Anoxygenic Photosynthesis

In the article we are going to study about oxygenic photosynthesis. Here we are also going to discuss oxygenic photosynthesis bacteris.at last we are going to discuss some important questions related to the topics.

Anoxygenic phototrophic bacteria (APB) are a diverse phylogenetic group of bacteria that use a variety of organic/inorganic electron donors to perform anoxygenic photosynthesis. They have several desirable properties, including anaerobic growth, low energy requirements, diverse modes of metabolism, low growth and maintenance costs, and can be used for a variety of environmental biotechnology applications.

They can also be used to generate a variety of valuable metabolic and cellular products. However, in comparison to algae and cyanobacteria, APB-based applications have received far less attention. Despite the fact that several life cycle assessment studies have highlighted the unsustainable nature of algae-derived bioenergy and the potential risks of unwanted cyanotic in production in mixed algal cultures. As a result, a paradigm shift is required, with a focus on identifying and developing techniques for harvesting bioenergy and value-added substances from APB.

Bacterial photosynthetic activity can be either oxygenic or anoxygenic. They differ in terms of the chlorophyll and carotenoid pigments they contain, the photosynthetic electron donor they use, and the composition of their photosynthesis in the machinery. Without the evolution of oxygen, anoxygenic phototrophic bacteria can perform photosynthesis. These bacteria can grow in anaerobic conditions, and the presence of oxygen inhibits the formation and function of most groups of anoxygenic phototrophs’ photosynthetic machinery and pigments.

Phylogeny and types of anoxygenic phototrophic bacteria

APB are a phylogenetic and photosynthetically diverse group of organisms that share two common characteristics that set them apart from the more conserved oxygenic phototrophic bacteria. The first is that they use bacteriochlorophylls as the primary photo pigment rather than chlorophyll; and the second is that they do not oxidise water, but rather use sulphide, hydrogen, organics, or similar electron donors as reducing power for photosynthesis. Photosynthesis is carried out by ABP bacteria using either a type I or a type II reaction center. Organisms of the phylum Chlorobi, Firmicutes, and Acidobacteria have type I reaction centers, whereas organisms of the phylum Chloroflexi, Proteobacteria, and Gemmatimonadetes have type II reaction centres.APB harvest light at a wavelength of 740–1020 nm, allowing multiple anoxygenic phototrophs to coexist in the same environment. The bacteria, Chloroflexus aurantiacus, have high levels of bacteriochlorophyll c and were initially classified as a distinct lineage called as” green non-sulfur bacteria” by Oyaizu et al., along with the closely related filamentous chemotrophic, Herpetosiphon aurantiacus, and a non-motile chemotrophic, Thermomicrobium roseum. However, because these bacteria differed from green sulphur bacteria in both phylogeny and physiological characteristics, they are now classified as filamentous anoxygenic phototrophic bacteria.

Aerobic anoxygenic APB are obligate heterotrophs that use bacteriochlorophyll (BChl) and as their primary light-harvesting pigment. This group of bacteria, unlike APB, requires oxygen for growth and photosynthetic electron transfer, and they lack Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), an important photosynthetic enzyme that allows atmospheric carbon dioxide to be fixed into carbon-rich organic compounds. These organisms will not be discussed in this review because they are quite distinct from APB and have not been well studied for their applications. Table 1 lists the various APB groups and highlights their diverse metabolic characteristics and habitats.

Bacteria and Archaea

Green Sulphur bacteria (GSB), red and green filamentous phototrophs (FAPs, such as Chloroflexi), purple bacteria, acidobacteria, and heliobacteria are among the bacteria that can perform anoxygenic photosynthesis.

Some archaea (for example, Halobacterium) capture light energy for metabolic function and are thus phototrophic, but none have been found to “fix” carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and an electron transport chain, proteins like halo rhodopsin use triterpenes to move ions against a gradient and produce ATP via chemiosmosis in the same way that mitochondria.

Pigments

Anaerobic photosynthesis pigments are similar to chlorophyll but differ in molecular detail and peak wavelength of light absorbed. Within their natural membrane milieu, bacteriochlorophylls a through g absorb the most electromagnetic radiation in the near-infrared. This is in contrast to chlorophyll a, the most common plant and cyanobacteria pigment, which has a peak absorption wavelength that is approximately 100 nanometers shorter (in the red portion of the visible spectrum).

Type 1 reaction centers

The reaction center bacteriochlorophyll pair, P840, is used in the electron transport chain of green Sulphur bacteria, such as that found in the model organism Chlorobaculum tepidum. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential and readily donates an electron to bacteriochlorophyll 663, which then passes it down an electron transport chain. The electron travels through a series of electron carriers and complexes before being used to reduce NAD+ to NADH. P840 regeneration is accomplished by cytochrome oxidizing a sulphide ion from hydrogen sulphide (or hydrogen or ferrous iron).

Type 2 reaction centers

Although the type II reaction centers are structurally and sequentially similar to Photosystem II (PSII) in plant chloroplasts and cyanobacteria, known anoxygenic photosynthesis organisms lack a region analogous to PSII’s oxygen-evolving complex..

When the reaction Center bacteriochlorophyll pair, P870, is excited by light absorption, the electron transport chain of purple non-sulfur bacteria begins. Excited P870 will then donate an electron to bacteriopheophytin, which will then pass it down the electron chain to a series of electron carriers. During the process, an electrochemical gradient is formed, which can then be used to synthesize ATP via chemiosmosis. P870 must be regenerated (reduced) before a photon can reach the reaction-center and restart the process. In the bacterial environment, the most common electron donor is molecular hydrogen.

Conclusion

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 is known as phototrophy. Phototrophy is classified into two types: chlorophyll-based chloro phototrophy and rhodopsin-based retinal photography. Anoxygenic photosynthesis is a phototrophic process that captures and converts light energy to ATP without producing oxygen; water is thus not used as an electron donor. Anoxygenic phototrophic bacteria (APB) are a diverse phylogenetic group of bacteria that use a variety of organic/inorganic electron donors to perform anoxygenic photosynthesis. They have several desirable properties, including anaerobic growth, low energy requirements, diverse modes of metabolism, low growth and maintenance costs, and can be used for a variety of environmental biotechnology applications. Bacterial photosynthetic activity can be either oxygenic or anoxygenic. Green Sulphur bacteria (GSB), red and green filamentous phototrophs (FAPs, such as Chloroflexi), purple bacteria, acidobacteria, and heliobacteria are among the bacteria that can perform anoxygenic photosynthesis. Anaerobic photosynthesis pigments are similar to chlorophyll but differ in molecular detail and peak wavelength of light absorbed.

Anoxygenic Photosynthesis