Morphological and ultrastructural variability of planktonic Cyanophyceae in relation to seasonal periodicity

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Abstract
Morphology, cytology and ultrastructure of vegetative cells, filament polarity, heterocyst differentiation and akinete development in Gloeotrichia echinulata are described under natural conditions for the first time by following wild populations of the alga over two successive field seasons. The ageing sequence of vegetative cells is studied in space and time by comparing cells from one end of a filament to the other and by comparing filaments through a complete cycle of growth and development. Heterocysts are characterized by massive sheaths and absence of membranous polar elaborations; healthy gas vesicles are retained in the heterocyst, even into senescence. Akinetes develop a “spore sheath” by modification of the filament sheath. As the akinete develops, a dense stratified deposit accumulates externally to the adjacent chain of meristematic cells. These become isolated and eventually detached, presumably as a hormogonial form of perennation.