Studies on Morphological Changes of the Blue-green Alga Nostoc muscorum A with Special Reference to the Role of Light

Morphological changes of Nostoc muscorum A were studied withspecial reference to growth conditions. According to Lazaroff(1973), N. muscorum A has a life cycle dependent on the lightconditions; cells of a coccoid form grow in the dark (aseriatestage) while cells of a filamentous form grow in light (seriatestage). The conversion from the aseriate to the seriate stageis photocontrolled by red stimulation and green suppression.We reexamined (i) the light effect on the morphological formsof N. muscorum A growing under various conditions, includingN₂-fixing and non-fixing, and (ii) the light effect on the controlof the aseriate to seriate conversion in the dark in relationto the effect on the dark growth. Results for (i) indicatedthat cell forms did not necessarily depend on the light butwere determined by growth conditions; under conditions sufficientfor supporting rapid growth, cells grew at the seriate stage,and under insufficient conditions, at the aseriate stage. Thelight effect to induce and support filamentous growth was explainedas the photosynthetic energy supply for improving the growth.Results for (ii) revealed that red light induced the dark aseriateto seriate conversion and at the same time enhanced the darkgrowth after the illumination. Green light suppressed both theconversion and the dark growth stimulated by red light. Thered-green photocontrol of the dark conversion was explainedby the enhancing effect of red light and the inhibitory effectof green light on the dark growth. Morphological changes betweencoccoid and filamentous forms of N. muscorum A are probablynot obligatory for continued growth. ¹Present address: Department of Botany, Faculty of Science,Kyoto University, Kitashirakawa, Kyoto 606, Japan. (Received September 22, 1980; Accepted December 6, 1980)