Photobehavior of microorganisms: a biophysical approach.

Since the beginning of the last century the study of the interaction between light and living matter has been one of the fundamental fields of man's investigation of the natural world. A whole section of modem science, photobiology. which involves scientists of different backgrounds such as biologists, chemists, and physi­ cists, is devoted to this subject. Light means energy for living organisms, even if not all organisms are capable o( directly utilizing luminous energy, which ability in fact seems limited to chlorophyll-containing organisms and to some colored bacteria. It is only because some organisms can utilize light energy to drive meta­ bolic processes that life on our planet is as we know it. Among the different photobiological phenomena, the ability of certain microor­ ganisms to react to light, and sometimes also to its direction, with variations in their motile properties has been extensively investigated for some decades. Excellent reviews have appeared in the last years on this subject (e.g. 9, IS, 18, 23, 35, 39, 49, 53, 70, 71, 77, 83, 92). The most important among these processes are phototropism, phototaxis, and photokinesis. Both phototropism and phototaxis can be described as a light-induced variation in the movement of living organisms. In phototaxis the light stimulus causes an oriented movement of freely moving organisms toward or away from a light source, whereas in phototropism the main body is fixed and the luminous signal induces a bending of part(s) of the organism toward or away from a light source. In photokinesis it is the speed of the freely moving organism that is controlled by the light intensity. From a teleological point of view it is understandable why living organisms have developed a capacity to search for the best light conditions for their survival. But the molecular processes that initiate and control this behavior are still largely