Photosynthetic bacteria Definition

• Photosynthetic bacteria are an essential group of prokaryotes which are capable of converting sunlight (from the sun), into chemical energy via a process called photosynthesis.
• Because they can generate their own energy from inorganic materials found around them, they are called photoautotrophs. They are therefore not dependent on organic material from other organisms for energy.
• There are many types of photosynthetic bacteria, which are widespread in terrestrial and aquatic environments. They can be found in soils, oceans, sludges, and lakes. 
• These organisms are also primary producers in these habitats (e.g., marine habitats with the highest concentration of photosynthetic bacteria) and play an important role in the food chain.
• Photosynthetic bacteria plays a crucial role in both carbon fixation and oxygen production.

Photosynthetic bacteria Examples

There are many types of bacteria that can photosynthesize, as we have already mentioned. They can be divided into five main groups, which include:

Cyanobacteria

Cyanobacteria, also known as “blue green algae”, are Gram-negative bacteria that are commonly found in aquatic ecosystems. While there have been more than 2000 species of Cyanobacteria described, it is estimated that over 6,000 species are distributed in various habitats around the world.

These species can live in a symbiotic relationship to other plants and lichens due to their ability to fix nitrogen. Many other species, however, can also be found in soil and water.

Photosynthesis in Cyanobacteria

Photosynthesis in Cyanobacteria, like plants and algae is characterized as the released of oxygen. This is called oxygenic photosynthesis. These bacteria are classified as oxygenic photostrophs. They have been found to be flexible and diverse in their pigmentation.

Most species contain Chl. a, phycocyanin and allophycocyanin. However, some species also contain phycoerythrin. Other species, such as Prochlorococcus species and Acaryochloris species, contain Chl. b and Chl. d. Studies have shown that Chl. a can be transformed into Chl. b by changing certain environmental conditions in some species.

Photosynthesis in Cyanobacteria is a linear transfer of electrons. This is also the case for other oxygenic phototrophs. The electrons are transferred to NADP (Nicotinamide adenine diucletide phosphate) by the PSII and PSII photosystems, resulting in the production oxygen and ATP.

Photosynthesis in Cyanobacteria generally involves these steps:

1. The PSII system, which is located on the Cyanobacteria Thylakoid surface, absorbs light energy. This breaks down water and releases oxygen and electrons.
2. The electrons are then transferred from the first step by plastoquinone, an isoprenoid quone molecule implicated in the electron transport chain, into the thylakoid membrane and to the cyt complex (cytochrome).
3. These electrons are taken from the b6f compound and transferred to the PSI(Photosystem I) via plastocyanin, (PC), through the thylakoid lumin
4. PSI is a process in which electrons are excited by light again before they reduce NADP+
5. The thylakoid lumen is where the protons from this process are released, which promotes the synthesis and use of ATP by the enzyme ATP synase. However, electrons from PSI reenter PQ pool. This contributes to the production more ATP than NADPH.

Examples of Cyanobacteria 

Here are some examples of Cyanobacteria types:

• Synechococcus elongatus
• Microcystis aeruginosis
• Nostoc punctiforme
• Anabaena variabilis
• Arthrospira platensis

Proteobacteria (purple bacteria)

The phylum Proteobacteria is the second group of photosynthetic bacteria. Proteobacteria is a Gram-negative bacteria that is widely distributed around the world, just like Cyanobacteria. This phylum, however, is not Cyanobacteria. It contains many pathogenic species, including Salmonella, Escherichia and Vibrio.

According to studies, there are over 460 genera within the Phylum Proteobacteria with over 1,600 species within five (5) main classes; Gammaproteobacteria, Alphaproteobacteria, Epsilonproteobacteria, Betaproteobacteria, and Deltaproteobacteria.

Photosynthesis in Proteobacteria

• Cyanobacteria is an oxygen-photosynthetic bacteria. However, Proteobacteria members are more closely associated with anoxygenic photosynthetic bacteria. However, there are some aerobic species that have been found in various habitats around the globe.
• Comparable to plants and other photosynthetic bacteria, (e.g. Cyanobacteria), Purple bacteria capable of photosynthesis have been shown to only consist of a single photosystem (bacteriochlorophyll) within the intracellular membrane. These bacteria are unable split water and release oxygen due to the simplicity of their photosynthetic devices.
• Simple organic acids and hydrogen disulfide are common electron donors in anaerobic environments. These electrons are then carried through different electron carriers, before the protons are pumped through membrane to create a proton gradient. The enzyme ATP synthase then uses the gradient to create ATP.
• Purple sulfur bacteria uses hydrogen and sulfide as electron donors. Non-sulfur bacteria rely on organic carbon compounds.

Examples of photosynthetic Proteobacteria

Here are some examples of photosynthetic Proteobacteria varieties:

• Rhodobacter sphaeroides
• Rhodobacter capsulatus
• Chromatium okenii
• Rhodopseudomonas palustris
• Alorhodospira halophila

Heliobacteria

• Heliobacteria was first described in 1983. It is a Gram-positive bacteria belonging to the phylum Firmicutes. Most members of this group, like Cyanobacteria are capable of nitrogen fixation. Many Heliobacteria species are found in rice paddy soil, while some species can also be found in soils other than that (rarely found within aquatic environments).
• Heliobacteria, which are obligate anaerobes, are very sensitive to oxygen. They are capable of photosynthesis, but not producing oxygen. Heliobacteria are capable of photosynthesis but not autotrophic growth. Photosynthesis is therefore crucial for nitrogen fixation.
• Heliobacteria have a unique photosynthesis pigment, bacteriochlorophyll (BChl) g. This is unlike other bacteria that can photosynthesise.
• Heliobacteria photosynthetic growth is inhibited in molecular oxygen.
• Heliobacteria have a unique reaction centre that uses iron-sulfur as the terminal electron acceptors, which is different from other photosynthetic bacteria. This type of reaction center can be classified as a type 1, which is different than those found in purple bacteria.
• The electrons are generally transferred from cytochrone C (a membrane-bound structure), to a chlorophyll-a-like molecule via the P800 special couple before reaching the iron–sulfur centre.
• Some stages of the electron processing (e.g. Transfer from cytochrome C to P800 has been shown to depend on the environment conditions such as temperature and viscosity.
• Heliobacterium mobiliz and Heliobacterium modicaldum are two of the most well-known photosynthetic Heliobacteria.

Chlorobi

• Chlorobi, also known as the green sulfur bacteria is a phylum which consists of anaerobic mandatory bacteria that can only grow under anoxic conditions. This group is often found in low-light environments and sulfur-rich habitats, as their name implies.
• They can be found in the lowest part of the photic area, which is located in ocean environments. The photic area is the zone or layer that lies near the ocean’s surface. They are capable of capturing light energy necessary for photosynthesis.
• The localization of green sulfur bacteria at the lowest part of the photic area not only allows them access to light energy but also protects them against atmospheric oxygen. They are extremely sensitive to oxygen.
• Green sulfur bacteria is an obligate phototroph, which means that they do not depend on any other sources of energy.
• They are often found in low-light environments, which is why they have large light-harvesting structures (photosynthetic antenna complex), also known as the chlorosome.
• These complexes contain the Bacteriochlorophylls, BChl.c, BChl.d and BChl.e. They can hold approximately 200,000 Bacteriochlorophylls molecules. This is important for these bacteria because it ensures they capture enough light photons to enable photosynthesis.

Photosynthesis in Green Sulfur Bacteria

• Green sulfur bacteria, anoxygenic phototrophs, require anoxic conditions for photosynthesis. They use sulfur (or other forms sulfur-like elemental sulfur, or thiosulfate), instead of water to donate electrons (splitting water releases oxygen).
• These electrons can then be used to fix carbon via the noncyclic electron transportation chain (which is also associated with NADPH production).
• The electrons that are transported through the electron chain in the noncyclic system end up as NADPH (by reducing NADP), which is used to fix carbon.

Examples of photosynthetic Chlorobi 

Some examples of photosynthetic Chlorobi species include:

• Chlorobium tepidum
• Chlorobium phaeovibrioides
• Pelodictyon phaeoclathhratiforme 
• Chlorobium phaeobacteroides 

Chloroflexi

• Chlorobacteria is also known as Chlorobacteria. It is a phylum that includes bacteria with different characteristics. Some species require oxygen to live a chemoheterotrophic lifestyle. Others are anoxygenic photoautotrophs and can photosynthesise light energy.
• This phylum is ecologically and physiologically diverse. It can be found in many habitats, including hot springs and sediments. The filamentous morphology is characteristic of most classes in the phylum Chloroflexi.
• Only the class Chloroflexia, also known as filamentous green bacteria non-sulfur bacteria, within the phylum Chloroflexi, contains phototrophic bacteria.
• These bacteria have light-harvesting complexes that are called chlorosomes. They contain bacteriochlorophyll (c). The inner side of the cell membrane is where the chlorosomes attach. This includes some light-harvesting bacteria bacteriochlorophyll (d) and some bacteriochlorophyll (a).
• Complex III a (menaquinol), which is involved in electron transport during photosynthesis, has been replaced by cytochrome BC in other photosynthetic organisms.
• The photosynthesis process in these organisms is not well understood. Photosynthetic species are classified under the class Chloroflexia.

Photosynthetic Bacteria Vs Chemosynthetic Bacteria

Chemosynthetic bacteria are bacteria species that use chemical energy from organic molecules such as hydrogen sulfide and inorganic materials like methanol to produce organic compounds. This is a great method for bacteria living in harsh environments (e.g. in deep ocean vents).

Some bacteria that depend on chemosynthesis to get energy include:

• Sulfur bacteria – E.g. Paracoccus denitrificans
• Methanobacteria – E.g.  Methanobacterium kanagiense
• Hydrogen-oxidizing bacteria – E.g. Venenivibrio stagnispumantis 

Environments such as the ocean floor, e.g. Living organisms cannot access light energy in environments such as the ocean floor (e.g. deep-sea vent). Many microorganisms depend on chemical energy to make organic compounds they and their surroundings can use.

Chemosynthesis requires several elements. These elements include a carbon source (e.g. These include a carbon source (e.g. carbon dioxide), an electron donors, and an energy resource. As an example, hydrogen sulfide can be used to convert carbon courses such as carbon dioxide into simple sugars by chemosynthetic bacteria living in giant tubeworms deep within hydrothermal vents.

Some of these byproducts could include water, sulfur, and so on. Depending on the energy source,

Sulfide, methane and methane are two of the most important electron donors, especially in deep-sea.

The process can be either aerobic or anaerobic depending on the location of the chemosynthetic bacteria. In aerobic chemosynthesis oxygen is the primary electron acceptor. However, other compounds such as carbon dioxide, nitrates and sulfates are also used. Anaerobic chemosynthesis uses electron acceptors.

Similarities

Produce their own food

Photosynthetic and chemosynthetic bacteria can produce their own food by using elements such as water, carbon dioxide and an energy source. These bacteria are called primary producers. They can produce their own food and do not heavily depend on any other organisms or organic matter (e.g. carbohydrates) for energy

Unicellular organisms 

The majority of bacteria that is involved in photosynthesis and chemosynthesis are single-celled organisms, unlike plants. Some species, however, form long filaments because they don’t separate after cell division.

Both photosynthetic as well as chemosynthetic bacteria require three elements to synthesize organic compounds. These elements include an energy source, electron donor, and a carbon matter (carbon dioxide).

Differences between Photosynthetic and Chemosynthetic Bacteria

Source of energy

The source of energy needed for carbon fixation is one of the major differences between photosynthetic and chemosynthetic bacterium.

While photosynthetic bacteria uses light energy (energy from the sun) to photosynthesise, chemosynthetic organisms use energy from inorganic materials like hydrogen sulfide. These bacteria store energy in chemical bonds within these materials.

Phototrophs are photosynthesis bacteria that use light energy. Chemosynthetic bacteria are referred to as chemotrophs.

Plastids

Photosynthetic bacteria must capture light energy. They have special structures called chloroplasts, which contain chlorophyll. Chemosynthetic bacteria on the other side do not require plastids or chlorophyll to capture light energy.

Oxygen as a byproduct 

Because some photosynthetic bacteria must split water during photosynthesis (oxygen) is one of the major by-products. Many chemosynthetic bacteria do not release oxygen gas as a byproduct because they don’t use water the same way.