Ecophysiological Studies On The Edible Cyanobacteria, Nostoc Sphaeroides

Nostoc sphaeroides, Ge-xian-mi, is an economic blue-green alga, being used as herbal ingredients and food. Much left to be investigated in order to improve the method of mass culture of this economic alga, though some of its biological characteristics have been documented. This study investigated the morphological development, hormogonia differentiation, growth and ecophysiology of this economic species. The main results are as follows:Study on the colonial formation indicated that a colony was formed from a hormogonium via the following steps: the gas vesicles disappeared, with the formation of sheath-enclosed filament, heterocysts differentiated, and then seriate filaments formed, which developed into aseriate colonies, then to filamentous colonial stage. Such a development was faster in the presence of NCV or under aeration. NOs" reduced the heterocyst formation, but did not completely prevent it. Cultured without the presence of NCV and with aeration, the hormogonia developed into the spherical colonies, while cultured without aeration and with NCV, the developed colonies were elongated. Hormogonial differentiation was affected by the initial biomass density of the colonies. It reached the maximum when the relative volume ratio of colonial volume to that of the total medium was 5%, while almost no hormogonia were produced when the ratio was over 20%.The rates of photosynthesis and growth of Ge-xian-mi were related with its colonial sizes. The bigger the colony, the less the relative growth rate was. The relative growth rate increased with the increasing light density to the maximal value at about lOOumol m-V1. Regardless of the colonial size, the relative growth rateincreased with the increasing temperature from 10 C to 30 C, reaching the highest value at 30 C and then decreased. The photosynthesis rate increased with theincreasing temperature, too, while attained its maximal at 35 C. Smaller colonies were associated with higher net photosynthetic rates, higher affinities to dissolved inorganic carbonate. In a view of drought resistance, photochemical efficiency in abig colony decreased dramatically after losing 84% of the total water, while it did not decrease before 93% of its total water was lost in a small one. In terms of photoinhibition, photochemical efficiency was more inhabited in the bigger ones when exposed to high light, and the former also recovered faster.Ge-xian-mi is resistant to UV-radiation. When exposed to it for a long period of 12 days, their photochemical efficiency changed insignificantly. UV-radiation induced the synthesis of UV-absorbing compounds, the content of which was higher in the colony and its UV tolerance was stronger, compared with the hormogonia.