Nomen omen: Euglena gracilis possesses a rhodopsin‐based photoreceptor

• Published in: Photochemistry and photobiology Vol. 101; no. 2; pp. 350 - 358
• Main Authors: Lorenzetti, Giulia, Barsanti, Laura, Birindelli, Lorenzo, Gualtieri, Paolo, Legnaioli, Stefano
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 01.03.2025
• Subjects: Algae; Animals; Cattle; Correlation coefficient; Correlation coefficients; Embedding; Euglena; Euglena gracilis; Euglena gracilis - chemistry; Euglena gracilis - metabolism; Photoreception; Photoreceptors; Raman spectra; Raman spectroscopy; retina; retinal; Rhodopsin; Rhodopsin - chemistry; Rhodopsin - metabolism; Spectrum Analysis, Raman; Vertebrates; Visual system; rhodopsin; visual system; retinal; Raman spectroscopy; retina; Euglena gracilis
• ISSN: 0031-8655; 1751-1097; 1751-1097
• DOI: 10.1111/php.13999

The unicellular microalga Euglena gracilis has always been considered the ideal alga to investigate photoreceptive responses and systems, and it has been the subject of hundreds of articles. Moreover, because of its detectable photoreceptor, it has been given a key role in the evolution of photoreception, from single and simple cells to complex visual system of higher organisms. In this article, we report the Raman spectra recorded in vivo on photoreceptors of E. gracilis and Bos taurus retina. The almost perfect superimposability (correlation coefficient r = 0.955) of these spectra states that the Euglena possesses a photoreceptor with the same structural characteristic of a vertebrate photoreceptor, i.e. a stack of membrane layers embedding rhodopsin‐like proteins. Raman spectra recorded in vivo on photoreceptors of E. gracilis after hydroxylamine treatment further confirm our findings, which should lead to a reconsideration of most of the scientific literature on algae photoreception and eye evolution. Euglena possesses a photoreceptor with the same structural characteristics of a vertebrate photoreceptor, i.e. a stack of membrane layers embedding rhodopsin‐like proteins. The comparison between the Raman spectrum of Bos taurus retinas (black line) and that of Euglena photoreceptor (red line) highlights their very high degree of similarity, further confirmed by statistical analysis (Pearson's coefficient r = 0.955) and identification of retinal peaks in the photoreceptor spectrum. The present findings also exclude the presence of FAD in the photoreceptor and should lead to a reconsideration of most of the scientific literature on algae photoreception and eye evolution.