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Soil Biol. Switzer, L. Spirulina the Whole Food Revolution. Tiedje, J. Cyanobacteria live in the water, and can manufacture their own food through "photosynthesis. Individual cyanobacteria are very small and usually just single cells, either round, ovoid, or stringlike in shape. Some types grow in colonies that can be large. These colonies are built of many layers, and are called stromatolites if more or less dome-shaped or oncolites if round.
The characteristic layered structure of fossilized stromatolites advertises their presence, helping scientists locate them and identify their age through radioactive dating of the surrounding rocks. To the far left is a fossil colony, and to the left a living one from Schopf , showing how this pattern has remained stable for billions of years!
Detailed comparisons of individual fossils of ancient bacteria also show them to be virtually identical to those found in living colonies. Much of what we describe about ancient stromatolites is based on our observations of the behavior of living ones, but it appears that this line of evidence should be quite reliable. Stromatolites appear to be the ultimate "living fossils", life forms that have survived for 3. Stromatolites ruled the earth for billions of years.
One line of evidence is the huge deposits of fossils they have left. Indeed, because of their striking appearance, they have been used as ornamental stonework.
To the left is a children's playground slide in China, made of 1. To the right is the Chinese parliamentary building; the columns are stromatolite fossils of the same age; note the people and van for scale picture by G.
Other examples in the literature describe cyanobacteria participating in a two-step system for the production of CH 4 either by producing nutrients used by methanogenic bacteria or by removing CO 2 from biogas and thereby improving its quality. Sustainable CH 4 production has been achieved in Synechococcus PCC from atmospheric CO 2 and solar energy, where the photosynthetic products including glucose or acetic acid were used as nutrients by a methanogenic bacterium in the CH 4 generation Koshland Another species, Arthrospira platensis has also been involved in biogas production by removing CO 2 from the biogas formed by AD of sewage sludge Converti et al.
The algae will be harvested from the Gulf of Gdansk and the Vistula submersion. The project is expected to be completed in BiofuelDigest From an economic point of view, the cost of CH 4 from cyanobacteria is still far more expensive when compared to the CH 4 derived from fossil fuels. In the coming years, this situation might change with efforts to improve the current technology resulting in economically reasonable prices of the gas.
The process of CH 4 production alone or coupled to other bioenergy producing processes, therefore needs further investment in research Rittmann It has recently been reported that microorganisms can convert light into electric energy with the use of photoelectrochemical cells. In these cells, high-energy electrons produced by the light excitation in the photosystems are transferred to an electron mediator, which in turn transfers them to an electrode and thereby producing electricity.
Examples in the literature have studied strains like Anabaena Yagishita et al. Cyanobacterial species can also act as natural resources of H 2 in fuel cells Dawar et al. Behera and co-workers employed Spirulina in fuel cells for H 2 production. This species possess a known high nutritional value that could reduce the production costs of the energy production by this mean Behera et al. In the dark phase of photosynthesis, commonly known as the Calvin cycle, these molecules are used to produce sugars and other organic compounds from CO 2 and water.
Glucosephosphate G6P is in turn formed from 3PG via gluconeogenesis. Finally, RuBP is recovered from fructose 6-phosphate, G3P and dihydroxyacetone phosphate in a sequence of reactions similar to the non-oxidative branch of the pentose phosphate pathway.
Enzymes involved in the Calvin cycle are encoded by genes known as cbb genes. Gibson and Tabita showed that these genes are regulated by a common promoter activated by a LysR-type transcription factor, CbbR.
Knockout strains with mutations affecting CbbR were impaired in the expression of cbb genes Gibson and Tabita It is well known that cyanobacterial Rubisco possesses a relative low affinity for CO 2 when compared to other algae or higher plants. The CCM contains two carbon-fixing enzymes, Rubisco and carbonic anhydrases, stored in carboxysomes. These storage microcompartments are thought to increase the CO 2 level surrounding Rubisco away from the competing O 2.
A detailed understanding about the regulation of the main CCM constituents may enable the manipulation of this system to optimize the CO 2 fixation. A recent study has shown that in cyanobacteria, carboxysomes possess a specific organization through the cell, not found in other prokaryotes, and this distribution is closely linked to the CO 2 fixation efficiency.
It seems that two cytoskeletal proteins, par A involved in chromosome and plasmid segregation and merB involved in cell morphology , are involved in the organization of this specific carboxysomal pattern Savage et al.
The overexpression of carbonic anhydrase, insertion of a more efficient Rubisco or multiple copies of Ci transporters could in principle increase CO 2 fixation levels in cyanobacteria. Although Rubisco is the main enzyme responsible for the C fixation, cyanobacteria possess an additional assimilation mechanism operated by phosphoenolpyruvate carboxylase PEPC and malic acid to assist them in the fixation of large amounts of CO 2 which is similar to the C 4 pathway in plants Yang et al.
The 2-phosphoglycolate P-glycolate is a noxious by-product of the Rubisco oxygenase activity which inhibits important enzymes in the Calvin cycle phosphofructokinase and triosephosphate isomerase. Besides photorespiration, in cyanobacteria, this molecule is degraded via the glycerate and decarboxylation pathway. Certain compounds derived from these pathways, including glycolate and glycine, seem to be potential candidates to control the Ci level in cyanobacteria Eisenhut et al.
Some cyanobacteria strains are capable of assimilating some sugars and growth in dark conditions as facultative heterotrophs. Nevertheless, cyanobacteria grown in the dark have shown lower growth rates than when grown under light conditions Stanier and Cohenbazire Unlike most other phototrophs, in cyanobacteria, photosynthesis and respiration co-occur in a single compartment within the cell, the thylakoid membrane. In addition, constituents from both electron transfer chains such as the redox carriers cytochrome bf complex, plastoquinone, cytochrome c 6 and plastocyanin are shared as well.
Although they have common elements, some of them are still specifically associated to one of the pathways. Sugars are the main and most common source of metabolic energy among living organisms.
Sugar catabolic pathways [glycolysis, the oxidative pentose phosphate pathway OPP and tricarboxylic acid cycle TCA ] are active mainly during the dark phase of the light—dark cycle.
These pathways are responsible for producing NAD P H and other biosynthetic metabolites involved in the normal cellular functions. The major route of glc degradation is the OPP cycle and is considered as the main CO 2 fixation mechanism in cyanobacteria.
Glucosephosphate dehydrogenase controlled at the level of gene expression is especially interesting from a regulation point of view. In addition, low RuBP levels significatively reduce this enzyme activity Kaplan et al. Modulation of sugar catabolic pathways in cyanobacteria during the light—dark transition has been reviewed by Osanai et al.
In Synechocystis sp. PCC , enzymes that participate in the sugar catabolism are stimulated by the light—dark shift and by the circadian rhythm.
Regulatory proteins including histidine kinase, Hik8 and the RNA polymerase sigma factor, Sig E are involved in this activation. In addition, reduced N 2 concentrations also trigger the transcription of sugar catabolic genes via NtcA, the major N 2 mediator. A detailed analysis of the transcriptional network in central metabolism during light periods has been provided by microarray data from Cyanothece Stockel et al. This study revealed that the glycogen accumulated in diurnal periods is later degraded via glycolysis, OPP and the TCA cycle during dark or C depletion conditions.
However, cyanobacteria possess an incomplete TCA cycle unable to work properly as a respiratory chain discussed in the next section Stanier and Cohenbazire Synechocystis sp. PCC was shown to be able to grow under dark conditions with periodic light pulses at glc expenses, a phenomenon known as light-activated heterotrophic growth LAHG.
PCC Tabei et al. Central sugar metabolism differs among photoautotrophic, heterotrophic and photomixotrophic growth conditions reviewed by Kaplan et al.
Previous studies revealed that genomic and metabolomic data provide enough information to model the central metabolism and estimate the flux balance during different conditions in Synechocystis PCC Hong and Lee Modelling results were correlated with experimental data and may be extrapolatable to the whole cell metabolism in this organism. Sugars and other organic compounds from the central metabolism participate in the biosynthesis of diverse cellular metabolites. Here, only the biosynthesis of major classes of carbohydrates will be discussed: glycogen, sucrose and the carbohydrates involved in salt stress response, trehalose and glucosylglycerol and cell wall polysaccharides.
Glycogen is the main carbon and energy storage polysaccharide in cyanobacteria. In the cell, glycogen is synthesised during light periods from assimilated CO 2 Ball and Morell The enzyme glycogen phosphorylase glgP , EC 2. The enzyme phosphoglucomutase pgm , EC 5. Different conditions such as N 2 depletion Yoo et al. In addition, a role as storage and signalling molecule has also been associated to this disaccharide Desplats et al. A schematic representation of sucrose metabolism is shown in Fig.
The main enzymes involved in the sucrose synthesis are sucrose-phosphate synthase sps , EC 2. Sucrose is later hydrolyzed into glc and fru in an irreversible reaction catalyzed by invertases EC 3. Genomic analysis revealed the importance of these enzymes under N 2 -fixing conditions in filamentous cyanobacteria Vargas et al. Sucrose metabolic pathway in cyanobacteria adapted from Curatti et al. Sucrose synthase has only been found in heterocyst-forming cyanobacteria.
Genes encoding metabolic enzymes: agp , ugp , sps , spp , sus , inv , ces. This enzyme seems to be present only in filamentous N 2 -fixing cyanobacteria Curatti et al.
Other osmoprotectants like glucosylglycerol GG, composed of a sugar and a polyol and trehalose also occur in cyanobacteria and they are related to the salt tolerance of the strains. TreY encodes maltooligosyltrehalose synthase mts , EC 5.
Gene disruption experiments indicated that trehalose, like sucrose, plays a crucial role in dehydration Asthana et al. The cyanobacterial cell wall combines features from gram-positive and gram-negative bacteria. From inside to outside a cytoplasmic membrane, a highly cross-linked peptidoglycan layer and an outer membrane with lipopolysaccharides LPSs are the main constituents of this cell wall Hoiczyk and Hansel Peptidoglycan strands in cyanobacteria consist of repeating subunits of the aminosugars N -acetylmuramic acid and N -acetylglucosamine.
In addition, this strand contains cross-linked peptides and it is complexed with specific polysaccharides in its structure. Peptidoglycan biosynthesis, in bacteria, is mediated by the genes murA - murG , mraY and pbp and a schematic representation can be found in Garcia et al.
The LPSs in cyanobacteria have not been extensively documented. Previous studies indicated the different composition of the LPS in marine Synechococcus sp. In addition, it has been suggested that a gene homologue of lpxC , alr participates in the LPS lipid A biosynthesis Nicolaisen et al. Many cyanobacteria are also able to secrete diverse extracellular polymeric substances EPS into their immediate surroundings of the cell reviewed by Pereira et al.
Cellulose has been found as the main constituent of the EPS in several cyanobacteria Nobles et al. As mentioned above, the sus gene seems to be involved in the cellulose biosynthesis Curatti et al. Recently, Synechococcus sp. A possible strategy to enhance the lipid content for biofuel purposes could include knocking out the genes involved in the biosynthesis of storage or osmotic protectant substances. In addition, the overexpression of genes involved in the degradation of these compounds could also increase the lipid production in the cell.
Previous studies on eukaryotic algae indicate that certain starch-impaired strains accumulate higher amounts of PUFA or TAG under N 2 starvation for a review see Radakovits et al. Redirecting the C flux to the cellulose synthesis would be another approach in order to increase the fuel content in cyanobacteria. Currently, ethanol production is derived mainly from the fermentation of cellulose.
The overexpression of cesA plus a knockout of the agp , spp and sps genes could hypothetically cause an increase of the cellulose content in the cell. Besides respiration, other polysaccharides could also in principle be catabolised by fermentation. Previous studies have already attempted to enhance the ethanol levels by developing genetically engineered organisms Deng and Coleman ; Dexter and Fu Fatty acid and protein biosynthetic pathways possess phosphoenolpyruvate PEP as common substrate Fig.
PEP is converted to pyruvate by pyruvate kinase pyk , EC 2. In addition, pyruvic acid can be converted to alanine and thus participates in protein metabolism.
PEPC from Synechococcus vulcanus was strongly activated by fructose-1,6-diphosphate while aspartate acted as a strong suppressor. PEPC seems to divert the carbon flux away from fatty acid biosynthesis. PCC , has led to a lipid content increase in this organism Song et al. Simplified overview of the fatty acid biosynthesis and some of the competing pathways in cyanobacteria adapted from Liu et al.
Pathways not present in cyanobacteria or those which are unknown are indicated with dashed lines. As pointed out before, the first committed reaction in the fatty acid biosynthesis is an enzymatic reaction catalysed by ACCase.
Previous studies have already supported the important role that ACCase possesses directing the C flow towards fatty acid synthesis Lykidis and Ivanova ; Song et al. In plants, fatty acyl-ACP synthesised in the plastids is transformed to free fatty acids by acyl-ACP thioesterases and then transported from the chloroplast to the cytoplasm, thus the possibilities of influencing the control of fatty acid biosynthesis are remote.
In the FAS II system, each reaction is catalyzed by an individual enzyme, while its eukaryotic counterpart is composed of a single multifunctional enzymatic entity FAS I. As mentioned earlier, ACCase activity is inhibited by this acyl-ACP and thereby this end product plays a fundamental role in the fatty acid synthesis control. The formed fatty acyl-ACPs are later directed to the synthesis of the membrane glycerolipids including monogalactosyldiacylglycerol, digalactosyldiacylglycerol, phosphatidyl glycerol PG and sulfoquinovosyldiacylglycerol Weier et al.
The first step in lipid biosynthesis is the formation of a 1-acyl- sn -glycerolphosphate lysophosphatic acid. In microorganisms, this compound is known to be produced by two different mechanisms.
In this system, a molecule of phosphate is added to the fatty acyl group derived from a fatty acyl-ACP chain by PlsX and then transferred to GP in a reaction catalyzed by PlsY Lu et al. It may be of interest to develop a similar strategy in cyanobacteria-overexpressing pls XYC genes.
Nutrient-limited conditions in E. This response is modulated by guanosine tetra- ppGpp and pentaphosphate pppGpp which are included in a group of compounds called alarmones. These compounds mediate a wide spectrum of metabolic reactions.
Previous results indicated that GPAT is negatively regulated by ppGpp leading to a decrease in the lipid synthesis and consequently a decrease in the fatty acid production through the accumulation of fatty acyl-ACPs, responsible of the inhibition of several steps previously mentioned in fatty acid synthesis.
Additionally, in plants and yeasts an acyl-CoA independent reaction catalysed by a phospholipid-diacylglycerol acyltransferase leads also to the TAG synthesis Dahlqvist et al. This in vivo production of fuel molecules circumvents the necessity of industrial transesterification of TAG.
These levels were lower than initially expected. Fatty acid production was improved by overexpressing a native thioesterase and fadD , together with a heterologous acyl-CoA ligase and an ester synthase atfA.
The same authors used genetic engineering to over produce fatty alcohols in E. Recently, similar strategies have been followed by Lu in Synechocystis sp. PCC Lu , although no yields have been reported. Radakovits engineered eukaryotic algae with specific thioesterases for the production of C 12 —C 14 fatty acids.
In cyanobacteria, thioesterases overexpression results on the enhancement of fatty acid secretion Roessler et al. As previously mentioned, fatty acyl-ACP thioesterase has the capacity to uncouple fatty acid from lipid synthesis.
As pointed out before, an increase in fatty acid production has been achieved already in E. In addition, long-chain acyl-CoA seems also to control the expression of genes encoding enzymes of fatty acid catabolism by interacting with the FadR transcriptional regulator.
Previous results indicated that FadR is able to bind to specific DNA sequences inhibiting transcription of genes involved in fatty acid catabolism. These approaches have already been studied in plants and yeasts. In addition, many enzymes involved in the lipid metabolism possess common activities constraining the possibilities of eliminating single steps.
In yeast, deletion of the fatty acid catabolism-encoding genes shows a fatty acids rise in the cells. The fatty acid secretion seems to be also stimulated in these mutants Radakovits et al. Thus, similar strategies could be developed in cyanobacteria.
Any extra amount of energy or C produced during the cell growth is accumulated in storage products in the organisms. Although, as mentioned earlier, glycogen is the main carbohydrate reserve in cyanobacteria, these organisms also produced polyhydroxyalkanoates to store their excess of energy and C.
There are three main enzymes participating in the cyanobacterial glycogen synthesis, an ADP-glucose pyrophosphorylase agp glgC , EC 2. PCC , it seems that mutation of the agp gene is linked to a higher accumulation of PHB compared to the wild type during photoautotrophic growth Wu et al. Mixotrophic growth in presence of either glc or acetate resulted in an enhancement of the glycogen in the wild type and increased growth in both wild type and mutant and PHB in mutant and wild type contents, respectively.
Since acetate provides the acetyl subunits required to form acetyl-CoA, the levels of PHB are higher when compared to photoautotrophic conditions. Nevertheless, acetate unlike glc did not seem to stimulate the cellular growth in a significant way. It has been suggested, that the produced acetyl-CoA is not incorporated into pathways that contributes to the basic functions of the cells.
The participation of acetyl-CoA in the rate-limiting step of the lipid biosynthesis catalysed by ACCase might provide a partial explanation to these findings. These results indicate that agp mutants are able to harness the chemical energy from photosynthesis more efficiently at low light conditions; at higher intensities impaired glycogen synthesis would cause photosynthesis inhibition feedback inhibition.
The same authors later found that during salt stress, these modified organisms produce sucrose rather than GG as osmoprotectant Miao et al. This example further illustrates the complexity of metabolic networks and their modulation. Under constant conditions of light, the growth, photosynthesis and respiration rates were particularly low in these mutants.
Although during salt and oxidative stress they were able to produce sucrose PCC does not accumulate GG , these modified organisms presented an additional growth reduction compared to wild type. These results indicate that glycogen is indirectly linked to the strains ability to successfully deal with stress conditions, probably due to the correlation between glycogen and ATP availability.
Previous studies in eukaryotic algae have shown that the inactivation of genes involved in the biosynthesis of starch, another storage compound, causes an enhanced lipid production Ramazanov and Ramazanov Unfortunately, the lipid and protein contents were not examined in the glycogen-deficient mutants of Synechococcus. The TCA cycle is in most organisms an essential aerobic pathway for the final oxidation of carbohydrates and fatty acids. In contrast, cyanobacteria possess a non-complete cycle mainly orientated to the 2-oxoglutarate 2-OG synthesis, which is involved in amino acid biosynthesis and N 2 fixation.
The 2-OG is derived from citrate in a reaction catalyzed by isocitrate dehydrogenase icd , EC 1. Thus, in cyanobacteria, the reducing power is mainly generated during photosynthesis Muro-Pastor and Florencio The first enzymatic step of the cycle is mediated by a citrate synthase gltA , EC 2.
OAA serves as a precursor of the biosynthesis of several amino acids including aspartate. It should be pointed out that PEPC lacks oxygenase activity and is therefore more effective in the C assimilation than Rubisco where carboxylase and oxygenase activities compete with each other Gillion This fact could explain why C flux, in cyanobacteria, is primarily allocated to the protein biosynthesis.
Usually, TCA intermediates cause the suppression of pyruvate kinase activity, the main modulating enzyme in carbohydrate catabolism Lin et al. As stated above, cyanobacteria possess an incomplete TCA cycle where two key enzymes are missing, the 2-oxoglutarate dehydrogenase ogdh , EC 1. Previous modelling studies in Synechocystis PCC evaluated the addition of these missing enzymes to the metabolomic network Shastri and Morgan Their results suggested that the lack of a completed TCA cycle does not diminish the growth significantly in this microorganism.
In water columns some cyanobacteria float by forming gas vesicles, like in archaea. These vesicles are not organelles as such. They are not bounded by lipid membranes but by a protein sheath. Cyanobacteria use the energy of sunlight to drive photosynthesis, a process where the energy of light is used to split water molecules into oxygen, protons, and electrons. As with any prokaryotic organism, cyanobacter does not show nuclei nor internal membranes; many cyanobacter species have folds on their external membranes which function in photosynthesis.
Cyanobacteria get their color from the bluish pigment phycocyanin, which they use to capture light for photosynthesis. Photosynthesis in cyanobacteria generally uses water as an electron donor and produces oxygen as a by-product, though some species may also use hydrogen sulfide as occurs among other photosynthetic bacteria. Carbon dioxide is reduced to form carbohydrates via the Calvin cycle.
In most forms the photosynthetic machinery is embedded into folds of the cell membrane, called thylakoids. Because of their ability to fix nitrogen in aerobic conditions they are often found in symbiontic partnerships with a number of other groups of organisms, including but not limited to fungi lichens , corals, pteridophytes Azolla , and angiosperms Gunnera.
Many cyanobacteria are able to reduce ambient levels of nitrogen and carbon dioxide under aerobic conditions, a fact that may be responsible for their evolutionary and ecological success. In anaerobic conditions, they are also able to use only PS I—cyclic photophosphorylation—with electron donors other than water for example hydrogen sulfide , in the same way as the purple photosynthetic bacteria.
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