Photosensory Behavior in Procaryotes

Most motile microorganisms actively search for a suitable environment for survival and growth by using external factors such as photic (121, 189), magnetic (201), mechanical (246), chemical (26), thermal (117, 164), galvanic (278), and gravitational (25) stimuli as clues for directional migration. It is trivial to state that light is the single most important factor for photosynthetic organisms, which depend on the availability of visible (in some cases also near-ultraviolet [UV] or infrared) radiation (190). However, the photoreceptor pigments for orientation are not necessarily identical to the pigments for light harvesting, comprising only a subset of the photosynthetic pigments in some cases and beinig totally different in others (94, 185, 186). Some nonphotosynthetic microorganisnis also use light as a reference for orientation (119, 123, 266). In these organisms the selection advantage of photoorientation is not always obvious. Although movement responses in motile microorganisms controlled by light were first observed and described more than a century ago; the biophysical and molecular mechanisms underlying photoreception, signal transduction, and control of the motor apparatus are still obscure in most cases. Treviranus (273) is believed to have been one of the first to describe photomovement of eucaryotic flagellates, and Cohn (54) and Engelmann (73, 74) were among the first to observe light-dependent movement of procaryotes. In spite of the fact that the basic phenomena were already described and analyzed by the beginning of this century (38, 58, 214), the phenomenological characterization of the response and the terminology of the behavior are still controversial in some cases. Therefore, without going into the historical development of the terminology and its controversial usage (see reference 185), a set of currently widely accepted definitions of light-dependent motor responses are given here (61). (i) Phototaxis (formerly also called phototopotaxis) describes a movement oriented with respect to light direction. Like many eucaryotic organisms, cyanobacteria show a directed movement toward the light source (positive phototaxis) at low fluence rates (measured in watts per square meter] and a movement away from the light source (negative phototaxis) at high fluence rates (68). At intermediate fluence rates, populations often split into subpopulations moving in either direction. Pieper (211) found a movement perpendicular to the incident light rays which is called diaphototaxis. Recently, a number of authors have used the term phototaxis in a wider sense to describe a photomovement unrelated to direction of the incident light (250, 264): Halobacterium halobium has been found to respond to sudden changes in the fluence rate (125), which is defined as a photophobic response but which may lead to accumulation of cells in a light field or an orientation in a light gradient (188). This usage of the term is in parallel to the usage of "chemotaxis" for bacteria which move up or down a chemical gradient by a trial-and-error mechanism but not by directed movement (4). (ii) Photokinesis describes the dependence of the speed of movement on the fluence rate of irradiation. In this case the direction of the light beam is irrelevant and the organisms detect the absolute, steady-state irradiation (183). When an organism moves faster at a given fluence rate than in the dark control, this behavior is defined as positive photokinesis; wheni it moves slower, it is called negative photokinesis (94). Sotne organisms stop in darkness or at very high fluence rates. (iii) A sudden change in the fluence rate (dIldt) causes a photophobic response in many organisms. The individual response is species specific and may constitute a reversal of movement, a temporary stop, or a directional change (148, 186, 190). Either a step-up or a step-down in the fluence rate can cause a photophobic reaction. Some organisms respond to a step-down at low fluence rates and to a step-up at high fluence rates (99, 184, 279; D.-P. Hader, in P. Fay and C. Van Baalen, ed., Cyanobacteria, Current Research, in press). The two responses may be mediated by the same or by different photoreceptor systems (126). The photophobic response can be induced by either a temporal change in the