bacteria, [ 24 ] fungi, [ 25 ] slime molds, [ 26 ] viruses, [ 27 ] diatoms [ 28 ] and macroalgae. [ 29 ] The biological signifi cance of structural color is mainly studied and understood in insects [ 30,31 ] and other animals [ 32,33 ] in terms of intra-and inter-specifi c communication, [ 34 ] The copyright line of this paper was changed 7 March 2017 after initial publication. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.…”

“…For example, in the red alga, Chondrus crispus Stackhouse [ 53 ] (Rhodophyta), it has been shown that structural color is produced by a multilayered structure in the cuticle with refractive-index periodicity. [ 29 ] Furthermore, virtually no studies have attempted to investigate the vivid and diverse array of structural color patterns found within the brown algae (Phaeophyceae). It has been suggested that intracellular quasi-ordered spherical inclusions in the epidermal cells termed 'iridescent bodies' are responsible for the observed structural color, [ 54 ] however there is no clear experimental evidence that correlates these bodies with the optical appearance.…”

“…[ 32 ] Subsequently, only a few studies have attempted to work on and identify the mechanisms responsible for producing structural color and they have mainly focused on the red algae (Rhodophyta), where intracellular [ 52 ] and extracellular [ 29 ] structures have been observed. For example, in the red alga, Chondrus crispus Stackhouse [ 53 ] (Rhodophyta), it has been shown that structural color is produced by a multilayered structure in the cuticle with refractive-index periodicity.…”