Oxygen-evolving system and secondary quinonic acceptors are highly reduced in dark adapted Euglena cells: A thermoluminescence study

Abstract

Characteristics of thermoluminescence glow curves were compared in three types of Euglena cells: (i) strictly autotrophic, Cramer and Myers cells; (ii) photoheterotrophic cells sampled from an exponentially growing culture containing lactate as substrate repressing the photosynthetic activity; (iii) semiautotrophic cells, sampled when the lactate being totally exhausted, the photosynthesis was enhanced.

In autotrophic and semiautotrophic cells, composite curves were observed after series of two or more actinic flashes fired at −10°C, which can be deconvoluted into a large band peaking in the range 12–22°C and a smaller one near 40°C, This second band presents the characteristics of a typical B band (due to S_2/3Q_B ^- recombination), whereas the first one resembled the band, shifted by -15–20°C, which is observed in herbicide resistant plants. The amplitude of this major band, which was in all cases very low after one flash, exhibited oscillations of period four but rapidly damping, with maxima after two and six flashes. In contrast, photoheterotrophic Euglena displayed single, non-oscillating curves with maxima in the range 5–10°C.

In autotrophic and semiautotrophic cells, oxidizing pretreatments by either a preillumination with one or more (up to twenty-five) flashes, or a far-red preillumination in the presence of methylviologen, followed by a short dark period, induced thermoluminescence bands almost single and shifted by +3–5°C, or +12°C, respectively. In autotrophic cells, far-red light plus methyl viologen treatment induced a band peaking at 31°C, as in isolated thylakoids from Euglena or higher plants, while it had barely any effect in photoheterotrophic cells.

Due to metabolic activities in dark-adapted cells, a reduction of redox groups at the donor and acceptor sides of PS II dark-adapted cells is supposed to occur. Two different explanations can be proposed to explain such a shift in the position of the main band in dark-adapted autotrophic control. The first explanation would be that in these reducing conditions a decreasing value of the equilibrium constant for the reaction: S_nQ_A ^-Q_B⇌S_nQ_AQ_B ^-, would determine the shift of the main TL band towards low temperatures, as observed in herbicide resistant material. The second explanation would be that the main band would correspond to ‘peak III’ already observed in vivo and assigned to S_2/3Q_B ^2- recombinations.