PHOTOBIOLOGY OF BACTERIA

The field of photobiology is concerned with the interactions between l ight and living matter. For Bacteria this interaction serves three rec ognisable physiological functions: provision of energy, protection aga inst excess radiation and signalling (for motility and gene expression ). The chemical structure of the primary light-absorbing components in biology (the chromophores of photoactive proteins) is surprisingly si mple: tetrapyrroles, polyenes and derivatised aromats are the most abu ndant ones. The same is true for the photochemistry that is catalysed by these chromophores: this is limited to light-induced exciton- or el ectron-transfer and photoisomerization. The apoproteins surrounding th e chromophores provide them with the required specificity to function in various aspects of photosynthesis, photorepair, photoprotection and photosignalling. Particularly in photosynthesis several of these proc esses have been resolved in great detail, for others at best only a ph ysiological description can be given. In this contribution we discuss selected examples from various parts of the field of photobiology of B acteria. Most examples have been taken from the purple bacteria and th e cyanobacteria, with special emphasis on recently characterised signa lling photoreceptors in Ectothiorhodospira halophila and in Fremyella diplosiphon.