STEADY HYPSOCHROMIC SHIFT OF PEAK RESPONSIVITY TO ATTRACTANT LIGHT FROM 590 nm TO 560 nm IN Halobacterium halobium*

Abstract

Abstract— Peak responsivity of photoattraction in Halobacterium halobium cells shows steady hypsochromic shift from 590 nm wavelength under low irradiance conditions to 560 nm under high irradiance conditions. Inversion of the photoattractant response, as dependent on blue vs red background light, is compatible with the known properties of photochromic sensory rhodopsin‐I (SR‐I) with ground state maximum absorption at 587 nm. Relaxation of the photoattractant response in H. halobium, as a function of wavelength and irradiance, gives a hint at an antagonistic pigment or intermediate state, different from ground state SR‐I, with peak sensitivity at 620 nm or even above. The less sensitive photoattractant response at 560 nm persists without photorelaxation and represents the peak responsivity under high irradiance conditions.