A Microbial Biorealm page on the genus Nostoc punctiforme
Higher order taxa
Genus : Nostoc
Species : punctiforme
Description and significance
Nostoc punctiforme is of the genus Nostoc; which is also known as cyanobacteria and formerly known as blue green algae. N. punctiforme is a filamentous, motile cyanobacterium with a very complex and unique life cycle. This life cycle, which allows for the bacteria to differentiate into one of three forms in response to various environmental factors, is partly accountable for the enormity of the genome: it must allow for numerous cellular response mechanisms which allow for this microbe to exist in drastically different environments (3). The genome, which has been completed by the Doe Joint Genome Institute, is one of the largest sequenced bacterial genomes to date.
N. punctiforme’s ability to differentiate into three distinct cellular forms places it in a unique group of microbes. The three forms, which are dependent on environmental factors, are heterocysts, akinetes, and hormogonia filaments (see cell structure and metabolism). Comprehension on its ability to respond to environmental ques so efficiently may prove to become a biotechnological advantage in the future.
Being one of the most ancient and well known aerobes, N. punctiforme has been found to participate in numerous symbiotic events. In these relationships, N. punciforme increases its rate of nitrogen fixation for the use of fixed nitrogen by its partner.
Nostoc punctiforme was isolated from symbiotic association with the gymnosperm cycad Macrozamia sp.(4) and it is larger than any of the other sequenced cyanobacterial genome, with an astounding current base count of 9,757,495 (3). The enormity of the genome is necessary for the cell to be able to adapt to numerous drastically different environments. It is a circular genome that consists of 45.2 mol % Guanine and Cytosine pairs and 54.8 mol % Adenine and Thymine pairs (4). Each cell contains multiple copies of the circular chromosome (5) which is hypotheisezed to be one of the reasons that N. punctiforme are able to withstand DNA mutagenisis due to prolonged exposure to UV light (6).
Of the 5000 recognized open reading frames (ORFs), 62% encode for proteins with a known or probable function. Curiously enough, one fourth of the genome encodes ORFs that cannot be associated with any previously sequenced ORF. Of the recognized ORFs, 5%, the largest proportion of genes, code for signal transduction mechanisms which can be understood by the cell’s extensive ability to sense and respond to environmental factors. Cell envelope synthesis, cell division and chromosome segregation account for 4% of the ORFs, as do amino acid transport and metabolism. Organic carbon metabolism codes for 3% of the ORFs (3). Apart from the coding DNA, the genome for N. punctiforme has been found to contain numerous insertions sequences and multilocus repeats as well as genes that code for transposase and DNA modification enzymes (2). These are characteristics that contribute to the genomic variation within the species.
Cell structure and metabolism
Nostoc punctiforme is a gram positive prokaryote capable of survivng in radically different environments. Under optimal growth conditions, it can grow as filaments several millimeters long. In order to grow in diverse conditions, however, it must be capable of multiple metabolic pathways. N. punctiforme is usually an autotroph; however, it can behave as a heterotroph if it is removed from light for an extended period of time (3). In this state, the cell relies heavily on the oxidative pentose phosphate pathway. In conditions that provide available light, the cell has not only the compound chlorophyll a (standard in most autotrophs), but also light harvesting pigments which allow it to thrive in competitive habitats or in cases of constant UV light (2).
Of all the metabolic processes it is capable of, N. punctiforme is most recognized for its efficient nitrogen fixation capablilities. It can utilize various inorganic and organic nitrogen sources which reduces its competition in its habitat. However, when nitrogen sources are poor or nonexistant, N. punctiforme will begin heterocyst differentiation, the first of three possible cellular forms for the prokaryote. This results in the termination of oxygenic photosynthetic reactions and conversion to heterotrophic metabolic mode. There will also be a sharp increase in respiration rate(3). In this state, the cells are 6-10 um in diameter (4). In an environment where there is a limitation on nutrient sources other than nitrogen, the cell will convert into a spore, which for this particular species is known as an akinete. In an akinete form, with a diameter between 10 and 20 um(4), the bacteria can remain viable for hundreds of years. However, there is little information on their metabolic pathways at this time. The third potential form for N. punctiforme takes place under high stress. When this occurs, the cell takes on the form of a hormogonium filament. In this state, the cells will undergo division without the usual precursors of an increase in cell biomass and DNA replication. These cells become short, motile filaments that express photo- and chemotactic behaviors. They also produce gas vesicles in order to create buoyancy and a gliding motility in soil or water(3). The hormogonium filaments are 1.5 to 2 um in diameter (4).
The ability of N. punctiforme to differentiate between cell structures and accommodate metabolic pathways accordingly is the reason it is one of the most versatile and adaptatious microbes understood today.
As one of the oldest known aerobes, with a fossil record dating back from 3 to 3.5 billion years ago, ancestors to N. punctiforme are known to have drastically changed the ancient biosphere in two ways. The first is in the photosynthetic oxygen production and release. This oxygen saturated reactive chemicals near it and increased the concentration of oxygen in the atmosphere. The second way is its formation of endosymbiotic associations with eukaryotic cells that led to the evolution of chloroplast-containing algae and terrestrial plants.
Today, N. punctiforme is still participating in symbiotic relationships with fungi and terrestrial plants, including gymnosperms, angiosperm, and bryophytes (2). In these partnerships, N. punctiforme decreases its rate of photosynthesis an dincreases the rate of both its nitrogen fixation and its heterotrophic metabolism. In return, its partner will produce molecules which regulate the differentiation of the microbe into one of its three forms: heterocyst, akinete, or hormogonia filament(3).
Being a very versatile species, largely due to its cabpability of morphing between these three forms, N .punctiforme has a variety of possible habitats. Though it is most often found in terrestrial habitats, it can also survive in fresh water, tropical, temperate and polar terrestrial systems. In the aquatic and terrestrial ecosystems, N. punctiforme will often be found as a colondy of filaments in a gelatinous matrix (4).
There are no known diseases caused by N. punctiforme in any species.
Application to Biotechnology
Further studies of N. punctiforme are likely to produce information on global regulation of multiple developmental pathways, symbiotic associations, and regulation of carbon and nitrogen fixation (4). This may become crucial information due to the dense growing “blooms” of N. punctiforme; of which many have been found to be toxic. These toxic populations are a growing concern for public health. Full comprehension and understanding of the species and its genome will help us manage and maintain their populations (6)
Also, due to the unique diversity and versatility of the genome of N. punctiforme, it serves as a novel place to further delve into the signal transduction pathways and cellular response mechanisms.
One recent study explored the effect of Hydrogenase on gas exchange withing N. punctiforme. During nitrogen fixation, N. punctiforme produces hydrogen which is then taken up by a hydrogenase. It was found that both hydrogen production and nitrogen fixation rates are dependent on the light source and intensity. In wild type N. punctiforme, and increase in light leads to an increase in hydrogen production, whilst there is no observable effect on the rate of nitrogen fixation. In prolonged exposure to light, N. punctiforme will actually decrease the reate of nitrogen fixation. However, when a hydrogenase deficient N. punctiforme mutant is analyzed, it can be seen that in prolonged exposure to light it will rather increase the hydrogen production. These findings are important for understanding nitrogenase-based systems to be able to direct cells towards hydrogen production as opposed to nitrogen fixation (9).
Another study recently took interest in the role of extracellular polysaccharides (EPS) in stress tolerance of N. punctiforme. In times of dessication, also known as dehydration, N. punctiforme ceases all photosynthetic pathways, and therefore does not produce oxygen. However, immediately upon rehydration of the cells, their cell walls are able to evolve oxygen at a high rate. This was found only to be true of samples that had EPS in their extracellular environment: their recovery rate was much higher. Also, only cells with EPS were able to withstand a freeze-thaw treatment. Concludingly, EPS is essential for the cell to survive conditions of extreme stress, such as dessication, freezing and thawing (10).
In one recent study, acyl-lipid desaturase isolated from N. punctiforme, was found to convert a C-C single bond into a C=C double bond in fatty acids that are bound to glycerolipids on the membrane. This may provide to be an essential characteristic for N. punctiforme’s ability to survive in drastically different habitats (11).
Furthermore, N. punctiforme’s transcriptome, or the number of genes being transcribed and used , has been analyzed in different states of the cell. When grown in a ammonia rich media, the transcriptome consists of 2,935 genes; nearly double that of the steady state proteome. When grown in dinitrogen rich media, the cells most often differentiate into their heterocyst form to increase nitrogen fixation. In this state, they are using 495 genes. In low nutrient media, N. punctiforme will differentiate into a spore, known as an akinete. This cell structure utilized 497 genes. In circumstances of extreme stress to the cell, it will convert into its hormogonia filamentous form. This calls for unregulated cell division and requires 1,827 genes in the genome to be in use. The large differences in transcriptomes are indicitave of distinct upstream regulation mechanisms that must be activated for each cell differentiation (5).
1. NCBI: National Center for Biotechnology Information site: Nostoc Punctiforme, Accessed August 27 2007, “http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=272131&lvl=3&lin=f&keep=1&srchmode=1&unlock”
2. Meeks, J., Elhai, J., Thiel, T., Potts, M., Larimer, F., Lamerdin, J., Predki, P., Atlas, R.; “An Overview of the genome of Nostoc punctiforme, a multicellular, symbiotic cyanobacterium” Photosynthesis Research v. 70:80-106. 2001.
3. T. Thiel, J. Meeks, J. Elhai, M. Potts, F. Larimer, J. Lamerdin, P. Predki, R. Atlas; ”Nitrogen Fixation: Analysis of the Genome of the Cyanobacterum Nostoc punctiforme” University of Missouri Department of Biology
4. “Nostoc punctiforme PCC 73102” Doe Joint Genome Institute. http://genome.jgi-psf.org/draft_microbes/nospu/nospu.home.html
5. Elsie L. Campbell, Michael L. Summers, Harry Christman, Miriam E. Martin, and John C. Meeks; “Global Gene Expression Patterns of Nostoc punctiforme in Steady-State Dinitrogen-Grown Heterocyst-Containing Cultures and at Single Time Points during the Differentiation of Akinetes and Hormogonia” Journal of Bacteriology v.189 p. 5247-5256: July 2007.
6. Wright DJ, Smith SC, Joardar V, Scherer S, Jervis J, Warren A, Helm RF, Potts M.; “UV irradiation and desiccation modulate the three-dimensional extracellular matrix of Nostoc (Cyanobacteria)” Journal of Biological Chemistry v.280 p.40271-40281: December 2005.
7. Meeks, J., Wong, F.; “Establishment of a functional symbiosis between the cyanobacterium Nostoc punctiforme and the bryophyte Anthoceros punctatus requires genes involved in nitrogen control and initiation of heterocyst differentiation” Microbiology v.148 p.315-323: 2002.
8. Paula Tamagnini, Rikard Axelsson, Pia Lindberg, Fredrik Oxelfelt, Röbbe Wünschiers, and Peter Lindblad; “Hydrogenases and Hydrogen Metabolism of Cyanobacteria” Microbiology and Molecular Biology Reviews p.1-20: March 2002.
9. Pia Lindberg, Peter Lindblad, Laurent Cournac; “Gas Exchange in the Filamentous Cyanobacterium Nostoc punctiforme Strain ATCC 29133 and Its Hydrogenase-Deficient Mutant Strain NHM5” Applied and Environmental Microbiology p. 2137-2145: April 2004.
10. Yoshiyuki Tamaru, Yayoi Takani, Takayuki Yoshida, Toshio Sakamoto; “Crucial Role of Extracellular Polysaccharides in Desiccation and Freezing Tolerance in the Terrestrial Cyanobacterium Nostoc” Applied and Environmental Microbiology p. 7327-7333: November 2005.
11. Suresh Chintalapati, Jogadhenu Shyam, Sunder Prakash, Pratima Gupta, Shuji Ohtani, Iwane Suzuki, Toshio Sakamoto, Norio Murata, Sisinthy Shivaji; “A novel Δ9 acyl-lipid desaturase, DesC2, from cyanobacteria acts on fatty acids esterified to the sn−2 position of glycerolipids” Biochem journal v. 398 p. 207-214: September 2006.
Edited by student of Rachel Larsen: Bobbi Leal
Edited by KLB