A New Dual-Beam Microphotometer for Determination of Action Spectra of Light-Induced Phaeoplast Movements in Dictyota dichotoma

Abstract

A microphotometer with a variable stimulus light source and a modulated measuring light beam has been developed to measure the transmittance changes caused by the movement of Dictyota phaeoplasts from the low intensity to the high intensity arrangement. These transmittance changes were used to determine the fluence rate-response curves and to calculate the action spectrum of phaeoplast displacements. The measuring light was set to a constant fluence rate and a fixed wavelength of 672 nm, while the stimulus light was varied in wavelength and fluence rate. Phaeoplast movements occurred at wavelengths between 333 nm and 509 nm. For those wavelengths at which the applied fluence rates were not sufficient to produce the complete high intensity arrangement, a mathematical model was developed to fit the measured values to a theoretical function of the transmittance changes. This way the transmittance maxima could be determined by extrapolation. Action spectra were derived from the fitted fluence rate-response curves in two different ways, using either the directness of movement (steepness of slope) or the fluence rate which causes 40 % of the maximal transmittance change as a measure of the chromatophore displacement. The action spectra display two major maxima at 360 nm and at 466 rim. Depending on the method chosen, a major minimum is found at 402 nm or at 381 nm and at 443 nm. The strong effect of UV around 360 nm suggests that a flavoprotein rather than a carotenoid might be the photoreceptor responsible for the high intensity movement of phaeoplasts in Dictyota dichotoma . In addition, the lacking effectiveness of wavelengths around 545 nm indicates that at least the carotenoid fucoxanthin is not involved in the perception of the active light.