Power transmission along trichomes of filamentous cyanobacteria Phormidium uncinatum has been studied with the use of ethylrhodamine fluorescence as a probe for the transmembrane electric potential difference (delta psi). It is found that agents preventing the light-induced delta psi formation (photosynthetic redox chain inhibitor dibromothymoquinone) or dissipating delta psi (uncoupler tetrachlorotrifluoromethylbenzimidazole) strongly decrease the fluorescence of the ethyl-rhodamine-stained trichomes. K+-H+ antiporter nigericin converting delta pH to delta psi increases the fluorescence. These relationships are in agreement with the assumption that ethylrhodamine electrophoretically accumulates inside the cyanobacterial cells. Illumination of a single cell in the P. uncinatum trichome gives rise to quenching of the fluorescence in this cell and usually in one or two neighbor cells, whereas the rest of trichome remains fluorescing. A small light spot (5% of the trichome length) causes an increase in the ethylrhodamine fluorescence not only in the illuminated but also in the nonilluminated parts of the trichome up to the laser-treated cell or its neighbor(s). It is concluded ethylrhodamine can be used to monitor the power transmission which was previously demonstrated by microelectrode studies of the cyanobacterial trichomes. In certain trichomes, several "dark" cells appear during the storage of the trichomes without energy sources. Illumination for several minutes results in dark cells becoming fluorescing. Thus some cells or cell clusters can be reversibly excluded from the lateral delta psi-transmitting system of the trichome, the rest being still electrically connected. This means that filamentous cyanobacteria possess mechanisms to transmit power along the trichome and to switch off this transmission.

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