Fluorescence microscopy reveals variations in cellular composition during formation of akinetes in the cyanobacterium Aphanizomenon ovalisporum

Abstract

Akinetes are differentiated, thick-walled, resting cells produced by many species of the filamentous heterocystous cyanobacteria belonging to the Nostocales and Stigonematales. They are commonly developed after the end of exponential growth and are normally resistant to low temperatures and to desiccation for extended periods. Akinetes maintain a low level of metabolic activities such as photosynthesis, respiration and de novo synthesis of proteins and lipids. In a previous study we reported that mature akinetes of Aphanizomenon ovalisporum maintained only residual photosynthetic activity. However the cellular abundance of PSI and PSII reaction centres increased in akinetes relative to exponentially grown vegetative cells while the ratio of PSI to PSII remained relatively unchanged. In the present study we provide spectral and biochemical evidence that akinetes lose their phycobilisomes during their development. DAPI stained trichomes of exponentially grown cultures emitted fluorescence signals attributed to nucleic acids and polyphosphate granules. Akinetes that developed in akinete-induced cultures were preferentially enriched with nucleic acids and deprived of polyphosphate bodies, whereas adjacent vegetative cells were loaded with polyphosphate bodies. Examination of akinetes from akinete-induced cultures of A. ovalisporum by spectral confocal laser scanning microscopy indicated reduction of the phycobilisome fluorescence signal. Mature isolated akinetes could be divided into two distinct populations: (i) a majority of akinetes that lost their phycobilisome fluorescence signal, but maintain chlorophyll fluorescence attributed to the reaction centres, and (ii) a minor group of mature akinetes that maintained a high phycobilisome fluorescence signal.