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Cyanobacteria respond to a decrease in light intensity by reversing their direction of gliding. The sensitivity of the phototactic response in Phormidium uncinatum increased two- to threefold under anaerobic conditions. Light-dependent changes in the membrane potential (∆ψ), as measured by tetraphenylphosphonium distribution, were also found to be larger in anaerobic conditions, suggesting that the photophobic response is governed by sensing of the protonmotive force (
). The uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP), when added to P. uncinatum trichomes, also evoked a phobic response in a spatial gradient assay. The extent of repulsion by different concentrations of CCCP correlated with its ability to decrease ∆ψ. A viscous environment, exudates of an old culture, or high concentrations of Ca2+ (plus the ionophore A23187) caused oscillatory reversals and a partial asynchronization of cells within a trichome. EGTA or CCCP in high concentrations restored synchronization. Ethionine inhibited reversals and the addition of 10−6
m-Ca2+ (plus A23187) restored photophobic sensitivity. A depolarizing electrical potential spread from the leading end (the ‘head’) of the trichomes following a decrease in light intensity. It is suggested that sensing of
or chemoeffectors leads to a methylation-requiring step followed by a taxic signal in the form of simultaneous changes in ∆ψ and Ca2+ concentration.
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