Nonproteobacteria Gram-Negative Bacteria

Cyanobacteria

The cyanobacteria, also known as blue-green bacteria, are a phylum of bacteria that obtain their energy through photosynthesis.

Learning Objectives

Describe the characterstics associated with Cyanobacteria including:  cell types, forms of motility and metabolic properties

Key Takeaways

Key Points

  • Cyanobacteria can be found in almost every terrestrial and aquatic habitat.
  • Cyanobacteria include unicellular and colonial species.
  • 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.
  • Many cyanobacteria are able to reduce nitrogen and carbon dioxide under aerobic conditions, which may be responsible for their evolutionary and ecological success.

Key Terms

  • cyanobacteria: Cyanobacteria, also known as blue-green bacteria, blue-green algae, and Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis.
  • photosynthesis: The process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts.
  • heterocyst: A specialized nitrogen-fixing cell formed by some filamentous cyanobacteria.

Cyanobacteria, also known as blue-green bacteria, blue-green algae, and Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The ability of cyanobacteria to perform oxygenic photosynthesis is 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 the endosymbiotic theory, chloroplasts in plants and eukaryotic algae have evolved from cyanobacterial ancestors via endosymbiosis.

DISTRIBUTION AND EFFECT ON ECOSYSTEMS

Cyanobacteria can be found in almost every terrestrial and aquatic habitat. Aquatic cyanobacteria are probably best known for the extensive and visible blooms that can form in both freshwater and the marine environment. These can have the appearance of blue-green paint or scum. The association of toxicity with such blooms has frequently led to the closure of recreational waters when blooms are observed. Cyanobacteria include unicellular and colonial species. Colonies may form filaments, sheets, or even hollow balls. Some filamentous colonies show the ability to differentiate into several different cell types, including:

image

Cyanobacteria: Cyanobacteria, also known as blue-green bacteria, blue-green algae, and Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis

image

Blue-green algae cultured in specific media: Cyanobacteria cultured in specific media. Cyanobacteria can be helpful in agriculture as they have the capability to fix atmospheric nitrogen to soil.

  • Vegetative cells, the normal, photosynthetic cells that are formed under favorable growing conditions.
  • Akinetes, the climate-resistant spores that may form when environmental conditions become harsh.
  • Thick-walled heterocysts, which contain the enzyme nitrogenase, vital for nitrogen fixation. Heterocysts may also form under the appropriate environmental conditions (anoxic) when fixed nitrogen is scarce.

Heterocyst-forming species are specialized for nitrogen fixation and are able to bind nitrogen gas to ammonia (NH3), nitrites (NO−2) or nitrates (NO−3). These molecules can be absorbed by plants and converted into protein and nucleic acids.

HORMOGONIA

Many cyanobacteria form motile filaments called hormogonia, that travel from the main biomass to bud and form new colonies elsewhere. The cells in a hormogonium are often thinner than those found in the vegetative state, and the cells on either end of the motile chain may be tapered. To break away from the parent colony, a hormogonium often must tear a weaker filament cell, called a necridium.

CYANOBACTERIUM CELLS AND MOTILITY

Individual cells of a cyanobacterium typically have a thick, gelatinous cell wall. They lack flagella, but hormogonia and some species may move about by gliding along surfaces. Many of the multi-cellular filamentous forms of Oscillatoria are capable of a waving motion; the filament oscillates back and forth. 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.

PHOTOSYNTHESIS AND OTHER METABOLIC PROCESSES

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. The water-oxidizing photosynthesis is accomplished by coupling the activity of photosystem (PS) II and I (Z-scheme). 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.

They also share an archaeal property, the ability to reduce elemental sulfur by anaerobic respiration in the dark. Their photosynthetic electron transport shares the same compartment as the components of respiratory electron transport. Their plasma membrane contains only components of the respiratory chain, while the thylakoid membrane hosts both respiratory and photosynthetic electron transport.

CLASSIFICATION

The cyanobacteria were traditionally classified by morphology into five sections, referred to by the numerals I-V. The first three–Chroococcales, Pleurocapsales, and Oscillatoriales–are not supported by phylogenetic studies. However, the latter two–Nostocales and Stigonematales–are monophyletic, and make up the heterocystous cyanobacteria. Some cyanobacteria produce toxins, called cyanotoxins. This results in algal blooms, which can become harmful to other species including humans if the cyanobacteria involved produce toxins.

Anoxygenic Photosynthetic Bacteria

Some groups of bacteria can capture light energy and convert it to ATP without the production of oxygen through anoxygenic photosynthesis.

Learning Objectives

Describe the mechanisms that specific bacteria use to undergo anoxygenic photosynthetic bacteria, including: green sulfur and purple sulfur bacteria

Key Takeaways

Key Points

  • There are several groups of bacteria that undergo anoxygenic photosynthesis: green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic acidobacteria, and phototrophic heliobacteria.
  • Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls similar to chlorophyll found in eukaryotes.
  • Green sulfur bacteria are a family of obligately anaerobic photoautotrophic bacteria most closely related to the distant Bacteroidetes which are adapted to a narrow range of energy-limited conditions, an ecology shared with the simpler cyanobacteria.
  • Purple sulfur bacteria are a group of Proteobacteria capable of photosynthesis, anaerobic or microaerophilic, and often found in hot springs or stagnant water.

Key Terms

  • anoxygenic: That does not involve the production of oxygen
  • photosynthesis: The process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts.
  • photoautotroph: An organism, such as all green plants, that can synthesize its own food from inorganic material using light as a source of energy

Overview of Photosynthesis

Phototrophy is the process by which organisms trap light energy (photons) and store it as chemical energy in the form of ATP and/or reducing power in NADPH. There are two major types of phototrophy: chlorophyll-based chlorophototrophy and rhodopsin-based retinalophototrophy. Chlorophototrophy can further be divided into oxygenic photosynthesis and anoxygenic phototrophy.

Oxygenic and anoxygenic photosynthesizing organisms undergo different reactions, either in the presence of light or with no direct contribution of light to the chemical reaction (colloquially called “light reactions” and “dark reactions”, respectively). Anoxygenic photosynthesis is the phototrophic process where light energy is captured and converted to ATP, without the production of oxygen; water is, therefore, not used as an electron donor. There are several groups of bacteria that undergo anoxygenic photosynthesis: green sulfur bacteria, green and red filamentous anoxygenic phototrophs (FAPs), phototrophic purple bacteria, phototrophic acidobacteria, and phototrophic heliobacteria.

ANOXYGENIC PHOTOTROPHS

Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls; these are similar to chlorophyll found in eukaryotes. Bacteriochlorophyll a and b have wavelengths of maximum absorption at 775 nm and 790 nm, respectively. Unlike oxygenic phototrophs, anoxygenic photosynthesis only functions using either one of two possible types of photosystem. This restricts them to cyclic electron flow; they are therefore unable to produce O2 from the oxidization of H2O.

image

Bacteriochlorophyll a: Bacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria.

GREEN SULFUR BACTERIA

The green sulfur bacteria are a family of obligately anaerobic photoautotrophic bacteria most closely related to the distant Bacteroidetes. They are non-motile with the exception of Chloroherpeton thalassium, which may glide. They come in sphere, rods, and spiral forms. Photosynthesis is achieved using bacteriochlorophyll (BChl) c, d, or e, in addition to BChl a and chlorophyll a, in chlorosomes attached to the membrane. The electron transport chain (ETC) of green sulfur bacteria uses the reaction centre bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it either returns to P840 or is used to reduce NAD+. If the electron leaves the chain to reduce NAD+, P840 must be reduced for the ETC to function again. The green sulfur bacterias’ small dependence on organic molecule transporters and transcription factors indicates that these organisms are adapted to a narrow range of energy-limited conditions, and fit into an ecology shared with the simpler cyanobacteria,

image

Green d winogradsky: A column containing green sulfur bacteria which uses anoxygenic photosynthesis.

PURPLE SULFUR BACTERIA

The purple sulfur bacteria are a group of Proteobacteria capable of photosynthesis. They are anaerobic or microaerophilic, and are often found in hot springs or stagnant water. Unlike plants, algae, and cyanobacteria, they do not use water as their reducing agent, and so do not produce oxygen. Instead, they use hydrogen sulfide, which is oxidized to produce granules of elemental sulfur. This in turn may be oxidized to form sulfuric acid.The purple sulfur bacteria are divided into two families: the Chromatiaceae and Ectothiorhodospiraceae, which respectively produce internal and external sulfur granules, and show differences in the structure of their internal membranes.

Purple sulfur bacteria are generally found in illuminated anoxic zones of lakes and other aquatic habitats where hydrogen sulfide accumulates. They are also found in “sulfur springs” where geochemically or biologically produced hydrogen sulfide can trigger the formation of blooms of purple sulfur bacteria. Anoxic conditions are required for photosynthesis; these bacteria cannot thrive in oxygenated environments. The electron transport chain of purple non-sulfur bacteria begins when the reaction center bacteriochlorophyll pair, P870, becomes excited by the absorption of light. Excited P870 will then donate an electron to Bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate a proton motor force (PMF) which can then be used to synthesize ATP by oxidative phosphorylation. The electron returns to P870 at the end of the chain so it can be used again once light excites the reaction-center.

Prochlorophytes

Picoplankton is the fraction of plankton which can be photosynthetic or heterotrophic.

Learning Objectives

Describe the characteristics associated with Prochlorophytes, a member of the Picoplankton

Key Takeaways

Key Points

  • Picoplankton are responsible for the majority of the primary productivity in oligotrophic gyres, and are different from nanoplankton and microplankton.
  • Because they are small, they have a greater surface-to-volume ratio. This enables them to obtain the scarce nutrients in these ecosystems.
  • Prochlorophyta are a photosynthetic prokaryote member of the phytoplankton group Picoplankton. They are abundant in nutrient poor tropical waters and use a unique photosynthetic pigment, divinyl-chlorophyll, to absorb light and acquire energy.

Key Terms

  • prochlorophyta: a photosynthetic prokaryote member of the phytoplankton group Picoplankton
  • picoplankton: plankton composed of cells between 0.2 and 2 micrometers that are either photosynthetic or heterotrophic

Picoplankton is the fraction of plankton, composed by cells between 0.2 and 2 μm, that is either photosynthetic (photosynthetic picoplankton; ) or heterotrophic (heterotrophic picoplankton). Some species are also mixotrophic. Picoplankton are responsible for the majority of the primary productivity in oligotrophic gyres, and are different from nanoplankton and microplankton. Because they are small, they have a greater surface-to-volume ratio, which enables them to obtain scarce nutrients in these ecosystems.

image

Prokaryotes vs. Eukaryotes: Picoplankton observed by epifluorescence microscopy, a technique which allows the detection of certain groups of cells possessing fluorescent pigments; and example would be Synechococcus, which possess phycoerythrin

Prochlorophyta are a photosynthetic prokaryote member of the phytoplankton group Picoplankton. These oligotrophic organisms are abundant in nutrient-poor tropical waters and use a unique photosynthetic pigment, divinyl-chlorophyll, to absorb light and acquire energy. These organisms lack red and blue Phycobilin pigments and have staked thylakoids, both of which make them different from Cyanophyta ( Cyanobacteria ). Prochlorophyta were initially discovered in 1975 near the Great Barrier Reef and off the coast of Mexico. The following year, Ralph A. Lewin, of the Scripps Institution of Oceanography, assigned them as a new algal sub-class.

In addition to Prochlorophyta, other phytoplankton that lack Phycobilin pigments were later found in freshwater lakes in the Netherlands, by Tineke Burger-Wiersma. These organisms were termed Prochlorothrix. In 1986, Prochlorococcus was discovered by Sallie W. Chisholm and his colleagues. These organisms might be responsible for a significant portion of the global primary production.

Prochlorophytes are very small microbes generally between 0.2 and 2 µm (Photosynthetic picoplankton). They morphologically resemble Cyanobacteria, formally known as Blue Green Algae. Members of Prochlorophyta have been found as coccoid (spherical) shapes, like Prochlorococcus, and as filaments, like Prochlorothrix.