Cells Acting as Lenses: A Possible Role for Light in the Evolution of Morphological Asymmetry in Multicellular Volvocine Algae

Abstract

The volvocine green algae have been extensively used to address various questions related to the evolution of multicellularity and cell differentiation, in terms of the genetics, developmental constraints, and underlying selective forces specific to this group. More recently, physical characteristics of the environment and of the emerging multi-celled entities have also been considered as potential contributors to the evolution of multicellularity in this lineage. However, the role of light in the evolution of multicellularity—beyond its direct photosynthetic role—has not been explored. The objectives of this work are (1) to show that algal cells, in both unicellular and multicellular algae, concentrate incident light, and (2) to suggest that this concentrated light might have contributed to the evolution of multicellularity in volvocine algae. We show that single algal cells can act as lenses and concentrate light from a remote source (e.g., the Sun) into beams, by a combination of standard refractive imaging of transmitted light and diffractive Arago-Poisson imaging of the light surrounding the cells. In the spheroidal multicellular volvocine algae, the peripheral cells facing the Sun can concentrate incident sunlight towards the interior of the colony. We suggest that the evolution of morphological asymmetries associated with the anterior-posterior polarity exhibited by multicellular spheroidal volvocine algae may have been influenced by this phenomenon. Whether the effect of these light beams is still important to extant spheroidal volvocine algae remains to be investigated. Future experiments are also needed to assess the relative contributions of the two light concentrating mechanisms by algal cells.

Appendix

Figure 7 demonstrates the spot of diffracted light that occurs behind approximately circular opaque objects, namely, powdered graphite. The black background permits a clear view of the bright spot behind the small particles. The larger, odd shaped graphite fragments are too irregular to generate a bright spot by addition of wavelets. In Figure 8 we used 21 μm diameter polystyrene spheres (Bangs Labs) to demonstrate complex focusing and diffraction. The interior of these spheres is surely less complex than the interior of algal cells. The images show focusing of the incident light in apparently two distinct steps, followed by a diffraction pattern that is similar to the diffraction patterns observed with the Arago-Poisson effect (Sommargren and Weaver 1990; Kolodziejczyk et al. 2002). It is too difficult to show unambiguously the contribution of the Arago-Poisson diffractive imaging to the light being concentrated by the algal cells, because of their internal structure, surface irregularities and deviations from sphericity. Both experimental and theory investigations are currently underway. In particular, we have shown that by restricting the area of the illumination source, the beam propagating the image of the light source becomes longer and its cross section increases with distance, as would be expected from the Arago effect. These experiments, indicating the dependence on geometry of the source, are preliminary to investigate imaging of the Sun by algal cells.

Fig. 7

Graphite powder viewed from below with a 40 × objective. Light is incident from above. a In panel a all graphite particles appear black because they do not transmit light. A diffraction ring is seen outside the particles. b In panel b the objective was moved downwards by a few micrometers. The smaller particles show the Arago bright spot that is produced by diffracted light originating near the particles’ edge. The larger particles do not show the spot because their irregular surface eliminates coherent addition of wavelets

Fig. 8

The light emerging below a 21 μm diameter polystyrene sphere surrounded by water, and illuminated from above. The images are obtained with a 40 × objective being moved away from the sphere. a, b and c are in sequence. The light spot is a section through the concentrated beam emerging from the sphere. For image in panel d, showing a diffraction pattern, the objective was lowered a few micrometers away from its position for image in panel c