Influence of phosphorus on morphology of Calothrix parietina (Cyanophyta) in culture

A strain of Calothrix parietina (Durham No. 550) was isolated from an upland stream whose physical and chemical environment is already known in some detail. Morphological changes were followed in batch culture in a medium lacking combined nitrogen and under conditions which eventually led to phosphorus deficiency. Four different stages were recognized for descriptive purposes with light microscopy. Stage I commences with hormogonia and ends with the development of a heterocyst at one end of the trichome. Stage II commences with the formation of a mature heterocyst; this is followed rapidly by a tapering of the trichome, first by a narrowing at the apical end and then by a broadening at the basal end. Under phosphorus-rich conditions the apical part of this tapered trichome can continue to give rise to a gas-vacuolate hormogonium, but with increasing phosphorus deficiency hormogonium synthesis ceases and the terminal cells start to differentiate into a hair. This latter event is recognized as the start of stage III. When further phosphate is added to a population with hairs a characteristic series of events occurs which is recognized as stage IV. Several gas-vacuolate hormogonia start to differentiate at the apical end of the chlorophyll-containing part of the trichome; the hair falls off; the hormogonia are released by gliding from the sheaths of the old filaments and then float to the surface. A discussion of the role of the hair as a “sacrificial” structure is included.