A Microbial Biorealm page on the genus Nostoc flagelliforme
Higher order taxa
Bacteria; Cyanobacteria; Cyanophyceae; Nostocales; Nostocaceae
Description and significance
Nostoc flagelliforme, one type of blue-green alga, lives in a specific niche with highly varying temperatures, low rainfall, and limited nutrients, but under such conditions it thrives in colonial filaments or alone as single, free-living cells. Its adaptability to its erratic environment is what makes it an intriguing and useful organism. It can be found in arid and semiarid locations within Algeria, China, the former Czechoslovakia, France, Mexico, Mongolia, Morocco, Russia, Somalia, and the United States. In N. flagelliforme’s typical habitat, the changes in temperature from just night to day can be extreme. In Yongden, of the Gansu Province in China, temperatures from night to day differed by 11.0-16.4 degrees Celsius during the summer months. These conditions are important in demonstrating the adaptability of the organism; however, N. flagelliforme’s greatest survival tool may be its ability to desiccate in high temperatures and light exposure for months to even years, and then to recover full photosynthetic activity within hours or days, depending on its surroundings. 
It is also important to note that N. flagelliforme is a terrestrial organism. Cultivation in a laboratory environment has proven difficult, if not impossible, for extended periods of time. Aquatic cultures of the organism, such as those used in the lab, may result in altering the chemical and physical properties of the sheath that surrounds its colonies. This difference in colonial sheath may be the cause of early disintegration of the sheath and cells by other bacteria. The sheath is presumed to be important for creating a more constant environment around N. flagelliforme colonies and retaining water. Disintegration of the sheath as happens in the lab appears to be the cause of colony death. 
Unfortunately there is very little genomic data on Nostoc flagelliforme available, but the few proteins that have been documented help to support the more widely disputed claims about the organism. Superoxide dismutase is a gene that has been found in the genome of N. flagelliforme at the locus ABR01632. The protein it encodes is 200 amino acids in length, and is responsible for catalyzing the conversion of superoxides radicals to hydrogen peroxide and molecular oxygen. The presence of this protein shows us that the organism uses oxygen as a final electron acceptor during respiration. 
One other protein found in N. flagelliforme is putative neutral trehalase. The gene that encodes this protein is found at the ABO31436 locus, and the protein is 469 amino acids long. It serves to aid in the protection of other proteins and membranes against stresses such as heat shock. This protein demonstrates the strong survivability and adaptability of the organism. 
Cell structure and metabolism
The primary carbon source of Nostoc flagelliforme is carbon dioxide. Its main metabolic pathway is the combination of photosynthesis and dark respiration, but it can also utilize the nitrogen fixation pathway. Due to its terrestrial nature, the organism’s photosynthetic activity is highest when slightly desiccated, or dried. Photosynthesis peaks at 30% water loss, most likely due to a decreased aqueous diffusion barrier for carbon dioxide. Further desiccation results in decreased photosynthetic activity, likely due to loss of intracellular water. Dark respiration is able to increase in productivity until approximately 70% water loss, at which point it lowers to half activity until it stops completely. The further away from the optimal 30% water loss, the less light is required for photosynthesis. It is believed that this is due to inefficient use of high irradiance rather than efficient use of low irradiance. The organism’s ability to perform tasks such as light harvesting, energy conversion, and carbon assimilation decline as desiccation increases. 
N. flagelliforme is known to have a significantly low carbon dioxide compensation point in comparison to other blue-green algae. This means that less carbon dioxide is required for optimal photosynthetic uptake and release. This may be because it possesses a carbon dioxide-concentrating mechanism (CCM) that staggers carbon dioxide use as required. However, it appears that N. flagelliforme’s CCM may be less efficient than that of other organisms. This is important because increased carbon dioxide has been found to augment photosynthetic activity for N. flagelliforme. It is believed that in cultivation attempts, increased carbon dioxide levels will help the bacteria flourish. As carbon dioxide levels increase over time, as is happening across the Earth, it will no longer be a limit for the organism’s growth. Higher temperatures and irradiances will also become less imposing obstacles. (It is important to note that while temperature sensitivity may be present for enzymes involved in photophosphorylation, electron transport, and plastoquinone diffusion, net photosynthetic output varies little within the range of 15-35 degrees Celsius. Such a broad temperature range actually encourages a larger photosynthetic output for the organism, but temperatures in its habitat extend outside of this range.) 
After prolonged drought and full desiccation of the organism, its metabolic reactivation goes in order of respiration, photosynthesis, and finally nitrogen fixation. Photosynthetic recovery requires several hours after rehydration. Several factors play a role in recovery time. These factors include the amount of water available for rehydration, and the amount of exogenous potassium ions available. Nutrients like phosphorous play only a minor role in recovery rates. Potassium ions were observed to dramatically increase these rates. Rehydration in a BG11 medium containing adequate K+ resulted in recovery 16 times faster than in distilled water. Peak photosynthetic recovery rates were observed with 345 M K+; however, higher levels of potassium ions have little effect. In the wild, dew may supply the organism with sufficient water while keeping it slightly desiccated, but potassium supplied by rain is limited by evaporation and lack of rain, meaning that N. flagelliforme growth may be limited by K+ levels. K+ is believed to be so important because if its role as a regulatory cation for intracellular pH. It is also a factor in maintaining an ionic environment suitable for preserving the structure of necessary enzymes. 
The disintegration of colonial sheath in Nostoc flagelliforme has been associated with the propagation of other bacteria. These bacteria may inhibit the organism’s growth, or even actively decompose its cells. In the lab, it may be possible that the aquatic sheath provides nutrients for bacterial growth. Desiccation appears to maintain or enhance textural elasticity of the sheath. In an experiment involving bacteria-exposed cultures of N. flagelliforme, disintegration of the colonies occurred within a day with no desiccation. With one long desiccation of 3.5 hours, disintegration was postponed for 5 days, and increasing the frequency but retaining the same amount of time of desiccation postponed disintegration for a total of 7 days. In cultures in which other bacteria were not present, or were present in controlled numbers, N. flagelliforme was able to survive without disintegration for up to 20 days, regardless of desiccation. 
N. flagelliforme may form colonial filaments or stay as free-living cells, depending on the environmental conditions. The amount of light plays a large role, while change in temperature has little effect. Under low light conditions (20-60 µmol photons•m-2•s-1), colonies formed but growth rate is hindered. With higher light intensity (180 µmol photons•m-2•s-1), growth rate was much higher, but colonial filaments did not form. Photoinhibition is not the cause of the lack of colonies, because N. flagelliforme has been shown to grow in up to 1140 µmol photons•m-2•s-1. It is possible that sheath-forming factors are inhibited in such extreme light exposure. This information is important because of the overexploitation of N. flagelliforme as a food in China. Such overexploitation correlates to damage of the surrounding vegetation and deterioration of the environment, eventually resulting in sand storms of increasing magnitude. 
Nostoc flagelliforme has been found to be non-toxic in an acute study of 14 days, as well as a subacute study of 28 days, testing for toxicity through oral consumption by rats. Ophthalmological, haematological, serum biochemical and histopathological tests were run on the rats following the studies. They were also weighed, their food consumption was measured, and autopsies were completed in order to weigh organs and check that everything was normal. In the study, no major differences from the control group occurred. There is no significant evidence that N. flagelliforme has any effect on rats, though that is not evidence that it is completely non-toxic to humans. 
Nostoflan is an acidic polysaccharide found in Nostoc flagelliforme that has been found to have anti-Herpes Simplex Virus 1 properties. In early observations, viral sensitivity to nostoflan was believed to be at the highest during early stages of replication, such as the binding of the virus to the cell, and the penetration into the cell. After further research, it was found that nostoflan inhibits the virus’ ability to bind to the exterior of the cell. 
Phycobilisomes are structures within N. flagelliforme that harvest light of a specific wavelength and transfer it to the photosystems for photosynthesis. They were recently viewed using negative stain electron microscopy in conjunction with cryo-electron microscopy. These phycobilisomes consist of a tricylindrical core of allophycocyanin cylinders, appearing as a dual layer from the side. Such revealing images allow us to compare these harvesting structures with the ones of plants and other photosynthetic organisms. 
 Komárek, J., Kling, H. & Komárková, J. "Filamentous Cyanobacteria". Freshwater Algae of North America. (Wehr, J.D. & Sheath, R.G. Eds), Pages 177-196. San Diego: Academic Press.
 Qiu, B., and Gao, K. "Photosynthetic characteristics of the terrestrial blue-green alga, Nostoc flagelliforme". European Journal of Phycology. 2001. Volume 36. Pages 147-156.
 Gao, K., and Ye, C. "Culture of the terrestrial cyanobacterium, Nostoc flagelliforme (Cyanophyceae), under aquatic conditions". Journal of Phycology. 2003. Volume 39. Pages 617-623.
 Qiu, B., and Gao, K. "Dried field populations of Nostoc flagelliforme (Cyanophyceae) require exogenous nutrients for their photosynthetic recovery". Journal of Applied Phycology. 1999. Volume 11. Pages 535-541.
 Kanekiyo, K., Hayashi, K., Takenaka, H., Lee, J.B., and Hayashi, T. "Anti-herpes simplex virus target of an acidic polysaccharide, nostoflan, from the edible blue-green alga Nostoc flagelliforme". Biological & Pharmaceutical Bulletin. 2007. Volume 30. Pages 1573-1575.
 Yi, Z.W., Huang, H., Kuang, T.Y., and Sui S.F. "Three-dimensional architecture of phycobilisomes from Nostoc flagelliforme revealed by single particle electron microscopy". FEBS Letters. 2005. Pages 3569-73.
 Wang, Y., Chen, L.-P., Chen, X., Zhang, X., Yu, J. and Wang, Q.-X. "Cloning and expression of the gene which encodes SOD of Nostoc flagelliforme in E. coli". Unpublished.
 Takenaka, H., Yamaguchi, Y., Sakaki, S., Watarai, K., Tanaka, N., Hori, M., Seki, H., M. Tsuchida, M., Yamada, A., Nishimori, T., and Morinaga, T. "Safety evaluation of Nostoc flagelliforme (nostocales, cyanophyceae) as a potential food". Food and Chemical Toxicology. 1998. Volume 36, Issue 12. Pages 1073-1077.
Edited by Adam Northrup, student of Rachel Larsen
Edited by KLB