They’re the most numerous organisms on the planet. There are more of them on Earth than there are observable stars in the Universe and these little creatures are what enabled you – and every other complex living thing that has ever lived on the planet, from dinosaurs to daffodils – to exist.
The main source for food and oxygen are cyanobacteria and chloroplasts that do photosynthesis. Cyanobacteria are essential for the nitrogen cycle, and so to transform nitrogen in the atmosphere into useful form for organisms to make the basic building blocks for life. The end product of photosynthesis is glucose, - needed as food source for almost all life forms. For a proponent that life took millions of years to emerge gradually and biodiversity as well, and so cyanobacteria and chloroplasts, that came hundreds of millions of years after life started, that is a huge problem. No oxygen in the atmosphere and UV radiation would kill the organisms. Nor could they emerge without an adequate food source. Looking everything in that perspective, it makes a lot of sense to believe God created everything in six days. And created the atmosphere with oxygen, and the nitrogen cycle fully setup, and plants and animals like cyanobacteria, essential in the food chain and nitrogen cycle. That would solve the - problem of nutrition, - the problem of UV radiation - and the problem of the nitrogen source required for life.
Nitrogen fixation is extremely sensitive to oxygen—even modest concentrations of O2 inhibit this process.
The existence in the same organism of cyanobacteria of two conflicting metabolic systems, oxygen-evolving photosynthesis and oxygen-sensitive nitrogen fixation, is a puzzling paradox. Explanations are pure guesswork.
Researchers have long been puzzled as to how the cyanobacteria could make all that oxygen without poisoning themselves. To avoid their DNA getting wrecked by a hydroxyl radical that naturally occurs in the production of oxygen, the cyanobacteria would have had to evolve protective enzymes. But how could natural selection have led the cyanobacteria to evolve these enzymes if the need for them didn’t even exist yet? The explanations are fantasious at best.
Nick Lane describes the dilemma in the book Oxygen, the molecule that made the world:
Before cells could commit to oxygenic photosynthesis, they must have learned to deal with its toxic waste, or they would surely have been killed, as modern anaerobes are today. But how could they adapt to oxygen if they were not yet producing it? An oxygen holocaust, followed by the emergence of a new world order, is the obvious answer; but we have seen that there is no geological evidence to favor such a catastrophic history. In terms of the traditional account of life on our planet, the difficulty and investment required to split water and produce oxygen is a Darwinian paradox.
If there was a reduced atmosphere without oxygen some time back in the past ( which is btw quite controversial ) then there would be no ozone layer, and if there was no ozone layer the ultraviolet radiation would penetrate the atmosphere and would destroy the amino acids as soon as they were formed. If the Cyanobacterias however would overcome that problem ( its supposed the bacterias in the early earth lived in the water, but that would draw other unsurmountable problems ), and evolve photosynthesis, they would have to evolve at the same time protective enzymes that prevented them oxygen to damage their DNA through hydroxyl radicals. So what evolutionary advantage would there be they to do this ?
Cyanobacteria are the prerequisite for complex life forms. They are said to exist already 3,5 bio years, and did not change morphologically. They do oxygenic photosynthesis, where the energy of light is used to split water molecules into oxygen, protons, and electrons. It occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.
They have ATP synthase nano-motors. How could ATP synthase “evolve” from something that needs ATP, manufactured by ATP synthase, to function? Absurd “chicken-egg” paradox!
ATP Synthase is a molecular machine found in every living organisms. It serves as a miniature power-generator, producing an energy-carrying molecule, adenosine triphosphate, or ATP. The ATP synthase machine has many parts we recognize from human-designed technology, including a rotor, a stator, a camshaft or driveshaft, and other basic components of a rotary engine. This machine is just the final step in a long and complex metabolic pathway involving numerous enzymes and other molecules—all so the cell can produce ATP to power biochemical reactions, and provide energy for other molecular machines in the cell. Each of the human body’s 14 trillion cells performs this reaction about a million times per minute. Over half a body weight of ATP is made and consumed every day!
A rotary molecular motor that can work at near 100% efficiency.
We found that the maximum work performed by F1-ATPase per 120° step is nearly equal to the thermodynamical maximum work that can be extracted from a single ATP hydrolysis under a broad range of conditions. Our results suggested a 100% free-energy transduction efficiency and a tight mechanochemical coupling of F1-ATPase.
How could ATP synthase “evolve” from something that needs ATP, manufactured by ATP synthase, to function? Absurd “chicken-egg” paradox! Also, consider that ATP synthase is made by processes that all need ATP—such as the unwinding of the DNA helix with helicase to allow transcription and then translation of the coded information into the proteins that make up ATP synthase. And manufacture of the 100 enzymes/machines needed to achieve this needs ATP! And making the membranes in which ATP synthase sits needs ATP, but without the membranes it would not work. This is a really vicious circle for evolutionists to explain.
The history suggested by Proterozoic fossils is that cyanobacteria evolved early and quickly, and then just sat there, changing little over the eons. Many features of cyanobacterial biology are conserved across the entire phylum and so must already have been present when blue-greens began to diversify. Why should fossils from a 1.5-billionyear-old tidal flat look just like the cells observed in coastal mats today? The paleontological observation of long-term cyanobacterial stasis is particularly puzzling because we know that bacteria can evolve rapidly.
A tree showing evolutionary relationships among living cy anobacteria. Note that cy anobacteria with specialized cells fall on a fairly late branch of the tree. This means that fossils showing cell differentiation can place an upper bound on when the tree’s major branches formed.
Energy cycles, how did they "take off" ?
Photoautotrophs ( Photoautotrophs are autotrophs that use light as a source of energy to make organic molecules) like plants, cyanobacteria, and algae make a large proportion of the Earth’s organic molecules via photosynthesis, using light energy, carbon dioxide (CO2) in the atmosphere, and water (H2O). During this process, they also produce oxygen (O2). To supply their energy needs, both photoautotrophs and heterotrophs ( Heterotrophs must consumefood—organic molecules from their environment ) metabolize organic molecules via cellular respiration. Cellular respiration generates carbon dioxide and water and is used to make ATP ( the energy currency in the cell ). Oxygen is released into the atmosphere and can be reused by photoautotrophs to make more organic molecules such as glucose. In this way, an energy cycle between photosynthesis and cellular respiration sustains life on our planet.
Following biogeochemical Cycles are essential for advanced life on earth:
Hydrologic Cycle (Water Cycle)
Global Carbon Cycle
Phosphorus, Iron, and Trace Mineral cycles
That creates a huge problem for origin of life scenarios. How did these cycle get "off the hook"? This is a gigantic interdependent system, which, if one part of the cycle is missing, nothing goes.
That's why the origin of glucose is a huge problem, and one of the unanswered questions in origin of life research.
They have aerobic respiration and anaerobic fermentation which uniquely occurs together in these prokaryotic cells. They do photosynthesis through complex Photosystem I and II and other electron transport complexes. They have a carbon concentration mechanism, which increases the concentration of carbon dioxide available to the initial carboxylase of the Calvin cycle, the enzyme RuBisCO, and transcriptional regulation, which is the change in gene expression levels by altering transcription rates. They are capable of performing the process of water-oxidizing photosynthesis by coupling the activity of photosystem II and I, in a chain of events known as the Z-scheme. They metabolize Carbohydrates through the pentose phosphate pathway. They reduce Carbon dioxide to form carbohydrates through the Calvin cycle. Furthermore, they are able to reduce elemental sulfur by anaerobic respiration in the dark.
No nitrogen: no proteins, no enzymes, no life. We need nitrogen in our bodies, to form amino acids and nucleic acids. Cyanobacteria have the greatest contribution to nitrogen fixation. So in the beginning, not only was lack of oxygen a gigantic problem, but the lack of nitrogen was no less so. In order for the anaerobic organisms, whatever they might have been, to generate oxygen in quantity, they simply HAD to have nitrogen in their tissues (as enzymes etc). With nitrogen as unreactive as it is, then how did they fix it? N2 gas is a very stable compound due to the strength of the triple bond between the nitrogen atoms, and it requires a large amount of energy to break this bond. This is one of the hardest chemical bonds of all to break.The whole process requires eight electrons and at least sixteen ATP molecules. The process, nitrogenase, works in a more exact and efficient way than the clumsy chemical processes of human invention. Several atoms of iron and molybdenum are held in an organic lattice to form the active chemical site. With assistance from an energy source (ATP) and a powerful and specific complementary reducing agent (ferredoxin), nitrogen molecules are bound and cleaved with surgical precision. In this way, a ‘molecular sledgehammer’ is applied to the NN bond, and a single nitrogen molecule yields two molecules of ammonia. The ammonia then ascends the ‘food chain’, and is used as amino groups in protein synthesis for plants and animals. This is a very tiny mechanism but multiplied on a large scale it is of critical importance in allowing plant growth and food production on our planet to continue.
They are able to capture the energy of light with 95% efficiency. Recently it has been discovered, that they accomplish that through sophisticated quantum mechanics – an esoteric aspect of nature that even most scientists don’t understand. The use light harvesting antennas for that !!
They possess an autoregulatory transcriptional feedback mechanism called circadian clock and coordinate their activities such as sleep/wake behavior, body temperature, hormone secretion, and metabolism into daily cycles . This is an intrinsic time-keeping mechanism that controls the daily rhythms of numerous physiological processes. They control the expression of numerous genes, including those that code for the oscillator proteins of the clock itself.Cyanobacteria have 1,054 protein families !!!
In a BBC report, they said: Oxygenic photosynthesis is a very complicated metabolism and it makes sense that the evolution of such a metabolism would take perhaps two billion years.
Cyanobacteria (Rai et al., 1998) are known to orient their movement with respect to the Earth’s magnetic field.
Feel free to explain how Cyanobacteria got these amazing capabilities, amongst others, in a relatively short evolutionary timescale?
Adventures with cyanobacteria: a personal perspective
Contradictory Phylogenies for Cyanobacteria
The Biogeochemical Cycles of Trace Metals in the Oceans
Genomes of Stigonematalean Cyanobacteria (Subsection V) and the Evolution of Oxygenic Photosynthesis from Prokaryotes to Plastids
Light-driven oxygen production from superoxide by Mn-binding bacterial reaction centers
Biologie Uni Hamburg - Cyanobacteria
Cronodon Cyanobacteria, great !
Some Cyanobacteria adjust their buoyancy by means of gas vacuoles, enabling them to adjust their position in the water column, floating near the surface during the day for photosynthesis and sinking deeper at night to harvest nutrients. Nitrogen fixation requires anaerobic conditions, but Cyanobacteria are aerobes. They solve this problem by having specialized cells called heterocysts which have thick walls impermeable to oxygen and in which nitrogen fixation can occur. Smart, huh?
By producing oxygen as a gas as a by-product of photosynthesis, cyanobacteria are thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to endosymbiotic theory, the chloroplasts found in plants and eukaryotic algae evolved from cyanobacterial ancestors via endosymbiosis.
The Cyanobacteria: Molecular Biology, Genomics, and Evolution
Cyanobacteria are a fascinating and versatile group of bacteria of immense biological importance. Thought to be amongst the first organisms to colonize the earth, these bacteria are the photosynthetic ancestors of chloroplasts in eukaryotes, such as plants and algae. In addition, they can fix nitrogen, survive in very hostile environments (e.g. down to -60-degreesC), are symbiotic, have circadian rhythms, exhibit gliding mobility, and can differentiate into specialized cell types called heterocysts. This makes them ideal model systems for studying fundamental processes, such as nitrogen fixation and photosynthesis. In addition, cyanobacteria produce an array of bioactive compounds, some of which could become novel anti-microbial agents, anti-cancer drugs, UV protectants, etc. The amazing versatility of cyanobacteria has attracted huge scientific interest in recent years. Given that 24 genomes sequences have been completed and many more projects are currently underway, the point has been reached where there is an urgent need to summarize and review the current molecular biology, genomics, and evolution of these important organisms. This volume brings together the expertise and enthusiasm of an international panel of leading cyanobacterial researchers to provide a state-of-the art overview of the field. Topics covered include: evolution, comparative genomics, gene transfer, molecular ecology and environmental genomics, stress responses, bioactive compounds, circadian clock, structure of the photosynthetic apparatus, membrane systems, carbon acquisition, nitrogen assimilation, C/N balance sensing, and much more. This book will be essential for anyone with an interest in cyanobacteria, bacterial photosynthesis, bacterial nitrogen fixation, and symbiosis.
The Molecular Biology of Cyanobacteria summarizes more than a decade of progress in analyzing the taxonomy, biochemistry, physiology, cellular differentiation and developmental biology of cyanobacteria by modern molecular methods, especially molecular genetics. During this period cyanobacterial molecular biologists have been "studying those things that cyanobacteria do well," and they have made cyanobacteria the organisms of choice for detailed molecular analyses of oxygenic photosynthesis. Part 1 contains chapters describing the molecular evolution and taxonomy of the cyanobacteria as well as chapters describing cyanelles and the origins of algal and higher plant chloroplasts. Also included are chapters describing the picoplanktonic, oceanic cyanobacteria and prochlorophytes, "the other cyanobacteria." Part 2 is devoted to a detailed description of structural and functional aspects of the cyanobacterial photosynthetic apparatus. Included are chapters on thylakoid membrane organization, phycobiliproteins, and phycobilisomes, Photosystem I, Photosystem II, the cytochrome b6f complex, ATP synthase, and soluble electron carriers associated with photosynthetic electron transport. Structure as it relates to biological function, is heavily emphasized in this portion of the book. Part 3 describes other important biochemical processes, including respiration, carbon metabolism, inorganic carbon uptake and concentration, nitrogen metabolism, tetrapyrrole biosynthesis, and carotenoid biosynthesis. Part 4 describes the cyanobacterial genetic systems and gene regulatory phenomena in these organisms. Emphasis is placed on responses to environmental stimuli, such as light intensity, light wavelength, temperature, and nutrient availability. Cellular differentiation and development phenomena, including the formation of heterocysts for nitrogen fixation and hormogonia for dispersal of organisms in the environment, are described
Cyanobacteria are moss-like species that live in oxygen-poor environments bathed in light, such as in shallow bodies of water. They are the only bacteria that produce oxygen as a waste product07 -- which is an important task of this early life. They are exceedingly complex, far from what one would think to call primitive. They grow in long chains because when the cells reproduce they divide in half and tend to remain attached (Figure 2b). They secrete a kind of mucilage or slime which solidifies to form characteristic multi-layered dome-like structures called stromatolytes that grow in highly saline tidal basins -- shallow water between high and low tide. Living stromatolytes exist today in only a few locations worldwide, one being Hamelin Pool in Western Australia
If these fossils are cyanobacteria (or closely related ancestors), then it immediately poses a problem because -- as we will see -- cyanobacteia are advanced bacteria, not what one would assume to be representative of the earliest living species
Why bacteria and not archaea?
Some paleo-biologists insist that the earliest life was from the kingdom Archaea (indeed the name implies that they are the most ancient bacteria), based on the ability of archaea to manage in very hostile environments (which the early earth certainly was), and the claimed advantages of survival near deep water thermal vents.
It is not the purpose here to confirm or deny this possibility, but there are some good reasons to doubt that archaea could "be fruitful and multiply and fill the earth" [Gen. 1] to the degree required at this point in the earth's history: Archaea are too limited and specialized to fill that role. In addition, the genetic make-up of the archaea appears to be more advanced than that of bacteria, more akin to eukaryotes, and therefore (one would assume) a later development.
In the final analysis, though, it does not really matter whether the first living species were archaea; the first practical living species had to be bacteria -- oxygen-producing cyanobacteria (or close ancestors) -- and as a matter of fact, these were the first fossils preserved in the fossil record.
From the point of view that the main task of early life was to form a fit place for later life, it is significant that no known archaea species conduct photosynthesis or have oxygen as a waste product, and so they would be unable to convert the initial reducing environment to an oxidizing environment, required for advanced life.
Regarding the appearance of the first life, Alexandre Meinesz, How Life Began: Evolution's Three Geneses refers to "the strange fact that the ancestral bacteria were already highly diversified" when the first fossil evidence was found. He then continues, "The currently popular idea that life probably arose in warm subsurface waters along a mid-ocean ridge, the kind of environment where a great variety of heat-resisting bacteria thrive today, is a hypothesis without any scientific basis."
Could the oxygen and nitrogen cicle be explained by naturalistic means ? The reason for the abundance of oxygen in the atmosphere is the presence of a very large number of organisms which produce oxygen as a byproduct of their metabolism. Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis. They are one of the oldest bacteria that live on earth, said to exist perhaps as long as 3.5 billion years. And their capabilities are nothing more than astounding.
No cianobacteria, not enough oxygen, no higher life forms. These cianobacterias have incredibly sophisticated enzyme proteins and metabolic pathways, like the Z-scheme and electron transport chains, ATP synthase motors, circadian clock, the photosynthetic light reactions, carbon concentration mechanism, and transcriptional regulation , they produce binded nitrogen through nitrogenase, a highly sophisticated mechanism to bind nitrogen, used as a nutrient for plant and animal growth. The Nitrogen cycle is a lot more complex than the carbon cycle. Nitrogen is a very important element. It makes up almost 80% of our atmosphere, and it is an important component of proteins and DNA, both of which are the building blocks of animals and plants. Therefore without nitrogen we would lose one of the most important elements on this planet, along with oxygen, hydrogen and carbon. There are a number of stages to the nitrogen cycle, which involve breaking down and building up nitrogen and it’s various compounds.There is no real starting point for the nitrogen cycle. It is an endless cycle. Potential gaps in the system cannot be reasonably bypassed by inorganic nature alone.
It must have a degree of specificity that in all probability could not have been produced by chance. A given function or step in the system may be found in several different unrelated organisms. The removal of any one of the individual biological steps will resort in the loss of function of the system. The data suggest that the nitrogen cycle may be irreducibly interdependent based on the above criteria. No proposed neo-Darwinian mechanisms can explain the origin of such a system.The ultimate source of nitrogen for the biosynthesis of amino acids is atmospheric nitrogen (N2), a nearly inert gas. Its needed by all living things to build proteins and nucleic acids. This is one of the hardest chemical bonds of all to break. So, how can nitrogen be brought out of its tremendous reserves in the atmosphere and into a state where it can be used by living things?
To be metabolically useful, atmospheric nitrogen must be reduced. It must be converted to a useful form. Without "fixed" nitrogen, plants, and therefore animals, could not exist as we know them. This process, known as nitrogen fixation, occurs through lightening, but most in certain types of bacteria, namely cianobacteria. Even though nitrogen is one of the most prominent chemical elements in living systems, N2 is almost unreactive (and very stable) because of its triple bond (N?N). This bond is extremely difficult to break because the three chemical bonds need to be separated and bonded to different compounds. Nitrogenase is the only family of enzymes capable of breaking this bond (i.e., it carries out nitrogen fixation). Nitrogenase is a very complex enzyme system. Nitrogenase genes are distributed throughout the prokaryotic kingdom, including representatives of the Archaea as well as the Eubacteria and Cyanobacteria.With assistance from an energy source (ATP) and a powerful and specific complementary reducing agent (ferredoxin), nitrogen molecules are bound and cleaved with surgical precision.
In this way, a ‘molecular sledgehammer’ is applied to the NN bond, and a single nitrogen molecule yields two molecules of ammonia. The ammonia then ascends the ‘food chain’, and is used as amino groups in protein synthesis for plants and animals. This is a very tiny mechanism, but multiplied on a large scale it is of critical importance in allowing plant growth and food production on our planet to continue. ‘Nature is really good at it (nitrogen-splitting), so good in fact that we've had difficulty in copying chemically the essence of what bacteria do so well.’
If one merely substitutes the name of God for the word 'nature', the real picture emerges.These proteins use a collection of metal ions as the electron carriers that are responsible for the reduction of N2 to NH3. All organisms can then use this reduced nitrogen (NH3) to make amino acids. In humans, reduced nitrogen enters the physiological system in dietary sources containing amino acids. One thing is certain—that matter obeying existing laws of chemistry could not have created, on its own, such a masterpiece of chemical engineering.Without cyanobacteria - no fixed nitrogen is available.Without fixed nitrogen, no DNA, no amino-acids, no protein can be synthesised. Without DNA, no amino-acids,protein, or cyanobacteria are possible. So thats a interdependent system.
This time span was once considered too short for the emergence of something as complex as a living cell. Therefore, a number of people suggested that germs of life may have come to earth from outer space with cometary dust or even via a space probe sent out by some distant civilization.
A phylogenetic analysis based on protein data demonstrates a possibility of six classes of the linker family in cyanobacteria. Emergence, divergence, and disappearance of PBSs linkers among cyanobacterial species were due to speciation, gene duplication, gene transfer, or gene loss, and acclimation to various environmental selective pressures especially light.
Where is the demonstration ??
Strategies to Protect Nitrogenase against oxygen
Oxygen Relations of Nitrogen Fixation in Cyanobacteria
The coexistence of oxygen-evolving photosynthesis and oxygen-sensitive nitrogen fixation in diazotrophic cyanobacteria appears to be a remarkable achievement, especially when one considers that the two antagonistic processes may occur not simply in the same organism but indeed in the same cell. 1
Effect of oxygen on nitrogen fixation
Oxygen inactivates and destroys nitrogenase, and represses nitrogenase synthesis. The mechanisms that protect the enzyme system from the damaging effects of oxygen are rather varied. In many diazotrophs more than one mechanism may be present, and in cyanobacteria a whole range of devices seem to operate in an orchestrated fashion to protect nitrogenase from both atmospheric and intracellular sources of oxygen.
Obligate anaerobes, such as Clostndium pasteurianum and Desulfovibrio desulfuncans, are apparently devoid of any specific device to protect their nitrogenase, or indeed any other cell constituents, from the deleterious
effects of oxygen. Therefore they can live and fix nitrogen only in the complete absence of oxygen and are limited in their natural distribution to oxygen-free environments.
Facultative bacteria, for example Klebsiella pneumoniae, Bacillus polymyxa, and Rhodospirillum rubrum, are able to grow on combined nitrogen in both the presence and absence of oxygen but can fix nitrogen only anaerobically.
Microaerophilic bacteria, such as Azospirillum species, show a preference for subatmospheric levels of oxygen when fixing nitrogen. They are unable to fix nitrogen at high oxygen tensions or under anaerobic conditions.
Aerobic bacteria, represented by the Azotobacter species, are capable of growth on dinitrogen in air. Certain strains, however, may display oxygen sensitivity during induction of nitrogenase synthesis. Protective mechanisms have been shown to operate in the last three groups of nitrogen-fixing bacteria.
Protection methods of Nitrogenase from Oxygen
Both protein components of nitrogenase are extremely sensitive to oxygen and the bacteria fixing nitrogen aerobically have a variety of strategies to protect the nitrogenase from oxygen poisoning. Among the members of the
genus Azotobacter, there are three mechanisms for nitrogenase protection. These are respiratory protection, conformational protection, and oxygen regulation of nitrogenase synthesis.
The variety of mechanisms devised by the prokaryotes for protecting nitrogenase from O2 poisoning is an impressive example of the strategic versatility of the prokaryotes.
Some of the nonheterocystous cyanobacteria have solved the problem of nitrogen fixation and photosynthetic O2 evolution by separating the two processes in time rather than in space. Thus, nitrogenase is synthesized and nitrogen fixation takes place in the dark. During the photoperiod, the nitrogenase formed during the previous dark period is presumably destroyed. Indeed this was the first evidence for a biological clock in prokaryotes. It is still not
clear, however, how the filamentous nonheterocystous colonial cyanobacterium Trichodesmium fixes nitrogen during photosynthesis.
Metabolic Rhythms and Regulation in Cyanothece
Cyanothece sp. strain ATCC 51142 is a marine, unicellular, diazotrophic cyanobacterium. It appears that the strategy of temporal separation is used to protect oxygen-sensitive nitrogenase from photosynthetic oyxgen. When grown under alternating periods of 12-h light and 12-h dark (LD), photosynthetic oxygen evolution is limited to the light phase. The fixation of dinitrogen occurs during discrete periods in the dark phase. Dinitrogen fixation is a rather energy-expensive process, however, in the dark, photosynthesis cannot directly supply the energy and reducing power needed. In Cyanothece sp., it appears that dinitrogen fixation in the dark is powered by stored carbohydrates. During the latter part of the light phase, carbohydrates accumulate in large granules that form between the photosynthetic membranes (thylakoids). The carbohydrate granules can be visualized in the electron microscope. The number of granules and the amount of carbohydrate are highest just prior to the onset of the period of dinitrogen fixation. Most of the carbohydrate is degraded to fuel dinitrogen fixation, so there are very few granules remaining at the end of the dark phase. The conversion of carbohydrates to energy and reducing power is a process called respiration and requires oxygen. Thus, as the carbohydrates as used, oxygen within the cell is also being used. This in effect lowers the oxygen concentration within the cell and helps to protect nitrogenase from inactivation by oxygen.
Respiratory protection occurs because Azotobacter can consume oxygen much faster than its rate of entry into the cell. These unusually high rates of respiration thus result in maintaining the nitrogenase in an essentially anoxic environment. Indeed, limiting Azotobacter respiration increases their sensitivity to oxygen during nitrogen fixation. Azotobacter species have the highest known rate of respiratory metabolism of any organism, so they might protect the enzyme by maintaining a very low level of oxygen in their cells. Azotobacter species also produce copious amounts of extracellular polysaccharide (as do Rhizobium species in culture - see Exopolysaccharides). By maintaining water within the polysaccharide slime layer, these bacteria can limit the diffusion rate of oxygen to the cells. 14
In aerobic nitrogen-fixing organisms, the requirements for nitrogenase activity are generated in oxygen-dependent respiration under conditions that appear to conflict with the oxygen lability of nitrogenase. Azotobacter spp. and other aerobic diazotrophs are able to respond to increased concentrations of dissolved oxygen by increasing their rate of respiration, thereby maintaining low levels of intracellular oxygen and protecting their nitrogenase from inactivation. This adaptation may take place in response to an oxygen-sensing mechanism and probably involves several components of the respiratory system, such as NADH/NADPH dehydrogenases and cytochrome a2, as well as the main metabolic pathways. At higher oxygen tensions the respiratory response becomes nonlinear, which may indicate the involvement of additional protective mechanisms. Nevertheless, under natural conditions and within a limited range of dissolved-oxygen concentration, respiratory protection may be sufficient to scavenge excess oxygen and to maintain nitrogenase in a virtually oxygen-free cellular environment.
Conformational protection is a result of the ability of Azotobacter to synthesize another FeS protein that enters into an association with the nitrogenase complex and protects it from O2 inactivation
Under oxygen-stressed conditions (when the oxygen concentration approaches about 20 ,uM), nitrogenase in Azotobacter species is inactivated. The enzyme system will regain full activity upon removal of excess oxygen without new synthesis of nitrogenase proteins. It has been assumed that the observed inactivation (or switch-off) of nitrogenase occurs when the capacity for respiratory oxygen scavenging becomes inadequate to protect the nitrogenase
and that the reversible nature of this inactivation implies a transient change in the conformation of the enzyme complex. It is well documented that the protected, oxygen-tolerant form of nitrogenase in Azotobacter spp. is the result of an association between nitrogenase proteins and a protective 2Fe-2S protein, also called Shetna's protein II. The switch-off is apparently triggered by the oxidation of dinitrogenase reductase, mediated by Shetna's protein II, and is followed by the formation of an oxidized, oxygen-stable complex in which the three components are combined in a defined stoichiometric ratio.
This raises a few important questions: How did the protective enzyme emerge in the first place, how was the defined, correct stoichiometric ratio setup? since it has a defined value, not any value goes, which implies that any ratio out of the required one, will turn the protection inactive.
Restoration of nitrogenase activity (switch-on) is initiated by the reduction of the complex, followed by its dissociation. The switch-off-switch-on phenomenon has also been observed in Klebsiella pneumoniae and Rhodopseudomonas species. An alternative interpretation of this phenomenon considers the diversion of electrons from nitrogenase to oxygen or other electron acceptors to be the primary event that sets off the reversible inactivation of nitrogenase.
Nitrogen fixation in both freeliving organisms and symbiotic systems is accompanied by a variable amount of hydrogen evolution in a reaction catalyzed by nitrogenase. In the absence of a suitable substrate, such as N2, nitrogenase discharges protons to evolve hydrogen gas H2 in a reaction that consumes both ATP and reductant. H2 formation appears to be an intrinsic characteristic of the nitrogenase reaction and continues at a low level (1 mol of H2 per mol of N2) even under highly elevated oxygen tensions or in the presence of alternative substrates of nitrogenase (acetylene, cyanide, or azide). The function of uptake hydrogenase is multifold: it removes H2 inhibitory to N2 reduction, it acts as an oxygen scavenging device and augments respiratory protection, and it reduces the wastage of energy and reducing power inflicted by H2 production.
Enzymes Protecting against Reactive Forms of Oxygen
The production of reactive oxygen species, such as superoxide radical (02), hydrogen peroxide (H202), and hydroxyl radical (HO-), results from univalent reduction Of 02 and invariably accompanies aerobic respiration and oxygenic photosynthesis. Reactive forms of oxygen are extremely toxic to biological systems and would seriously damage not only nitrogenase but also many other essential cell constituents were there not enzymic mechanisms affecting their destruction. Superoxide dismutase, which catalyzes the reduction of superoxide radicals, is considered to be the primary defense mechanism against potential oxygen toxicity. Hydrogen peroxide can be eliminated by the action of catalase, which mediates its conversion to H20 and O2. Peroxidases, such as ascorbate peroxidase or glutathione peroxidase, reduce hydrogen peroxide and reinforce the action of catalase. Antioxidant enzymes appear to play an important part in complementing other devices in the protection of nitrogenase against oxygen inactivation in aerobic and microaerophilic diazotrophs.
Apparent incompatibility of photosynthesis and nitrogen fixation in cyanobacteria
Cyanobacterias have had to acquire efficient devices to protect nitrogenase first from oxygen generated as a result of their photosynthetic metabolism and later also from external oxygen stress. The former may be even more imperative when photoevolution of oxygen takes place in the proximity of nitrogenase activity. It has been established during the past 15 years of intensive research that cyanobacteria have mechanisms to protect their nitrogenase from oxygen. These range up to the most elaborate and efficient mechanisms represented by the specialized nitrogen-fixing cell, the heterocyst. There appear to be a variety of strategies that function more or less efficiently, alone or in combination, to protect the enzyme complex against both exogenous (atmospheric) and endogenous (photosynthetic) sources of oxygen.
1. Single celled cyanobacteria that do not form colonies cannot use the same strategies as the filamentous and colony-forming organisms. The unicellular cyanobacteria seperate in time the functions of photosynthesis and dinitrogen fixation. In the natural environment, these organisms photosynthesize and produce oxygen in the light during the day, and then fix dinitrogen in the dark at night.
2. Cyanobacterias with filamentous strains
Other cyanobacteria use different strategies to protect their nitrogenase from oxygen. Certain filamentous strains form bundles of filaments. In these bundles, only certain filaments will fix dinitrogen, the other filaments somehow shielding the N2-fixing filaments from oxygen.
Some cyanobacteria that form filaments (strings) of cells develop specialized cells that perform dinitrogen fixation, called heterocysts.The nitrogen-fixing cyanobacteria are presented with an even greater challenge than other aerobes because O2 is one of the main products of their photosynthetic metabolism. Anabaena and other related filamentous, nitrogen-fixing cyanobacteria solve the problem of O2 poisoning of nitrogenase by segregating the nitrogen-fixing enzymes in a specialized cell called “the heterocyst.” The heterocyst insulates the nitrogenase from O2 in two ways. First, it lacks photosystem II and thus does not generate any O2; photosystem I is still operative and continues to generate ATP by photophosphorylation. Second, it is surrounded by a laminated structure consisting of a series of unique glycolipids that seem to act as a physical barrier to prevent O2 from penetrating into the cell. Thus, the cell separates its nitrogenase both from endogenous as well as exogenous O2. The heterocyst can feed the fixed, reduced
nitrogen products to the adjoining vegetative cells, from which it receives the reducing power necessary to convert dinitrogen to amino acids. To add to the elegance of the solution, the heterocysts are interspersed along the filament, spaced so as to provide an optimum supply of fixed nitrogen to the growing and dividing vegetative cells. A peptide signal, similar to those used for quorum sensing by Gram-positive bacteria, is used to regulate this spacing.
The heterocysts form a thickened cell wall (glycocalyx) and plugs at the junction between cells in a filament to limit the diffusion of atmospheric oxygen into the heterocyst. Photosynthesis is turned off in heterocysts and they survive on sugars supplied by the other cells of the filament. These other cells, called vegetative cells, are repaid for their sugars with fixed nitrogen compounds made by the heterocyst. 13
Heterocysts provide a finely regulated anaerobic microenvironment for the efficient function and protection of nitrogenase. Heterocyst development results in the distinct spatial separation of the two contrasting metabolic activities of oxygenic photosynthesis and oxygen-sensitive nitrogen fixation.
Heterocyst differentiation involves profound structural and biochemical changes, which include the mobilization of granular inclusions and reserve products, the deposition of a multilayered envelope external to the cell wall, the formation of a narrow junction between the heterocyst and the adjacent vegetative cell, the disintegration and new formation of the intracytoplasmic membrane system, and protein degradation and synthesis of new proteins. In the heterocyst-forming cyanobacteria, nitrogen fixation and photosynthesis are spatially separated in different cell types. 2 Although bacteria are frequently considered just as unicellular organisms, there are bacteria that behave as true multicellular organisms. The heterocyst-forming cyanobacteria grow as filaments in which cells communicate. Intercellular molecular exchange is thought to be mediated by septal junctions. 3 These bacteria can be considered true multicellular organisms with cells exchanging metabolites and signaling molecules via septal junctions, involving the SepJ and FraCD proteins. 4 Heterocyst-forming cyanobacteria are true multicellular organisms with elaborated communication along the filament. Heterocysts obtain, from vegetative cells, reduced carbon in the form of -at least- sucrose and glutamate, the substrate of glutamine synthetase. In return, heterocysts deliver fixed nitrogen compounds, presumably including glutamine and β-aspartyl-arginine . The amidase AmiC2 from N. punctiforme perforates the septal peptidoglycan creating an array of nanopores, which appear to be essential for filament morphology, intercellular communication, and cell differentiation. That amidase is one of two proteins, AmiC1 and AmiC2, that are conserved in heterocyst-forming cyanobacteria.
Model of the cell–cell communication structure in filamentous cyanobacteria.
(A) Schematic filament of vegetative Anabaena cells with two sectioned cells (cell 1 and cell 2), allowing the view on top of the septal disks.
(B) Model of the septal disks between vegetative cells, showing AmiC drilling a nanopore, thereby forming the nanopore array. Nanopores containing septal junction complexes allow the exchange of molecules through the septal peptidoglycan. 4
To cross the septal cell wall, a nanopore array consisting of approximately 150 pores of 20 nm in diameter is present in the septal peptidoglycan 5 N-acetylmuramoyl-l-alanine amidases ( EC 126.96.36.199) are involved in nanopore formation.
Schematic view of N2 - fixation in heterocysts and carbon e nitrogen exchanges with vegetative cells in diazotrophic filamentous cyanobacteria.
PS I, photosystem I; PS II, photosystem II; RIB5P, ribulose-5-phosphate; 6PGLUC, 6-phosphogluconic acid; G6P, glucose-6-phosphate; GOGAT, glutamate synthase.
The diffusion of air into the heterocyst is a compromise between the maximum influx of dinitrogen gas while oxygen is kept sufficiently low to allow nitrogenase activity. This investigation tested the hypothesis that the heterocyst is capable of controlling the influx of air. 6 The heterocyst of Fischerella sp. is capable of controlling the influx of air. Nitrogenase is extremely sensitive for oxygen. Nitrogenase is irreversibly inactivated by exposure to even low concentrations of Oxygen. It is being claimed that Nitrogenase evolved during the pre-oxygenated state of the earth’s atmosphere 7 With the appearance of oxygenic photosynthesis and the resulting oxygenation of the atmosphere and large parts of the earth’s biosphere, aerobic diazotrophic organisms had to evolve mechanisms to protect nitrogenase from inactivation by oxygen.
How could that have happened without poisoning the organism, and stop nitrogenase activity ? The other question is how cyanobacterias got their energy previously, and why they developed such extremely complex molecular machinery.
The heterocyst differentiates from a vegetative cell at semi-regular distances along the trichome through a complex chain of events.
The coexistence of heterocysts and vegetative cells is essential for the survival of the filament since
(i) heterocysts lose their photosynthetic capacity so they need vegetative cells around to be provided with a source of fixed carbon and
(ii) cell division, i.e. reproduction, is only accomplished by vegetative cells. 8
From such a paradigmatic example it is clear that differentiation processes are the result of the interplay of complex regulatory networks acting inside the cell and external stimuli, coming from both the adjacent cells and the environment.
The formation of multicellular organisms from the assembly of single-celled ones constitutes one of the most striking and complex problems tackled by biology.
The most salient feature that characterizes multicellular organisms is the presence of different cell types, in such a way that the organism associates a different function to each cell type. In each of these cellular types, only a subset of the genes that constitute the genome of the organism (genotype) are expressed, which identify the function and morphology of the cell (phenotype). These processes are highly dynamical, directed by complex regulatory networks involving cell-to-cell interactions, and often triggered by external stimuli. As a result of the differentiation processes a rich cooperative pattern involving different cell types is established, increasing the complexity and adaptability of the organism. Nitrogenase, the enzyme that performs nitrogen fixation, is deactivated by oxygen so that nitrogen fixation cannot occur in its presence
Cyanobacteria solve the incompatibility of incorporating both oxygenic photosynthesis and nitrogen fixation by separating these processes (i) temporally, such as in the unicellular Cyanothece sp. strain ATCC 51142, which presents photosynthetic activity during the day and fixes nitrogen during the night , or (ii) spatially, by the generation of non-photosynthetic nitrogen-fixing cells distributed along the filament and acting as nitrogen suppliers. The resulting pattern forms one of the simplest and most primitive examples of a multicellular organism as a product of the interdependence between heterocysts and vegetative cells.
Multicellularity involves at least three welldefined processes: cell–cell adhesion, intercellular communication and cell differentiation. 10
The left cell represents a vegetative cell while the right a nitrogen-fixing heterocyst.
Red color indicates pseudogenes lacking functional counterpart in the No Azgenome. Orange indicates pseudogenes where a functional counterpart is present elsewhere in the genome. Fully functional gene(s) are illustrated (blue) only if their function is linked to other processes in the figure.The localization of pathways in vegetative cells or heterocysts is representative only for nitrogen fixation (heterocysts) and PSII activity(vegetative cells).Note that only amin or part of the nitrogen fixed in heterocysts is incorporate dusing the GS-GOGAT pathway and used for synthesis of amino acids, most is exported to the plant as NH3.Sugar is provided by the plant via the sugar phosphotransferase system (PTS). Function has been lost int he glycolyticpathway as the pfkA gene, encoding 6-phosphofructokinase, is a pseudogeneand sugar metabolism in the Azolla cyanobiont probably proceeds via the Oxidative Pentose Phosphate Pathway (OPPP). Extensive loss of function is evident among genes involved in uptake and transport of nutrients and NoAzhas lost the capacity to both import and metabolise alternative nitrogen sources.
2. The Cell Biology of Cyanobacteria, page 293
12. Advances in Biology and Ecology of Nitrogen Fixation, page 45
Last edited by Otangelo on Fri Nov 20, 2020 4:32 am; edited 70 times in total