Algal Eyes


‘Vision’ is defined in a very general sense as recognition of the ambient light pattern by a motile organism and its use for orientation in a local environment. The ‘eye’ is the organ or organelle in which the light absorption and transformation into a transient intracellular signal occurs. The pigmented eyespot is functioning as the optical system that operates in most motile green algae as an interference reflecting device (quarter wave stack). In Chlorophyceae the light sensors are light‐gated ion channels (Channelrhodopsin (ChR)), whereas in Euglenoida photo‐activated adenylyl cyclases (PAC) fulfil this function. ChR is located within the eyespot overlaying part of the plasma membrane whereas PAC is found as a paraflagellar swelling at the basis of the long flagalla.

Key Concepts:

  • Motile green algae are able to detect the light direction by means of a pigmented eye.

  • Algal eyes are designed for detection of defuse light.

  • Chlorophyceae use light‐gated ion channels (Channelrhodopsins) for fast depolarisation of the plasma membrane.

  • The depolarisation changes of the plasma membrane (PM) follow intensity changes recorded by the photoreceptor according to the orientation of the cells respective to the direction of the light source.

  • Depolarisation of the PM is continued to the flagellar membrane.

  • Flagallar membrane depolarisation causes activation of voltage gated Ca‐channel and influx of Ca2+.

  • Intraflagellar Ca2+ changes modulate the flagellar beating pattern resulting in directional changes of the swimming.

  • As a sensor, Euglenoids use a photo‐activated adenylyl cyclases (PAC) that produces cAMP in the light.

  • Since the PAC is attached to the long flagella, the produced cAMP is immediately sensed by the cAMP‐regulated ion channels at the flagaller base.

  • The intraflagallar Ca2+ changes alter the flagellar bending and the swimming direction accordingly.

Keywords: rhodopsin; opsin; retinal; phototaxis; vision

Figure 1.

(a) A scheme of a Chlamydomonas reinhardtii cell. (b) Magnification of the boxed region in (a). The pigmented eyespot functions as an optical device (quarter wave stack), which together with the channelrhodopsins (ChR1 and ChR2), and the secondary ion‐channel (probably of TRPC‐type) form the functional eye.

Figure 2.

(a) Schematic representation of Euglena gracilis. (b) Magnification of the boxed region in (a). Longitudinal section through the eye. The unstructured eyespot granules outside the chloroplast serve as a shading device. The PFS houses the photoreceptor system which, most likely, uses pterins for light harvesting and flavins as the functional redox component. (c) Cross‐section through the flagellum with attached photoreceptor. The photoreceptor molecules are highly ordered, forming a monoclinic photoreceptor crystal.

Figure 3.

(a) Diagram of a Chlamydomanas cell viewed from the flagellar pole. The mother and daughter basal bodies (black circles) which template the trans and cis flagellum, respectively, are each associated with two microtubule rootlets (blue lines) comprising either four (M4 and D4) or two (M2 and D2) microtubules. The eyespot (orange oval), containing the ChR1 photoreceptor in the plasma membrane, and the EYE2 and EYE3 proteins in the chloroplast, is associated with the daughter four membered rootlet (D4). (b) Immunoflourescence micrograph of two Chlamydomonas daughter cells stained with anti‐ChR1 (red) and anti‐acetylated tubulin (green). In each daughter, the eyespot (arrow) is associated with the nascent D4 acetylated microtubule rootlet.



Barsanti L, Passarelli V, Walne PL and Gualtieri P (1997) In vivo photocycle of the Euglena gracilis photoreceptor. Biophysical Journal 72: 545–553.

Berthold P, Tsunoda SP, Ernst OP et al. (2008) Channelrhodopsin‐1 initiates phototaxis and photophobic responses in Chlamydomonas by immediate light‐induced depolarization. Plant Cell 20: 1665–1677.

Boyd JS, Mittelmeier TM, Lamb MR and Dieckmann CL (2011) Thioredoxin‐family protein EYE2 and Ser/Thr kinase EYE3 play interdependent roles in eyespot assembly. Molecular Biology of the Cell 22: 1421–1429.

Braun F‐J and Hegemann P (1999) Two independent photoreceptor currents in the spheroidal alga Volvox carteri. Biophysical Journal 76: 1668–1678.

Diehn B (1969) Action spectra of the phototactic responses in Euglena. Biochimica Biophysica Acta 177: 136–143.

Foster KW, Saranak J, Patel N et al. (1984) A rhodopsin is the functional photoreceptor for phototaxis in the unicellular eukaryote Chlamydomonas. Nature 311: 756–759.

Foster KW and Smyth RD (1980) Light antennas in phototactic algae. Microbiological Reviews 44: 572–630.

Häder D‐P and Lebert M (1998) The photoreceptor for phototaxis in the photosynthetic flagellate Euglena gracilis. Photochemistry and Photobiology 68: 260–265.

Harz H and Hegemann P (1991) Rhodopsin‐regulated calcium currents in Chlamydomonas. Nature 351: 489–491.

Holmes JA and Dutcher SK (1989) Cellular asymmetry in Chlamydomonas reinhardtii. Journal of Cell Science 94: 273–285.

Iseki M, Matsunaga S, Murakami A et al. (2002) A blue‐light activated adenylyl cyclase mediates photoavoidance in Euglena gracilis. Nature 415: 1047–1051.

Lamb MR, Dutcher SK, Worley CK and Dieckmann CL (1999) Eyespot‐assembly mutants in Chlamydomonas reinhardtii. Genetics 153: 721–729.

Litvin FF, Sineshchekov OA and Sineshchekov VA (1978) Photoreceptor electric potential in the phototaxis of the alga Haematococcus pluvialis. Nature 271: 476–478.

Mittelmeier TM, Berthold P, Danon A et al. (2008) C2 domain protein MIN1 promotes eyespot organization in Chlamydomonas reinhardtii. Eukaryotic Cell 12: 2100–2112.

Mittelmeier TM, Boyd JS, Lamb MR and Dieckmann CL (2011) Asymmetric properties of the Chlamydomonas reinhardtii cytoskeleton direct rhodopsin photoreceptor localization. Journal of Cell Biology 193: 741–753.

Nagel G, Ollig D, Fuhrmann M et al. (2002) Channelopsin‐1, a light‐gated proton channel in green algae. Science 296: 2395–2398.

Nagel G, Szellas T, Huhn W et al. (2003) Channelopsin‐2, a light‐gated cation channel in green algae. Proceedings of the National Academy of Sciences of the USA 100(24): 13940–13945.

Ozawa S, Nield J, Terao A et al. (2009) Biochemical and structural studies of the large Ycf4‐photosystem I assembly complex of the green alga Chlamydomonas reinhardtii. Plant Cell 21: 2424–2442.

Piccinni E and Mammi M (1978) Motor apparatus of Euglena gracilis: ultrastructure of the basal portion of the flagellum and the paraflagellar body. Bollettino della Zoologica 45: 405–414.

Roberts DG, Lamb MR and Dieckmann CL (2001) Characterization of the EYE2 gene required for eyespot assembly in Chlamydomonas reinhardtii. Genetics 158: 1037–1049.

Schröder‐Lang S, Schwärzel M, Seifert R et al. (2007) Fast manipulation of cellular cAMP level by light in vivo. Nature Methods 4: 39–42.

Sineshchekov OA, Jung K‐H and Spudich JL (2002) Two rhodopsins mediate phototaxis to low‐ and high‐intensity light in Chlamydomonas reinhardtii. Proceedings of the National Academy of Sciences of the USA 99: 8689–8694.

Further Reading

Hegemann P (2008) Algal sensory photoreceptors. Annual Review of Plant Physiology 59: 167–189.

Monya Baker (2011) Light tools: optogenetics grows from an idea into a discipline. Nature Methods 8: 19–22.

Web Link

‘Optogenetics’ Method of the Year 2010:×7vHSHOE

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Hegemann, Peter, and Dieckmann, Carol(Sep 2011) Algal Eyes. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000318.pub3]