Vision in microalgae

Abstract

Flagellate green algae such as Chlamydomonas and related genera are guided by their eyes to places where light conditions are optimal for photosynthetic growth. These eyes constitute the simplest and most common visual system found in nature. The eyes contain optics, photoreceptors and the elementary components of a signal-transduction chain. Rhodopsin serves as the photoreceptor, as it does in animal vision. Upon light stimulation, its all-trans-retinal chromophore isomerizes into 13-cis and activates a photoreceptor channel which leads to a rapid Ca2+ influx into the eyespot region. At low light levels, the depolarization activates small flagellar currents which induce in both flagella small but slightly different beating changes resulting in distinct directional changes. In continuous light, Ca2+ fluxes serve as the molecular basis for phototaxis. In response to flashes of higher energy the larger photoreceptor currents trigger a massive Ca2+ influx into the flagella which causes the well-known phobic response. The identification of proteins contributing to this signalling system has just begun with the isolation and cloning of the opsins from Chlamydomonas and Volvox. These plant opsins are highly charged, are not typical seven-helix receptors, and are believed to form a protein complex with the photoreceptor channel. In Spermatozopsis, a G-protein has been found which interacts either directly with the rhodopsin or with the rhodopsin-ion channel complex. By using insertional mutagenesis, genes coding for proteins that are involved in signalling have been tagged. One of them is connected to the flagellar channel and crucial for the flagellar action potential. Elucidation of photoreception in flagellated algae will provide deeper insight into the development of visual systems, starting from single-celled organisms and moving up through higher animals.

Retinal biosynthesis in Eubacteria: in vitro characterization of a novel carotenoid oxygenase from Synechocystis sp. PCC 6803

“…Chlamydomonas , the all-transretinal polyene chromophore isomerizes upon light stimulation to 13-cis and activates a photoreceptor channel, leading to a rapid Ca 2 + influx into the eyespot region. The Ca 2 + fluxes represent the molecular basis for phototaxis (Hagemann, 1997). Recently, a new type of rhodopsin, channelrhodopsin, has been reported from Chlamydomonas reinhardtii .…”

Cyanobacteria blooms produce teratogenic retinoic acids

“…Recently, anomalous concentrations of RAs and 4-oxoRAs (up to 20 ng/L) were measured in some protected drinking water sources (30), which suggested that there might be unidentified sources of RAs in aquatic systems. There are some reports on the existence of retinal, one of the precursors of RAs, in algae, and isomerization of the all-trans-retinal chromophore into 13-cis-retinal activated the photoreceptor channel, whereas RAs were not associated with photoreceptors in algae (31)(32)(33). The nonanimal hydroxylase enzyme (CYP120A1) in the cyanobacteria Synechocystis was reported to be capable of converting RAs to corresponding hydroxyl derivatives (34), and a hydroxylated RA, 7-OH-RA, has recently been observed in several species of cyanobacteria (35).…”

Two rhodopsins mediate phototaxis to low- and high-intensity light in Chlamydomonas reinhardtii

“…Fitting with a sum of two saturating functions reveals a slower low-light-saturating current and faster high-light-saturating current (9-11, 16, 17), which were interpreted as being driven by either a single, or two different, photoreceptor proteins (18). Multiple photoreceptors have been considered, based on the complex shape of the photoreceptorcurrent action spectrum (8,11,14), and results from retinalreconstitution studies (19).For several years the most abundant protein in the eyespot membranes of Chlamydomonas and Volvox has been considered as the photoreceptor for photomotile responses in these algae (20,21). However, recently the so-called ''chlamyrhodopsin'' (21) has been ruled out as the photoreceptor pigment for either phototaxis or the photophobic response in Chlamydomonas (22).…”