PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Stress-induced filament fragmentation of Calothrix elenkinii (cyanobacteria) is facilitated by death of high-fluorescence cells. David Kaftan^{*,1, 2, 3}, Frantisek Adamec^{2, 3}, Ladislav Nedbal^{2, 3}, ¹ Photon System Instruments Ltd., Brno, Czech Republic² Laboratory of Applied Photobiology & Bioimaging, Nove Hrady, Czech Republic³ Institute of Physical Biology, Nove Hrady, Czech Republic

ABSTRACT- Irradiance power and spectral composition as well as nutrient availability strongly influence differentiation of filamentous cyanobacteria. When monitoring the life cycle of Calothrix elenkinii Kossinsk., we found that low nitrogen concentration and cultivation under green light lead to a transient appearance of high-fluorescence cells that rapidly bleach breaking the parental filament into shorter parts. The process dynamics was monitored in a microscope growth chamber by measuring transmission and chlorophyll fluorescence of individual cells by a high-sensitivity camera. Typically, the high-fluorescence cells appeared near the centre of the parental trichome signalled by a rapid, 2-3 fold rise in their fluorescence emission. By measuring the fluorescence excitation spectra with resolution of individual cells, we show that the elevated fluorescence emission was largely due to a high absorption by phycoerythrin and energy transfer to chlorophyll. Typically, after no more than 20 minutes, the fluorescence abruptly disappeared with transmission images indicating loss of pigmentation. The bleaching was a natural process that was not caused by the measuring light. Depending on the mechanical strain, the cell bleaching was followed by breaking of the parental filament. We propose that the high-fluorescence cells appear as a phase of programmed cell death allowing the fragmented filaments to escape from unfavourable environmental conditions. Also, after breaking away the peripheral parts of the filaments, the reduced colony can capture the low light more effectively and the basal heterocyte is providing nitrate for less vegetative cells.

KEY WORDS: cyanobacteria, chlorophyll fluorescence, nutrient stress, photosynthesis