Photophobic response

Fig. 15. Specific chemical inhibition of physiological bluelight responses 04) Swimming, inverse phobic response and direct photophobic response of Euglena as a function of iodide concentration118. (B) Phototropic and geotropic curvature of the avena coleoptile as function of azide concentration1S41...

A response is defined as any stimulus-induced alteration in the activity of the organism s motor apparatus which can (but need not always) result in an alteration of the movement or orientation of the organism. Where nature or occurrence of a response depends upon an increase or decrease in stimulus intensity, an increase may be indicated by the use of the term step-up and a decrease by the term step-down . Thus, a step-up photophobic response is a phobic response which occurs upon an increase in light intensity. The responses are classified as follows ... 

Diehn and Kint115) and Mikolajczyk and Diehn71) demonstrated specific inhibition of photoaccumulation and of the step-down photophobic response of Euglena gracilis, respectively, by potassium iodide. Iodide ions are known to de-excite flavins, but do not affect carotenoids87). For a description of the analogous experiments with corn coleoptiles, see Sect. 3.2 of the contribution by Schmidt in this volume. 

Fig. 2. Effect of DNP on photokinesis (closed circles) and photophobic response (open circles) of Rhodospirillum rubrum. Abscissa DNP concentration in mol Ordinate response in % of the uninhibited control (after1331)...

These experiments show that, as in the case of chlorophyll a and the carotenoids, the energy absorbed by the phycobiliproteins is utilized via the photosynthetic apparatus furthermore, they provide evidence that photophobic responses in blue-green algae are caused by sudden changes in the steady state of the photosynthetic electron transport, especially the non-cyclic one. 

Fig. 12. Action spectrum of light-induced potential changes measured with microelectrodes (after Hader46)) and action spectrum of photophobic response. Abscissa wavelength in nm Ordinates amplitude of light-induced potential changes in mV and phobic response in relative units respectively (after Nultsch88))...

Circular dichroism was used to study the structure of oxyblepharismin 40, the photoreceptor chromophore for the photophobic response of the blue form of Blespharisma japonicum <2005MI1343>. 

Takahashi T et al (1991) Photoisomerization of retinal at 13-ene is important for phototaxis of Chlamydomonas reinhardtii - simultaneous measurements of phototactic and photophobic responses. Biochem Biophys Res Com 178 1273-1279... 

So far all stimulatory processes of the halobacterial cell depend on fumarate as seen by the fact that smooth swimming mutant cells of strain M415 do not respond to any stimulus. This also holds true for the step-down photophobic response mediated by bacteriorhodopsin[5]. This points to the role of a central metabolite as an integrative tool for measurement of the metabolic state of the cell. Especially fumarate would allow us to measure and compare activity states of electron transport, fermentation and photosynthesis (see Fig. 2). 

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