The photoactivation energy of the visual pigment in two populations of Mysis relicta (Mysidacea, Crustacea) determined from temperature effects on spectral sensitivity

Abstract

Abstract: : Purpose: To study the relation between wavelength of maximum absorbance (λ_max) and photoactivation energy (E_a) in the visual pigments of two populations of the same subspecies of opossum shrimp (Mysis relicta Lovén). The two populations have been separated for 9000 years, adapting to different light environments (the Baltic "Sea" and "Lake" Pääjärvi), with A2 pigments peaking at 529 nm (Sea) and 555 nm (Lake), respectively. Methods: Spectral sensitivity was measured by electroretinogram recording in the intact eye, which allows accurate determination of the long–wavelength limb of the spectrum. From the temperature–dependence of spectral sensitivities in this range, the minimum energy required for photoactivation (E_a) can be estimated according to theory orignally proposed by Stiles¹. Results: The temperature effects were qualitatively in accordance with theory, as relative sensitivity to long wavelengths increased with rising temperature. The estimates of the minimum energy needed for photoactivation from this effect were E_a = 47.8 ± 1.8 kcal/mol for the Sea population and E_a = 41.5 ± 0.7 kcal/mol for the Lake population. Conclusions: The relative red–shift of λ_max in the Lake compared with the Sea population was associated with lower E_a and thus may carry a cost in terms of increased thermal noise^{2, 3}. The chromophore was the same (A2), so the difference must be due to differences in the amino acid sequence of the opsin. These are the first measurements of their kind in invertebrate visual pigments. ¹ Stiles, W.S. (1948). In: Transactions of the Optical Convention of the Worshipful Company of Spectacle Makers, pp 97–107. London: Spectacle Makers' Co. ² Barlow, H.B. (1957). Nature, 179:255–256. ³ Ala–Laurila et al. (2004). ARVO abstract 2004.

Keywords: photoreceptors • protein structure/function • electrophysiology: non–clinical