Water and Solute Transport in Cyanobacteria as Probed by Chlorophyll Fluorescence
In cyanobacteria, the transfer of excitation energy from phycobilisomes (PBS) to Photosystem (PS) I and PS II is subject to regulation (a) by the osmolality of the cell suspension, and (b) by the light acclimation state of cells. In hypo-osmotic suspension, or in the light acclimated state (state 1 cells), relatively more PBS excitation is delivered to PS II than in hyper-osmotic suspension, or in the dark-acclimated state (state 2 cells). Conversely, in hyper-osmotic cell suspension, or in state 2, PBS deliver relatively more excitation to PS I and less to PS II. As a result, cells in hypo-osmotic suspension, or in state 1 emit stronger chlorophyll a (Chi a) fluorescence (level F1 compared to cells in hyper-osmotic suspension, or in state 2 cells (level F2). The osmotically-induced Chi a fluorescence changes (∆F) and cell volume changes (∆V) obey Boyle-van t’ Hoff ’s law (∆F ∞ ∆ V = kOsmOUT-1), where OsmOUT is the external osmolality. ∆F is linearly proportional to ∆V These properties allow for quantitative analyses, by means of Chi a fluorometry, of difficult to determine static and dynamic cell properties, such as cytoplamic osmolality (OsmIN), cell turgor threshold, solute and water transport, and thermotropic phase transitions of cell membrane lipids, at unsurpassed time resolutions.
The osmotically-induced changes of Chi a in fluorescence in cyanobacteria are unrelated to changes in the redox poise of photosynthetic carriers or in the transthylakoid ∆pH. We propose that the ApcD-PS I site, where ApcD is the PBS core subunit that donates excitation to PS I, operates as an osmoregulated excitation transfer valve. In hypo-osmotic conditions the valve is open and PBS deliver excitation to PS II only; in hyper-osmotic conditions the valve is closed and PBS deliver excitation to both PS I and to PS II.
KeywordsThylakoid Membrane Glycine Betaine Cyanobacterial Cell Optical Cross Section Thermotropic Phase Transition
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