Photosynthetic Studies On The Terrestrial Cyanobacterium Nostoc Flagelliforme

Nostoc flagelliforme (Berk & Curtis) Bornet & Flash is a terrestrial cyanobacterium with high economic value, while its cultivation has not been successfully achieved. The recently developed aquatic-living colonies bear applied foreground, but little was known about their biological characteristics. Thus, this study compared the photosynthetic speciality of the two kinds of colonies, and investigated their responses to environmental stresses. The main results are as follows: Whether under submersed or emersed conditions, the optimal temperature for photosynthesis of both aquatic-living and natural colonies was 25℃. Both of the colonies showed activity of external carbonic anhydrase (CA), and that for the aquatic-living colonies was higher than that for natural ones. Desiccation of the colonies enhanced the CA activity. When emersed, the highest photosynthetic carbon fixation rate appeared at the water loss of 22-28% for aquatic-living colonies, and at the water loss of 43-48% for the natural colonies. Both forms of colonies exhibited a higher tolerance to desiccation stress, even at the water loss of 80%, they still maintained positive photosynthetic rates. The aquatic-living colonies had a higher photosynthetic carbon fixation capacity than the natural colonies. When incubated at different levels of salinity for 24h or 48h, the photosynthetic performance of the aquatic-living colonies was the highest at 20 mM NaC, though, the salt tolerance was lower compared with the reported values for the natural colonies. Natural colonies were insensitive to UV radiation. Its PSII photochemical activity could be reactivated under the sun light when re-hydrated, and was not affected by PAR, UV-A and UV-B. When re-dehydrated, the PSII activity decreased at the initial stage of water loss, then increased, and reached its highest value at the water loss of 70-90%, and then declined with further water loss under the high levels of solar irradiance. At 180, 360 and 720ppmv levels of CO2, photosynthetic carbon fixation responsed to solar UVR and desiccation in a similar way: increased with initial water loss till about 50% water content, then decreased with further water loss. High CO2 concentration enhanced the photosynthetic carbon fixation, but such an enhancement was not significantly affected by solar UVR. In nature, N. flagelliforme experiences repeated cycles of rehydration and dehydration, thereby, has developed the capacity to tolerate many environmental stresses such as desiccation, high solar irradiance and UVR. Although its colonies has been formed under aquatic conditions, they still maintained the capacity to tolerate environmental stresses, showed similar pattern of photosynthetic performance to the natural colonies during rehydration and dehydration. This indicates the feasibility of the aquatic-living colonies to be used as "seeds"for cultivation when shifted from the aquatic to terrestrial environments.