Fundamental questions and concepts about photoreception and the case of Euglena gracilis

Abstract

The ability to sense light can be considered the most fundamental and presumably the most ancient property of visual systems. This ability is the basis of phototaxis, one of the most striking behavioral responses of motile photosynthetic microorganisms (i.e. microalgae) to light stimuli, which allows them to move toward or away directional light. In order to fully exploit the information content of light (intensity, direction, distribution) microorganisms need proper perceiving devices, termed photoreceptors, which must act as sensors, to perceive wavelength and direction of light, as transducers, to convert the light signal into chemical and/or electrical information, but also as amplifiers and eventually as transmitters. This review describes the universal structural, behavioral and physiological features necessary for the proper functioning of these devices in algae, and how these features have been investigated by means of different analytical techniques such as for example microspectroscopy, digital fluorescence microscopy, two photons FLIM. The insight of the photoreceptive response mechanism is explained using the unicellular alga Euglena gracilis, in which the different structural, behavioral and physiological features combine to achieve a concerted, efficient response to light stimuli.

Photomovement in Euglena

“…Therefore it does not function as an interference reflector device which is found in many stigmata of Chlorophytes such as Chlamydomonas (Kreimer 1994). Even though the stigma does not harbor the photoreceptor, as suggested by earlier authors (France 1909;Schiff 1978, 1979), it seems to be involved in photoperception, functioning as a screening device which in lateral light casts a periodic shadow on the photoreceptor (the PAB, see below) as the cell rotates around its long axis when it is propelled by its long flagellum in a forward direction (Barsanti et al 2012). For comparison, we also shortly discuss several mutant strains of Euglena gracilis, some other species in the genus Euglena as well as the close relative E. longa (formerly Astasia longa) (Poniewozik 2014).…”

Trap it!

“…Euglena has a subcellular photoreceptor organelle that allows the cell to change its swimming direction under light stimulus. Phototaxis is dependent on the wavelength and intensity of the light; they accumulate in regions with adequate illumination and avoid areas with very dim or intense illumination [2,24,37]. The Euglena photoreceptor is known to strongly absorb blue wavelengths [2], prompting a instantaneous photophobic response upon intense blue stimuli that allows real-time interaction with human users.…”

Photosynthetic Euglenoids

“…However, several of the features associated with the green algal chloroplast are absent from euglenoids. The small green algal eyespot is about one micrometer long and is composed of two rows of carotenoid globules separated by thylakoid membranes and tightly packed under the chloroplast envelope (Barsanti et al, 2012). The small green algal eyespot is about one micrometer long and is composed of two rows of carotenoid globules separated by thylakoid membranes and tightly packed under the chloroplast envelope (Barsanti et al, 2012).…”